Related
I have a program in C, in which I initialize multiple number of arrays each with a bunch of lines. However, i'd like to avoid that since it increases the length of my main function. For example I have this;
int * pickup_Ind;
double *pickup_Val;
pickup_Ind = (int *) malloc(sizeof(int) * (size1));
pickup_Val = (double *) malloc(sizeof(double) * (size1));
int * lInd;
double *lVal;
lInd = (int *) malloc(sizeof(int) * size2);
lVal = (double *) malloc(sizeof(double) * size2);
int * simul_Ind;
double *simul_Val;
simul_Ind = (int *) malloc(sizeof(int) * (size3));
simul_Val = (double *) malloc(sizeof(double) * (size3));
I know I can reduce the number of lines by for example writing as:
int * pickup_Ind = (int *) malloc(sizeof(int) * (size1));
But still i will need to do this for every array. How to write this in a compact form with a function (which i will store in a header file), and then call this function from main. Not to mention i do not want to declare them as global variables, but to be able to use them in main. I tried the function below.
void initialize_bounds(int *arr1,int size1)
{
arr1= (int *) malloc(sizeof(int) * (size1));
for(int i=0;i<size1;i++)
arr1[i]=i;
}
But if i call this function via the following in main, i get error "Varuable test being used without initialized"
int* test;
initialize_bounds(test);
So to sum up, if i could write something like this, my problem is solved:
int *pickup_Ind,*pickup_Val,*lind,*lval;
int size1,size2;
initalize_bounds(pickup_Ind,pickup_Val,size1,size2);
You could write a function
void initialize_bounds(int **ind, double **val, int size) {
*ind = malloc(sizeof (**ind)*size);
for (int i = 0; i < size; i++) {
(*ind)[i] = i;
}
*val = malloc(sizeof (**val)*size);
}
and call it like
int * pickup_Ind;
double *pickup_Val;
initialize_bounds(&pickup_Ind, &pickup_Val, size1);
to initialize both arrays in one line. You still have to place one call to it per array-pair, however.
In the C language, arguments are passed to functions by value - so, actually, a copy is made and the original variable (in the calling code) cannot be changed. So, if you want a function to modify (say) an int argument, you pass it a pointer to that int.
Likewise, if you want a function to modify a pointer, you have to pass a pointer to that pointer.
So, in the case of the initialize_bounds function you have shown, you would need this:
void initialize_bounds(int** arr1,int size1) // 1st arg is a pointer to the pointer!
{
*arr1 = (int *) malloc(sizeof(int) * (size1)); // De-reference our `arr1` pointer
for(int i=0;i<size1;i++)
(*arr1)[i]=i;
}
Then, you can use this to initialize a pointer in your main function with a call like this:
int* test;
initialize_bounds(&test); // We need to pass the ADDRESS of the pointer we want to modify!
You can write a function that returns a freshly allocated and initialized array.
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
/* Header file */
int* AllocateArray(size_t size);
void DeallocateArray(int *array);
int main(void) {
const size_t size = 10;
int *const array = AllocateArray(size);
for (size_t i = 0; i < size; ++i) {
printf("%d ", array[i]);
}
printf("\n");
DeallocateArray(array);
return 0;
}
/* Implementation */
int* AllocateArray(size_t size) {
int *const array = malloc(size * sizeof(int));
if (array == NULL) {
// Allocation failed, handle it...
}
for (size_t i = 0; i < size; ++i) {
array[i] = i;
}
return array;
}
void DeallocateArray(int *array) {
if (array == NULL) {
return;
}
free(array);
}
I'd use something higher level, e.g. stretchy buffers. See this video for a live coding session that implements those - props to Per Vognsen for making this code, and for placing into public domain (i.e. completely free to use for any purpose, but I'm not a lawyer, so take anything I say with caution :).
You'd want to include bitwise/ion/common.c in your source file, and then the array allocation becomes simple. Stretchy buffers are perhaps the closest you get to the convenience of C++'s std::vector in C. They offer an API that doesn't feel like a C++ API transcribed in C - it is at the correct level, and lets you use plain pointers in a very sensible way (e.g. a buf_len of a NULL pointer is zero, not a crash, buf_push(mybuf, element) appends an element to the array and extends it if necessary, etc.
