I have to write a function that finds a product with given code from the given array. If product is found, a pointer to the corresponding array element is returned.
My main problem is that the given code should first be truncated to seven characters and only after that compared with array elements.
Would greatly appreciate your help.
struct product *find_product(struct product_array *pa, const char *code)
{
char *temp;
int i = 0;
while (*code) {
temp[i] = (*code);
code++;
i++;
if (i == 7)
break;
}
temp[i] = '\0';
for (int j = 0; j < pa->count; j++)
if (pa->arr[j].code == temp[i])
return &(pa->arr[j]);
}
Why don't you just use strncmp in a loop?
struct product *find_product(struct product_array *pa, const char *code)
{
for (size_t i = 0; i < pa->count; ++i)
{
if (strncmp(pa->arr[i].code, code, 7) == 0)
return &pa->arr[i];
}
return 0;
}
temp is a pointer which is uninitialized and you are dereferencing it which will lead to undefined behavior.
temp = malloc(size); // Allocate some memory size = 8 in your case
One more mistake I see is
if (pa->arr[j].code == temp[i]) // i is already indexing `\0`
should be
strcmp(pa->arr[j].code,temp); // returns 0 if both the strings are same
This code can completely be avoided if you can use strncmp()
As pointed out by others, you are using temp uninitialized and you are always comparing characters with '\0'.
You don't need a temp variable:
int strncmp ( const char * str1, const char * str2, size_t num );
Compare characters of two strings
Compares up to num characters of the
C string str1 to those of the C string str2.
/* Don't use magic numbers like 7 in the body of function */
#define PRODUCT_CODE_LEN 7
struct product *find_product(struct product_array *pa, const char *code)
{
for (int i = 0; i < pa->count; i++) {
if (strncmp(pa->arr[i].code, code, PRODUCT_CODE_LEN) == 0)
return &(pa->arr[i]);
}
return NULL; /* Not found */
}
When you write char* temp; you are just declaring an uninitialized pointer
In your case since you say that the code is truncated to 7 you could create a buffer
on the stack with place for the code
char temp[8];
Writing
temp[i] = (*code);
code++;
i++;
Can be simplified to:
temp[i++] = *code++;
In your loop
for (int j = 0; j < pa->count; j++)
if (pa->arr[j].code == temp[i])
return &(pa->arr[j]);
You are comparing the address of code and the character value of temp[i] which incidentally could be 8 and outside the array.
Instead what you want to do is compare what code points to and what temp contains:
for (int j = 0; j < pa->count; j++)
if (!strncmp(pa->arr[j].code, temp, 7)
return &(pa->arr[j]);
You should also return NULL; if nothing was found, seems you do not return anything.
Probably a good thing is also to make sure your temp[] always contains 7 characters.
Related
(in C, using visual studio 2022 preview), I have to do a program that link two strings together. Here's what I did:
I wrote two for-loops to count characters of first string and second
string,
I checked (inside the link function if the pointers are null (first and second). If they are null, then "return NULL".
I created "char *result". this is a new string and this is the string to be returned. I allocated enough memory to store nprime, nsecond, and 1 more character (the zero terminator). I used a malloc.
then, I checked if result is null. if it's null then "return NULL".
then, I wrote 2 for-loops to perform the linking between the first string and the second string. And here I got a compiler warning (because I think it's in compile time not in debug time). buffer overrun, the writable size is
"nprime+nsecond+1" but 2 bytes might be written.
my theory is that the program is trying to write outside the result-array, so there could be a loss of data, I tried to edit my code, therefore I write "nprime+nsecond+2" instead but it doesn't work, and it keeps showing me the same buffer overrun error.
#include <stdlib.h>
char* link( const char* first, const char* second) {
size_t nprime = 0;
size_t nsecond = 0;
if (first == NULL) {
return NULL;
}
if (second == NULL) {
return NULL;
}
for (size_t i = 0; first[i] < '\0'; i++) {
nprime++;
}
for (size_t i = 0; second[i] < '\0'; i++) {
nsecond++;
}
char* result = malloc(nprime + nsecond + 1);
if (result == NULL) {
return NULL;
}
for (size_t i = 0; i < nprime; i++) {
result[i] = first[i];
}
for (size_t i = 0; i < nsecond; i++) {
result[nprime + i] = second[i];
}
result[nprime + nsecond] = 0;
return result;
}
this is the main:
int main(void) {
char s1[] = "this is a general string ";
char s2[] = "this is a general test.";
char* s;
s = link(s1, s2);
return 0;
}
The warning is given due to the wrong conditions you defined in the first 2 for loops. The right loops should be as follows:
for (size_t i = 0; first[i] != '\0'; i++) {
nprime++;
}
for (size_t i = 0; second[i] != '\0'; i++) {
nsecond++;
}
With the conditions you defined (i.e. first[i] < '\0') you are just counting how many chars in the given string have an ASCII code lower than the ASCII code of \0 and exit the loop as soon as you find a char not fulfilling such condition.