#include <assert.h>
#include <string.h>
#include <stdlib.h>
// note that common.c includes nothing, so you have to set it up
#include "common.c"
#define buf_resize(b, n) ((n) <= buf_len(b) ? (b) : (((b) = buf__grow((b), (n), sizeof(*(b)), 0)), ((b) ? buf__hdr((b))->len = (n) : 0), (b)))
typedef struct {
int * pickup_Ind;
double *pickup_Val;
int * lInd;
double *lVal;
int * simul_Ind;
double *simul_Val;
} Data;
enum {
size1 = ...,
size2 = ...,
size3 = ...
}
Data make_Data(void) {
Data d;
memset(&d, 0, sizeof(d));
assert(buf_len(d->pickup_Ind) == 0);
buf_resize(d.pickup_Ind, size1);
buf_resize(d.pickup_Val, size1);
buf_resize(d.lInd, size2);
buf_resize(d.lVal, size2);
buf_resize(d.simul_Ind, size3);
buf_resize(d.simul_Val, size3);
}
int main(int argc, char **argv) {
Data d = make_Data();
assert(buf_len(d.pickup_Ind) == size1);
d.pickup_Ind[0] = 10;
assert(buf_len(d.pickup_Ind) == size1);
buf_push(d.pickup_Ind, 11);
assert(buf_len(d.pickup_Ind) == size1 + 1);
}
If you're building up the arrays by adding elements to them one-by-one, it'll make sense to reserve the capacity for the expected size of the array via buf_fit (it only reserves the memory but the buffer retains its length (e.g. zero)). The capacity reservation is entirely optional, though. It's there to prevent reallocation of the arrays while you add elements to them.
Thus:
Data make_Data(void) {
Data d;
memset(&d, 0, sizeof(d));
assert(buf_len(d->pickup_Ind) == 0);
buf_fit(d.pickup_Ind, size1);
buf_fit(d.pickup_Val, size1);
buf_fit(d.lInd, size2);
buf_fit(d.lVal, size2);
buf_fit(d.simul_Ind, size3);
buf_fit(d.simul_Val, size3);
}
int main(int argc, char **argv) {
Data d = make_Data();
assert(buf_len(d.pickup_Ind) == 0); // zero length: no data in the array (yet!)
assert(buf_cap(d.pickup_Ind) >= size1); // but it has the capacity we need
buf_push(d.pickup_Ind, 10);
buf_push(d.pickup_Ind, 11);
assert(buf_len(d.pickup_ind) == 2);
}
If you'll want to use stretchy buffers in multiple source files, you'll run afoul of the one declaration rule (ODR). Thus, you'll need to factor out macro definitions and function declarations out of common.c and into common.h.
If the Data is only allocated once, there's no need to free it prior to exiting the program: the operating system already does it for you. Otherwise, you may wish to add a function to do this job:
void free_Data(Data *d) {
buf_free(d.pickup_Ind);
buf_free(d.pickup_Val);
buf_free(d.lInd);
buf_free(d.lVal);
buf_free(d.simul_Ind);
buf_free(d.simul_Val);
assert(buf_len(d.pickup_Ind) == 0);
}
There are a lot of questions about converting a 2D array into a 1D array, but I am attempting just the opposite. I'm trying to partition a string into substrings of constant length and house them in a 2D array. Each row of this 2D matrix should contain a substring of the initial string, and, if each row were to be read in succession and concatenated, the initial string should be reproduced.
I nearly have it working, but for some reason I am losing the first substring (partitions[0] -- length 8*blockSize) of the initial string (bin):
int main (void){
char* bin = "00011101010000100001111101001101000010110000111100000010000111110100111100010011010011100011110000011010";
int blockSize = 2; // block size in bytes
int numBlocks = strlen(bin)/(8*blockSize); // number of block to analyze
char** partitions = (char**)malloc((numBlocks+1)*sizeof(char)); // break text into block
for(int i = 0; i<numBlocks;++i){
partitions[i] = (char*)malloc((8*blockSize+1)*sizeof(char));
memcpy(partitions[i],&bin[8*i*blockSize],8*blockSize);
partitions[i][8*blockSize] = '\0';
printf("Printing partitions[%d]: %s\n", i, partitions[i]);
}
for(int j=0; j<numBlocks;++j)
printf("Printing partitions[%d]: %s\n", j,partitions[j]);
return 0;
}
The output is as follows:
Printing partitions[0]: 0001110101000010
Printing partitions[1]: 0001111101001101
Printing partitions[2]: 0000101100001111
Printing partitions[3]: 0000001000011111
Printing partitions[4]: 0100111100010011
Printing partitions[5]: 0100111000111100
Printing partitions[0]: Hj
Printing partitions[1]: 0001111101001101
Printing partitions[2]: 0000101100001111
Printing partitions[3]: 0000001000011111
Printing partitions[4]: 0100111100010011
Printing partitions[5]: 0100111000111100
The construction of partitions in the first for loop is successful. After construction at read out, the string at partitions[0] contains garbage values. Can anyone offer some insight?