Since '\0' has ASCII value 0, your nprime and nsecond are never incremented, leading to a malloc with insufficient room for the chars you actually need.
Could you help please ?
When I execute this code I receive that:
AAAAABBBBBCCCCCBBBBBCOMP¬ıd┐╔ LENGTH 31
There are some weirds characters after letters, while I've allocate just 21 bytes.
#include <stdio.h>
#include <stdlib.h>
char * lineDown(){
unsigned short state[4] = {0,1,2,1};
char decorationUp[3][5] = {
{"AAAAA"},{"BBBBB"},{"CCCCC"}
};
char * deco = malloc(21);
int k;
int p = 0;
for(int j = 0; j < 4; j++){
k = state[j];
for(int i = 0; i < 5; i++){
*(deco+p) = decorationUp[k][i];
p++;
}
}
return deco;
}
int main(void){
char * lineDOWN = lineDown();
int k = 0;
char c;
do{
c = *(lineDOWN+k);
printf("%c",*(lineDOWN+k));
k++;
}while(c != '\0');
printf("LENGTH %d\n\n",k);
}
The function does not build a string because the result array does not contain the terminating zero though a space for it was reserved when the array was allocated.
char * deco = malloc(21);
So you need to append the array with the terminating zero before exiting the function
//...
*(deco + p ) = '\0';
return deco;
}
Otherwise this do-while loop
do{
c = *(lineDOWN+k);
printf("%c",*(lineDOWN+k));
k++;
}while(c != '\0')
will have undefined behavior.
But even if you will append the array with the terminating zero the loop will count the length of the stored string incorrectly because it will increase the variable k even when the current character is the terminating zero.
Instead you should use a while loop. In this case the declaration of the variable c will be redundant. The loop can look like
while ( *( lineDOWN + k ) )
{
printf("%c",*(lineDOWN+k));
k++;
}
In this case this call
printf("\nLENGTH %d\n\n",k);
^^
will output the correct length of the string equal to 20.
And you should free the allocated memory before exiting the program
free( lineDOWN );
As some other wrote here in their answers that the array decorationUp must be declared like
char decorationUp[3][6] = {
{"AAAAA"},{"BBBBB"},{"CCCCC"}
};
then it is not necessary if you are not going to use elements of the array as strings and you are not using them as strings in your program.
Take into account that your program is full of magic numbers. Such a program is usually error-prone. Instead you should use named constants.
In
char decorationUp[3][5] = {
{"AAAAA"},{"BBBBB"},{"CCCCC"}
};
your string needs 6 characters to also place the null char, even in that case you do not use them as 'standard' string but only array of char. To get into the habit always reverse the place for the ending null character
you can do
char decorationUp[3][6] = {
{"AAAAA"},{"BBBBB"},{"CCCCC"}
};
Note it is useless to give the first size, the compiler counts for you
Because in main you stop when you read the null character you also need to place it in deco at the end, so you need to allocate 21 for it. As before you missed the place for the null character, but here that produces an undefined behavior because you read after the allocated block.
To do *(deco+p) is not readable, do deco[p]
So for instance :
char * lineDown(){
unsigned short state[] = {0,1,2,1};
char decorationUp[][6] = {
{"AAAAA"},{"BBBBB"},{"CCCCC"}
};
char * deco = malloc(4*5 + 1); /* a formula to explain why 21 is better than 21 directly */
int k;
int p = 0;
for(int j = 0; j < 4; j++){
k = state[j];
for(int i = 0; i < 5; i++){
deco[p] = decorationUp[k][i];
p++;
}
}
deco[p] = 0;
return deco;
}
For an assignment in class, we have been instructed to write a program which takes a string and a delimiter and then takes "words" and stores them in a new array of strings. i.e., the input ("my name is", " ") would return an array with elements "my" "name" "is".