int numBlocks = strlen(bin)/(8*blockSize); // number of block to analyze
char** partitions = (char**)malloc((numBlocks+1)*sizeof(char)); // break text into block
for(int i = 0; i<numBlocks;++i){
partitions[i] = (char*)malloc((8*blockSize+1)*sizeof(char));
memcpy(partitions[i],&bin[8*i*blockSize],8*blockSize);
partitions[i][8*blockSize] = '\0';
printf("Printing partitions[%d]: %s\n", i, partitions[i]);
}
This all looks suspicious to me; it's far too complex for the task, making it a prime suspect for errors.
For reasons explained in answers to this question, void * pointers which are returned by malloc and other functions shouldn't be casted.
There's no need to multiply by 1 (sizeof (char) is always 1 in C). In fact, in your first call to malloc you should be multiplying by sizeof (char *) (or better yet, sizeof *partitions, as in the example below), since that's the size of the type of element that partitions points at.
malloc might return NULL, resulting in undefined behaviour when you attempt to assign into the location it points at.
Anything else (i.e. everything that isn't NULL) that malloc, calloc or realloc returns will need to be freed when no longer in use, or else tools such as valgrind (a leak detection program, useful for people who habitually forget to free allocated objects and thus cause memory leaks) will report false positives and lose part of their usefulness.
numBlocks, i, or anything else that's for counting elements of an array, should be declared as a size_t to follow standard convention (e.g. check the strlen manual, synopsis section to see how strlen is declared, noting the type of the return value is size_t). Negative values caused by overflows here will obviously cause the program to misbehave.
I gather you've yet to think about any excess beyond the last group of 8 characters... This shouldn't be difficult to incorporate.
I suggest using a single allocation, such as:
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define BLOCK_SIZE 8
int main(void) {
char const *bin = "00011101010000100001111101001101000010110000111100000010000111110100111100010011010011100011110000011010";
size_t bin_length = strlen(bin),
block_count = (bin_length / BLOCK_SIZE)
+ (bin_length % BLOCK_SIZE > 0); // excess as per point 6 above
char (*block)[BLOCK_SIZE + 1] = malloc(block_count * sizeof *block);
if (!block) { exit(EXIT_FAILURE); }
for (size_t x = 0; x < block_count; x++) {
snprintf(block[x], BLOCK_SIZE + 1, "%s", bin + x * BLOCK_SIZE);
printf("Printing partitions[%zu]: %s\n", x, block[x]);
}
for (size_t x = 0; x < block_count; x++) {
printf("Printing partitions[%zu]: %s\n", x, block[x]);
}
free(block);
exit(0);
}
Their are a few problems with your code.
You are allocating **partitions incorrectly.
Instead of:
char** partitions = (char**)malloc((numBlocks+1)*sizeof(char)); /* dont need +1, as numblocks is enough space. */
You need to allocate space for char* pointers, not char characters.
instead, this needs to be:
char** partitions = malloc((numBlocks+1)*sizeof(char*));
Also read Why not to cast result of malloc(), as it is not needed in C.
malloc() needs to be checked everytime, as it can return NULL when unsuccessful.
Once finished with the space allocated, it is always good to free() memory previously requested by malloc(). It is important to do this at some point in the program.
Here is some code which shows this:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define BLOCKSIZE 2
#define BLOCK_MULTIPLIER 8
int main(void) {
const char *bin = "00011101010000100001111101001101000010110000111100000010000111110100111100010011010011100011110000011010";
const size_t blocksize = BLOCKSIZE;
const size_t multiplier = BLOCK_MULTIPLIER;
const size_t numblocks = strlen(bin)/(multiplier * blocksize);
const size_t numbytes = multiplier * blocksize;
char **partitions = malloc(numblocks * sizeof(*partitions));
if (partitions == NULL) {
printf("Cannot allocate %zu spaces\n", numblocks);
exit(EXIT_FAILURE);
}
for (size_t i = 0; i < numblocks; i++) {
partitions[i] = malloc(numbytes+1);
if (partitions[i] == NULL) {
printf("Cannot allocate %zu bytes for pointer\n", numbytes+1);
exit(EXIT_FAILURE);
}
memcpy(partitions[i], &bin[numbytes * i], numbytes);
partitions[i][numbytes] = '\0';
printf("Printing partitions[%zu]: %s\n", i, partitions[i]);
}
printf("\n");
for(size_t j = 0; j < numblocks; j++) {
printf("Printing partitions[%zu]: %s\n", j,partitions[j]);
free(partitions[j]);
partitions[j] = NULL;
}
free(partitions);
partitions = NULL;
return 0;
}
Which outputs non-garbage values:
Printing partitions[0]: 0001110101000010
Printing partitions[1]: 0001111101001101
Printing partitions[2]: 0000101100001111
Printing partitions[3]: 0000001000011111
Printing partitions[4]: 0100111100010011
Printing partitions[5]: 0100111000111100
Printing partitions[0]: 0001110101000010
Printing partitions[1]: 0001111101001101
Printing partitions[2]: 0000101100001111
Printing partitions[3]: 0000001000011111
Printing partitions[4]: 0100111100010011
Printing partitions[5]: 0100111000111100
I have an ASCII file in which the entries of a vector are stored. I do not know the length (number of rows) of the file, nor do I have an estimation about its size as it may strongly vary from a few lines to some tens of thousands. I need an efficient way to read the data stored in that file and load them to a float* variable. The code should be in C.
My question is how to allocate memory for the vector which I need to create given that I do not know its size beforehand? Can you please give an example?
Finally, what is in your opinion the most appropriate prototype for such a function? Should it be something like:
load_data(const char* filename, float* data, int* vector_size);
?
Update 1.: While doing some initial tests, I wrote the following code:
void create_random_matrix(float* matrix, const int nrows) {
matrix = (float *) malloc(sizeof (float) * nrows);
short i;
for (i = 0; i < nrows; i++) {
matrix[i] = 7.0f;
}
}
which should return an array with all its elements equal to 7.0f. Instead, when I call it from my main.c:
float *a;
create_random_matrix(a, 10);
printf("%f",a[0]);
it prints 0.0f. How is that possible?!
Update 2. Was it not for your help, the following (working) code wouldn't have been written:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#define LINE_SIZE 10
#define ALLOCATION_CHUNK 50
int load_vector_data(const char* filename, float** vector, int* length) {
*vector = malloc(sizeof (float) * ALLOCATION_CHUNK);
int allocated_rows = ALLOCATION_CHUNK;
u_short i = 0;
FILE* fr = fopen(filename, "r");
if (fr == NULL) {
exit(FILE_NOT_FOUND);
}
char line[LINE_SIZE];
while (fgets(line, LINE_SIZE, fr) != NULL) {
if (i >= allocated_rows){
allocated_rows += ALLOCATION_CHUNK;
*vector = realloc(*vector, sizeof (float) * allocated_rows);
}
strip_newline(&line, LINE_SIZE);
(*vector)[i] = strtod(line, (char **) NULL);
i++;
}
*length = i;
*vector = realloc(*vector, sizeof (float) * i);
fclose(fr);
}
void strip_newline(char *str, int size) {
u_short i;
for (i = 0; i < size; ++i) {
if (str[i] == '\n') {
str[i] = '\0';
return;
}
}
}
I tried it with an 8000-lines file and seems to be working just fine! Please, feel free to comment.
fgets is you friend for reading the data from the file (If my assumption that each bit of data is on a new-line is correct). Read each line in 1 by 1 and use an strtof on the text you read. Reading text and converting to floats is inherently a slow process so I reckon the above is perfectly good enough.