Roughly, what I've attempted is to:
Use a separate helper called number_of_delimeters() to determine the size of the array of strings
Iterate through the initial array to find the number of elements in a given string which would be placed in the array
Allocate storage within my array for each string
Store the elements within the allocated memory
Include directives:
#include <stdlib.h>
#include <stdio.h>
This is the separate helper:
int number_of_delimiters (char* s, int d)
{
int numdelim = 0;
for (int i = 0; s[i] != '\0'; i++)
{
if (s[i] == d)
{
numdelim++;
}
}
return numdelim;
}
`This is the function itself:
char** split_at (char* s, char d)
{
int numdelim = number_of_delimiters(s, d);
int a = 0;
int b = 0;
char** final = (char**)malloc((numdelim+1) * sizeof(char*));
for (int i = 0; i <= numdelim; i++)
{
int sizeofj = 0;
while (s[a] != d)
{
sizeofj++;
a++;
}
final[i] = (char*)malloc(sizeofj);
a++;
int j = 0;
while (j < sizeofj)
{
final[i][j] = s[b];
j++;
b++;
}
b++;
final[i][j+1] = '\0';
}
return final;
}
To print:
void print_string_array(char* a[], unsigned int alen)
{
printf("{");
for (int i = 0; i < alen; i++)
{
if (i == alen - 1)
{
printf("%s", a[i]);
}
else
{
printf("%s ", a[i]);
}
}
printf("}");
}
int main(int argc, char *argv[])
{
print_string_array(split_at("Hi, my name is none.", ' '), 5);
return 0;
}
This currently returns {Hi, my name is none.}
After doing some research, I realized that the purpose of this function is either similar or identical to strtok. However, looking at the source code for this proved to be little help because it included concepts we have not yet used in class.
I know the question is vague, and the code rough to read, but what can you point to as immediately problematic with this approach to the problem?
The program has several problems.
while (s[a] != d) is wrong, there is no delimiter after the last word in the string.
final[i][j+1] = '\0'; is wrong, j+1 is one position too much.
The returned array is unusable, unless you know beforehand how many elements are there.
Just for explanation:
strtok will modify the array you pass in! After
char test[] = "a b c ";
for(char* t = test; strtok(t, " "); t = NULL);
test content will be:
{ 'a', 0, 'b', 0, 'c', 0, 0 }
You get subsequently these pointers to your test array: test + 0, test + 2, test + 4, NULL.
strtok remembers the pointer you pass to it internally (most likely, you saw a static variable in your source code...) so you can (and must) pass NULL the next time you call it (as long as you want to operate on the same source string).
You, in contrast, apparently want to copy the data. Fine, one can do so. But here we get a problem:
char** final = //...
return final;
void print_string_array(char* a[], unsigned int alen)
You just return the array, but you are losing length information!
How do you want to pass the length to your print function then?
char** tokens = split_at(...);
print_string_array(tokens, sizeof(tokens));
will fail, because sizeof(tokens) will always return the size of a pointer on your local system (most likely 8, possibly 4 on older hardware)!
My personal recommendation: create a null terminated array of c strings:
char** final = (char**)malloc((numdelim + 2) * sizeof(char*));
// ^ (!)
// ...
final[numdelim + 1] = NULL;
Then your print function could look like this:
void print_string_array(char* a[]) // no len parameter any more!
{
printf("{");
if(*a)
{
printf("%s", *a); // printing first element without space
for (++a; *a; ++a) // *a: checking, if current pointer is not NULL
{
printf(" %s", *a); // next elements with spaces
}
}
printf("}");
}
No problems with length any more. Actually, this is exactly the same principle C strings use themselves (the terminating null character, remember?).
Additionally, here is a problem in your own code:
while (j < sizeofj)
{
final[i][j] = s[b];
j++; // j will always point behind your string!
b++;
}
b++;
// thus, you need:
final[i][j] = '\0'; // no +1 !
For completeness (this was discovered by n.m. already, see the other answer): If there is no trailing delimiter in your source string,
while (s[a] != d)
will read beyond your input string (which is undefined behaviour and could result in your program crashing). You need to check for the terminating null character, too:
while(s[a] && s[a] != d)
Finally: how do you want to handle subsequent delimiters? Currently, you will insert empty strings into your array? Print out your strings as follows (with two delimiting symbols - I used * and + like birth and death...):
printf("*%s+", *a);
and you will see. Is this intended?
Edit 2: The variant with pointer arithmetic (only):
char** split_at (char* s, char d)
{
int numdelim = 0;
char* t = s; // need a copy
while(*t)
{
numdelim += *t == d;
++t;
}
char** final = (char**)malloc((numdelim + 2) * sizeof(char*));
char** f = final; // pointer to current position within final
t = s; // re-assign t, using s as start pointer for new strings
while(*t) // see above
{
if(*t == d) // delimiter found!
{
// can subtract pointers --
// as long as they point to the same array!!!
char* n = (char*)malloc(t - s + 1); // +1: terminating null
*f++ = n; // store in position pointer and increment it
while(s != t) // copy the string from start to current t
*n++ = *s++;
*n = 0; // terminate the new string
}
++t; // next character...
}
*f = NULL; // and finally terminate the string array
return final;
}
While I've now been shown a more elegant solution, I've found and rectified the issues in my code:
char** split_at (char* s, char d)
{
int numdelim = 0;
int x;
for (x = 0; s[x] != '\0'; x++)
{
if (s[x] == d)
{
numdelim++;
}
}
int a = 0;
int b = 0;
char** final = (char**)malloc((numdelim+1) * sizeof(char*));
for (int i = 0; i <= numdelim; i++)
{
int sizeofj = 0;
while ((s[a] != d) && (a < x))
{
sizeofj++;
a++;
}
final[i] = (char*)malloc(sizeofj);
a++;
int j = 0;
while (j < sizeofj)
{
final[i][j] = s[b];
j++;
b++;
}
final[i][j] = '\0';
b++;
}
return final;
}
I consolidated what I previously had as a helper function, and modified some points where I incorrectly incremented .
I am experiencing an issue where the invocation of realloc seems to modify the contents of another string, keyfile.
It's supposed to run through a null-terminated char* (keyfile), which contains just above 500 characters. The problem, however, is that the reallocation I perform in the while-loop seems to modify the contents of the keyfile.
I tried removing the dynamic reallocation with realloc and instead initialize the pointers in the for-loop with a size of 200*sizeof(int) instead. The problem remains, the keyfile string is modified during the (re)allocation of memory, and I have no idea why. I have confirmed this by printing the keyfile-string before and after both the malloc and realloc statements.
Note: The keyfile only contains the characters a-z, no digits, spaces, linebreaks or uppercase. Only a text of 26, lowercase letters.
int **getCharMap(const char *keyfile) {
char *alphabet = "abcdefghijklmnopqrstuvwxyz";
int **charmap = malloc(26*sizeof(int));
for (int i = 0; i < 26; i++) {
charmap[(int) alphabet[i]] = malloc(sizeof(int));
charmap[(int) alphabet[i]][0] = 0; // place a counter at index 0
}
int letter;
int count = 0;
unsigned char c = keyfile[count];
while (c != '\0') {
int arr_count = charmap[c][0];
arr_count++;
charmap[c] = realloc(charmap[c], (arr_count+1)*sizeof(int));
charmap[c][0] = arr_count;
charmap[c][arr_count] = count;
c = keyfile[++count];
}
// Just inspecting the results for debugging
printf("\nCHARMAP\n");
for (int i = 0; i < 26; i++) {
letter = (int) alphabet[i];
printf("%c: ", (char) letter);
int count = charmap[letter][0];
printf("%d", charmap[letter][0]);
if (count > 0) {
for (int j = 1; j < count+1; j++) {
printf(",%d", charmap[letter][j]);
}
}
printf("\n");
}
exit(0);
return charmap;
}
charmap[(int) alphabet[i]] = malloc(sizeof(int));
charmap[(int) alphabet[i]][0] = 0; // place a counter at index 0
You are writing beyond the end of your charmap array. So, you are invoking undefined behaviour and it's not surprising that you are seeing weird effects.
You are using the character codes as an index into the array, but they do not start at 0! They start at whatever the ASCII code for a is.
You should use alphabet[i] - 'a' as your array index.
The following piece of code is a source of troubles:
int **charmap = malloc(26*sizeof(int));
for (int i = 0; i < 26; i++)
charmap[...] = ...;
If sizeof(int) < sizeof(int*), then it will be performing illegal memory access operations.
For example, on 64-bit platforms, the case is usually sizeof(int) == 4 < 8 == sizeof(int*).
Under that scenario, by writing into charmap[13...25], you will be accessing unallocated memory.
Change this:
int **charmap = malloc(26*sizeof(int));
To this:
int **charmap = malloc(26*sizeof(int*));
I can't write a workable code for a function that deletes N characters from the string S, starting from position P. How you guys would you write such a function?
void remove_substring(char *s, int p, int n) {
int i;
if(n == 0) {
printf("%s", s);
}
for (i = 0; i < p - 1; i++) {
printf("%c", s[i]);
}
for (i = strlen(s) - n; i < strlen(s); i++) {
printf("%c", s[i]);
}
}
Example:
s: "abcdefghi"
p: 4
n: 3
output:
abcghi
But for a case like n = 0 and p = 1 it's not working!