As for your second question there are a couple of ways to do it. You could pass a float** in and make the malloc inside the function. Though this has the disadvantage of you needing to free it outside the function which is not exactly obvious. The only other way, i can think of, would be to scan through the file and count the number of new-lines then pre-allocate the array length for that.
Its hard to say whether doing a malloc and a bunch of reallocs would be more efficient than the scan through to count the number of lines, it would probably be worth trying both methods (neither is particularly hard) and seeing which one is faster for you.
I know how to create an array of structs but with a predefined size. However is there a way to create a dynamic array of structs such that the array could get bigger?
For example:
typedef struct
{
char *str;
} words;
main()
{
words x[100]; // I do not want to use this, I want to dynamic increase the size of the array as data comes in.
}
Is this possible?
I've researched this: words* array = (words*)malloc(sizeof(words) * 100);
I want to get rid of the 100 and store the data as it comes in. Thus if 76 fields of data comes in, I want to store 76 and not 100. I'm assuming that I don't know how much data is coming into my program. In the struct I defined above I could create the first "index" as:
words* array = (words*)malloc(sizeof(words));
However I want to dynamically add elements to the array after. I hope I described the problem area clearly enough. The major challenge is to dynamically add a second field, at least that is the challenge for the moment.
I've made a little progress however:
typedef struct {
char *str;
} words;
// Allocate first string.
words x = (words) malloc(sizeof(words));
x[0].str = "john";
// Allocate second string.
x=(words*) realloc(x, sizeof(words));
x[1].FirstName = "bob";
// printf second string.
printf("%s", x[1].str); --> This is working, it's printing out bob.
free(x); // Free up memory.
printf("%s", x[1].str); --> Not working since its still printing out BOB even though I freed up memory. What is wrong?
I did some error checking and this is what I found. If after I free up memory for x I add the following:
x=NULL;
then if I try to print x I get an error which is what I want. So is it that the free function is not working, at least on my compiler? I'm using DevC??
Thanks, I understand now due to:
FirstName is a pointer to an array of char which is not being allocated by the malloc, only the pointer is being allocated and after you call free, it doesn't erase the memory, it just marks it as available on the heap to be over written later. – MattSmith
Update
I'm trying to modularize and put the creation of my array of structs in a function but nothing seems to work. I'm trying something very simple and I don't know what else to do. It's along the same lines as before, just another function, loaddata that is loading the data and outside the method I need to do some printing. How can I make it work? My code is as follows:
# include <stdio.h>
# include <stdlib.h>
# include <string.h>
# include <ctype.h>
typedef struct
{
char *str1;
char *str2;
} words;
void LoadData(words *, int *);
main()
{
words *x;
int num;
LoadData(&x, &num);
printf("%s %s", x[0].str1, x[0].str2);
printf("%s %s", x[1].str1, x[1].str2);
getch();
}//
void LoadData(words *x, int * num)
{
x = (words*) malloc(sizeof(words));
x[0].str1 = "johnnie\0";
x[0].str2 = "krapson\0";
x = (words*) realloc(x, sizeof(words)*2);
x[1].str1 = "bob\0";
x[1].str2 = "marley\0";
*num=*num+1;
}//
This simple test code is crashing and I have no idea why. Where is the bug?
You've tagged this as C++ as well as C.
If you're using C++ things are a lot easier. The standard template library has a template called vector which allows you to dynamically build up a list of objects.
#include <stdio.h>
#include <vector>
typedef std::vector<char*> words;
int main(int argc, char** argv) {
words myWords;
myWords.push_back("Hello");
myWords.push_back("World");
words::iterator iter;
for (iter = myWords.begin(); iter != myWords.end(); ++iter) {
printf("%s ", *iter);
}
return 0;
}
If you're using C things are a lot harder, yes malloc, realloc and free are the tools to help you. You might want to consider using a linked list data structure instead. These are generally easier to grow but don't facilitate random access as easily.
#include <stdio.h>
#include <stdlib.h>
typedef struct s_words {
char* str;
struct s_words* next;
} words;
words* create_words(char* word) {
words* newWords = malloc(sizeof(words));
if (NULL != newWords){
newWords->str = word;
newWords->next = NULL;
}
return newWords;
}
void delete_words(words* oldWords) {
if (NULL != oldWords->next) {
delete_words(oldWords->next);
}
free(oldWords);
}
words* add_word(words* wordList, char* word) {
words* newWords = create_words(word);
if (NULL != newWords) {
newWords->next = wordList;
}
return newWords;
}
int main(int argc, char** argv) {
words* myWords = create_words("Hello");
myWords = add_word(myWords, "World");
words* iter;
for (iter = myWords; NULL != iter; iter = iter->next) {
printf("%s ", iter->str);
}
delete_words(myWords);
return 0;
}
Yikes, sorry for the worlds longest answer. So WRT to the "don't want to use a linked list comment":
#include <stdio.h>
#include <stdlib.h>
typedef struct {
char** words;
size_t nWords;
size_t size;
size_t block_size;
} word_list;
word_list* create_word_list(size_t block_size) {
word_list* pWordList = malloc(sizeof(word_list));
if (NULL != pWordList) {
pWordList->nWords = 0;
pWordList->size = block_size;
pWordList->block_size = block_size;
pWordList->words = malloc(sizeof(char*)*block_size);
if (NULL == pWordList->words) {
free(pWordList);
return NULL;
}
}
return pWordList;
}
void delete_word_list(word_list* pWordList) {
free(pWordList->words);
free(pWordList);
}
int add_word_to_word_list(word_list* pWordList, char* word) {
size_t nWords = pWordList->nWords;
if (nWords >= pWordList->size) {
size_t newSize = pWordList->size + pWordList->block_size;
void* newWords = realloc(pWordList->words, sizeof(char*)*newSize);
if (NULL == newWords) {
return 0;
} else {
pWordList->size = newSize;
pWordList->words = (char**)newWords;
}
}
pWordList->words[nWords] = word;
++pWordList->nWords;
return 1;
}
char** word_list_start(word_list* pWordList) {
return pWordList->words;
}
char** word_list_end(word_list* pWordList) {
return &pWordList->words[pWordList->nWords];
}
int main(int argc, char** argv) {
word_list* myWords = create_word_list(2);
add_word_to_word_list(myWords, "Hello");
add_word_to_word_list(myWords, "World");
add_word_to_word_list(myWords, "Goodbye");
char** iter;
for (iter = word_list_start(myWords); iter != word_list_end(myWords); ++iter) {
printf("%s ", *iter);
}
delete_word_list(myWords);
return 0;
}
If you want to dynamically allocate arrays, you can use malloc from stdlib.h.
If you want to allocate an array of 100 elements using your words struct, try the following:
words* array = (words*)malloc(sizeof(words) * 100);
The size of the memory that you want to allocate is passed into malloc and then it will return a pointer of type void (void*). In most cases you'll probably want to cast it to the pointer type you desire, which in this case is words*.
The sizeof keyword is used here to find out the size of the words struct, then that size is multiplied by the number of elements you want to allocate.
Once you are done, be sure to use free() to free up the heap memory you used in order to prevent memory leaks:
free(array);
If you want to change the size of the allocated array, you can try to use realloc as others have mentioned, but keep in mind that if you do many reallocs you may end up fragmenting the memory. If you want to dynamically resize the array in order to keep a low memory footprint for your program, it may be better to not do too many reallocs.
This looks like an academic exercise which unfortunately makes it harder since you can't use C++. Basically you have to manage some of the overhead for the allocation and keep track how much memory has been allocated if you need to resize it later. This is where the C++ standard library shines.
For your example, the following code allocates the memory and later resizes it:
// initial size
int count = 100;
words *testWords = (words*) malloc(count * sizeof(words));
// resize the array
count = 76;
testWords = (words*) realloc(testWords, count* sizeof(words));
Keep in mind, in your example you are just allocating a pointer to a char and you still need to allocate the string itself and more importantly to free it at the end. So this code allocates 100 pointers to char and then resizes it to 76, but does not allocate the strings themselves.
I have a suspicion that you actually want to allocate the number of characters in a string which is very similar to the above, but change word to char.