Thanks a lot!
A few people have shown you how to do this, but most of their solutions are highly condensed, use standard library functions or simply don't explain what's going on. Here's a version that includes not only some very basic error checking but some explanation of what's happening:
void remove_substr(char *s, size_t p, size_t n)
{
// p is 1-indexed for some reason... adjust it.
p--;
// ensure that we're not being asked to access
// memory past the current end of the string.
// Note that if p is already past the end of
// string then p + n will, necessarily, also be
// past the end of the string so this one check
// is sufficient.
if(p + n >= strlen(s))
return;
// Offset n to account for the data we will be
// skipping.
n += p;
// We copy one character at a time until we
// find the end-of-string character
while(s[n] != 0)
s[p++] = s[n++];
// And make sure our string is properly terminated.
s[p] = 0;
}
One caveat to watch out for: please don't call this function like this:
remove_substr("abcdefghi", 4, 3);
Or like this:
char *s = "abcdefghi";
remove_substr(s, 4, 3);
Doing so will result in undefined behavior, as string literals are read-only and modifying them is not allowed by the standard.
Strictly speaking, you didn't implement a removal of a substring: your code prints the original string with a range of characters removed.
Another thing to note is that according to your example, the index p is one-based, not zero-based like it is in C. Otherwise the output for "abcdefghi", 4, 3 would have been "abcdhi", not "abcghi".
With this in mind, let's make some changes. First, your math is a little off: the last loop should look like this:
for (i = p+n-1; i < strlen(s); i++) {
printf("%c", s[i]);
}
Demo on ideone.
If you would like to use C's zero-based indexing scheme, change your loops as follows:
for (i = 0; i < p; i++) {
printf("%c", s[i]);
}
for (i = p+n; i < strlen(s); i++) {
printf("%c", s[i]);
}
In addition, you should return from the if at the top, or add an else:
if(n == 0) {
printf("%s", s);
return;
}
or
if(n == 0) {
printf("%s", s);
} else {
// The rest of your code here
...
}
or remove the if altogether: it's only an optimization, your code is going to work fine without it, too.
Currently, you code would print the original string twice when n is 0.
If you would like to make your code remove the substring and return a result, you need to allocate the result, and replace printing with copying, like this:
char *remove_substring(char *s, int p, int n) {
// You need to do some checking before calling malloc
if (n == 0) return s;
size_t len = strlen(s);
if (n < 0 || p < 0 || p+n > len) return NULL;
size_t rlen = len-n+1;
char *res = malloc(rlen);
if (res == NULL) return NULL;
char *pt = res;
// Now let's use the two familiar loops,
// except printf("%c"...) will be replaced with *p++ = ...
for (int i = 0; i < p; i++) {
*pt++ = s[i];
}
for (int i = p+n; i < strlen(s); i++) {
*pt++ = s[i];
}
*pt='\0';
return res;
}
Note that this new version of your code returns dynamically allocated memory, which needs to be freed after use.
Here is a demo of this modified version on ideone.
Try copying the first part of the string, then the second
char result[10];
const char input[] = "abcdefg";
int n = 3;
int p = 4;
strncpy(result, input, p);
strncpy(result+p, input+p+n, length(input)-p-n);
printf("%s", result);
If you are looking to do this without the use of functions like strcpy or strncpy (which I see you said in a comment) then use a similar approach to how strcpy (or at least one possible variant) works under the hood:
void strnewcpy(char *dest, char *origin, int n, int p) {
while(p-- && *dest++ = *origin++)
;
origin += n;
while(*dest++ = *origin++)
;
}
metacode:
allocate a buffer for the destination
decalre a pointer s to your source string
advance the pointer "p-1" positions in your source string and copy them on the fly to destination
advance "n" positions
copy rest to destination
What did you try? Doesn't strcpy(s+p, s+p+n) work?
Edit: Fixed to not rely on undefined behaviour in strcpy:
void remove_substring(char *s, int p, int n)
{
p--; // 1 indexed - why?
memmove(s+p, s+p+n, strlen(s) - n);
}
If your heart's really set on it, you can also replace the memmove call with a loop:
char *dst = s + p;
char *src = s + p + n;
for (int i = 0; i < strlen(s) - n; i++)
*dst++ = *src++;
And if you do that, you can strip out the strlen call, too:
while ((*dst++ = *src++) != '\0);
But I'm not sure I recommend compressing it that much.