EDIT: Also keep in mind it makes a lot of sense to create functions to perform common tasks and enforce consistency so you don't copy code everywhere. For example, you might have a) allocate the struct, b) assign values to the struct, and c) free the struct. So you might have:
// Allocate a words struct
words* CreateWords(int size);
// Assign a value
void AssignWord(word* dest, char* str);
// Clear a words structs (and possibly internal storage)
void FreeWords(words* w);
EDIT: As far as resizing the structs, it is identical to resizing the char array. However the difference is if you make the struct array bigger, you should probably initialize the new array items to NULL. Likewise, if you make the struct array smaller, you need to cleanup before removing the items -- that is free items that have been allocated (and only the allocated items) before you resize the struct array. This is the primary reason I suggested creating helper functions to help manage this.
// Resize words (must know original and new size if shrinking
// if you need to free internal storage first)
void ResizeWords(words* w, size_t oldsize, size_t newsize);
In C++, use a vector. It's like an array but you can easily add and remove elements and it will take care of allocating and deallocating memory for you.
I know the title of the question says C, but you tagged your question with C and C++...
Another option for you is a linked list. You'll need to analyze how your program will use the data structure, if you don't need random access it could be faster than reallocating.
Your code in the last update should not compile, much less run. You're passing &x to LoadData. &x has the type of **words, but LoadData expects words* . Of course it crashes when you call realloc on a pointer that's pointing into stack.
The way to fix it is to change LoadData to accept words** . Thi sway, you can actually modify the pointer in main(). For example, realloc call would look like
*x = (words*) realloc(*x, sizeof(words)*2);
It's the same principlae as in "num" being int* rather than int.
Besides this, you need to really figure out how the strings in words ere stored. Assigning a const string to char * (as in str2 = "marley\0") is permitted, but it's rarely the right solution, even in C.
Another point: non need to have "marley\0" unless you really need two 0s at the end of string. Compiler adds 0 tho the end of every string literal.
For the test code: if you want to modify a pointer in a function, you should pass a "pointer to pointer" to the function. Corrected code is as follows:
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
typedef struct
{
char *str1;
char *str2;
} words;
void LoadData(words**, int*);
main()
{
words **x;
int num;
LoadData(x, &num);
printf("%s %s\n", (*x[0]).str1, (*x[0]).str2);
printf("%s %s\n", (*x[1]).str1, (*x[1]).str2);
}
void LoadData(words **x, int *num)
{
*x = (words*) malloc(sizeof(words));
(*x[0]).str1 = "johnnie\0";
(*x[0]).str2 = "krapson\0";
*x = (words*) realloc(*x, sizeof(words) * 2);
(*x[1]).str1 = "bob\0";
(*x[1]).str2 = "marley\0";
*num = *num + 1;
}
Every coder need to simplify their code to make it easily understood....even for beginners.
So array of structures using dynamically is easy, if you understand the concepts.
// Dynamically sized array of structures
#include <stdio.h>
#include <stdlib.h>
struct book
{
char name[20];
int p;
}; //Declaring book structure
int main ()
{
int n, i;
struct book *b; // Initializing pointer to a structure
scanf ("%d\n", &n);
b = (struct book *) calloc (n, sizeof (struct book)); //Creating memory for array of structures dynamically
for (i = 0; i < n; i++)
{
scanf ("%s %d\n", (b + i)->name, &(b + i)->p); //Getting values for array of structures (no error check)
}
for (i = 0; i < n; i++)
{
printf ("%s %d\t", (b + i)->name, (b + i)->p); //Printing values in array of structures
}
scanf ("%d\n", &n); //Get array size to re-allocate
b = (struct book *) realloc (b, n * sizeof (struct book)); //change the size of an array using realloc function
printf ("\n");
for (i = 0; i < n; i++)
{
printf ("%s %d\t", (b + i)->name, (b + i)->p); //Printing values in array of structures
}
return 0;
}
If you want to grow the array dynamically, you should use malloc() to dynamically allocate some fixed amount of memory, and then use realloc() whenever you run out. A common technique is to use an exponential growth function such that you allocate some small fixed amount and then make the array grow by duplicating the allocated amount.
Some example code would be:
size = 64; i = 0;
x = malloc(sizeof(words)*size); /* enough space for 64 words */
while (read_words()) {
if (++i > size) {
size *= 2;
x = realloc(sizeof(words) * size);
}
}
/* done with x */
free(x);
Here is how I would do it in C++
size_t size = 500;
char* dynamicAllocatedString = new char[ size ];
Use same principal for any struct or c++ class.
I am tring to create a sub-routine that inserts a string into another string. I want to check that the host string is going to have enough capacity to hold all the characters and if not return an error integer. This requires using something like sizeof but that can be called using a pointer. My code is below and I would be very gateful for any help.
#include<stdio.h>
#include<conio.h>
//#include "string.h"
int string_into_string(char* host_string, char* guest_string, int insertion_point);
int main(void) {
char string_one[21] = "Hello mother"; //12 characters
char string_two[21] = "dearest "; //8 characters
int c;
c = string_into_string(string_one, string_two, 6);
printf("Sub-routine string_into_string returned %d and creates the string: %s\n", c, string_one);
getch();
return 0;
}
int string_into_string(char* host_string, char* guest_string, int insertion_point) {
int i, starting_length_of_host_string;
//check host_string is long enough
if(strlen(host_string) + strlen(guest_string) >= sizeof(host_string) + 1) {
//host_string is too short
sprintf(host_string, "String too short(%d)!", sizeof(host_string));
return -1;
}
starting_length_of_host_string = strlen(host_string);
for(i = starting_length_of_host_string; i >= insertion_point; i--) { //make room
host_string[i + strlen(guest_string)] = host_string[i];
}
//i++;
//host_string[i] = '\0';
for(i = 1; i <= strlen(guest_string); i++) { //insert
host_string[i + insertion_point - 1] = guest_string[i - 1];
}
i = strlen(guest_string) + starting_length_of_host_string;
host_string[i] = '\0';
return strlen(host_string);
}
C does not allow you to pass arrays as function arguments, so all arrays of type T[N] decay to pointers of type T*. You must pass the size information manually. However, you can use sizeof at the call site to determine the size of an array:
int string_into_string(char * dst, size_t dstlen, char const * src, size_t srclen, size_t offset, size_t len);
char string_one[21] = "Hello mother";
char string_two[21] = "dearest ";
string_into_string(string_one, sizeof string_one, // gives 21
string_two, strlen(string_two), // gives 8
6, strlen(string_two));
If you are creating dynamic arrays with malloc, you have to store the size information somewhere separately anyway, so this idiom will still fit.
(Beware that sizeof(T[N]) == N * sizeof(T), and I've used the fact that sizeof(char) == 1 to simplify the code.)
This code needs a whole lot more error handling but should do what you need without needing any obscure loops. To speed it up, you could also pass the size of the source string as parameter, so the function does not need to calculate it in runtime.
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
signed int string_into_string (char* dest_buf,
int dest_size,
const char* source_str,
int insert_index)
{
int source_str_size;
char* dest_buf_backup;
if (insert_index >= dest_size) // sanity check of parameters
{
return -1;
}
// save data from the original buffer into temporary backup buffer
dest_buf_backup = malloc (dest_size - insert_index);
memcpy (dest_buf_backup,
&dest_buf[insert_index],
dest_size - insert_index);
source_str_size = strlen(source_str);
// copy new data into the destination buffer
strncpy (&dest_buf[insert_index],
source_str,
source_str_size);
// restore old data at the end
strcpy(&dest_buf[insert_index + source_str_size],
dest_buf_backup);
// delete temporary buffer
free(dest_buf_backup);
}
int main()
{
char string_one[21] = "Hello mother"; //12 characters
char string_two[21] = "dearest "; //8 characters
(void) string_into_string (string_one,
sizeof(string_one),
string_two,
6);
puts(string_one);
return 0;
}
I tried using a macro and changing string_into_string to include the requirement for a size argument, but I still strike out when I call the function from within another function. I tried using the following Macro:
#define STRING_INTO_STRING( a, b, c) (string_into_string2(a, sizeof(a), b, c))
The other function which causes failure is below. This fails because string has already become the pointer and therefore has size 4:
int string_replace(char* string, char* string_remove, char* string_add) {
int start_point;
int c;
start_point = string_find_and_remove(string, string_remove);
if(start_point < 0) {
printf("string not found: %s\n ABORTING!\n", string_remove);
while(1);
}
c = STRING_INTO_STRING(string, string_add, start_point);
return c;
}
Looks like this function will have to proceed at risk. looking at strcat it also proceeds at risk, in that it doesn't check that the string you are appending to is large enough to hold its intended contents (perhaps for the very same reason).
Thanks for everyone's help.