I am confused about how to use realloc to add space for other elements in the array nums
when the program first starts it has space for two elements but if the user wants to add more elements it will cause a segfault, this means we need to create a bigger array with 3 or more elements and add them one by one for the old one until index 1 and take a user-provided integer for the third element in the array
If the program is supposed to run in a while loop which never ends unless the user kills the process means we have to use realloc every time the array gets full that said my confusion starts here
do I have to make another array that will hold the address of realloc since we need to free it later on or can it use the same pointer for multiple realloc uses
int *nums[2];
int numsSize()
{
return sizeof(nums)/sizeof(int*);
}
//return index at which user added elements end
int numsIndex()
{
for (int i = 0 ; i < numsSize(); i++)
{
if (!nums[i])
{
return i;
}
}
return numsSize();
}
void numsResize()
{
// resize nums to have space for 4 elements
}
int main(void) {
nums[0] = 10 ;
printf("Size of array : %d \n", numsSize()); // outputs 2
printf("Index of last added element in array : %d\n", numsIndex()); // outputs 1
return 0;
}
If I understand the question correctly, you need to allocate a memory dynamically.
int *nums[2]; is fixed size array of pointer, you can't grow it dynamically.
nums[0] = 10 ; is invalid statement.
DEMO
#include <stdlib.h>
#include <stdio.h>
int *ptr = 0;
int size;
int currentIndex;
void
initialize ()
{
if (ptr)
return;
size = 2;
ptr = malloc (sizeof (int) * size);
if(!ptr)
exit(0);
currentIndex = 0;
}
void
unInitialize ()
{
if (ptr)
free (ptr);
size = 0;
currentIndex = 0;
}
void
doubleTheSize ()
{
ptr = realloc (ptr, sizeof (int) * size * 2);
if(!ptr)
exit(0);
size = size * 2;
}
void
AddElement (int element)
{
if (currentIndex >= size)
doubleTheSize ();
ptr[currentIndex] = element;
++currentIndex;
}
void
printInput ()
{
for (int i = 0; i < currentIndex; ++i)
{
printf ("%d ", ptr[i]);
} printf ("\n");
}
int
main (void)
{
initialize ();
int i = 0;
do
{
printf ("Enter Element :");
scanf (" %d", &i);
//condition to break the loop
if (i == -1)
break;
AddElement (i);
printInput ();
}
while (1);
unInitialize ();
return 0;
}
Related
#include <stdio.h>
#include <stdlib.h>
int find_lenght(int *arrr){
int i = 0;
while(arrr[i] != '\0'){
i++;
}
return i;
}
void init_array(int *arrr){
arrr=(int*)malloc(1*sizeof(int));
printf("New element:");
int lenght = find_lenght(arrr);
scanf("%d", &arrr[lenght]);
printf("Lenght = %d\n",lenght);
printf("Array elements are:\n");
for(int i = 0; i <= lenght; i++) {
printf("%d,", arrr[i]);
}
}
void print_array(int *arrr){
printf("Array elements are:\n");
int lenght = find_lenght(arrr);
for(int i = 0; i == lenght; i++) {
printf("%d,", arrr[i]);
}
}
int main() {
int *arr = NULL;
init_array(arr);
print_array(arr);
}
I don't know what am i missing here.
My point is to fill in and then print dynamic array
Also my taught is it's not filling the way it should, so it hasn't anything to print.
Your arr pointer in main is never assigned because your init_array assign the address of the allocated memory (the return value of malloc) to the input parameter arrr, which is, a local variable.
You have mainly two solutions to properly achieve what you want to do. The first one (the better one in my point of view), by making your init_array returning the allocated memory address to be assigned:
int* init_array()
{
int* retval = (int*)malloc(1*sizeof(int));
// ...
return retval;
}
int main()
{
int *arr = init_array(); //< assign arr with returned value
}
Another way is to make your init_array function taking a pointer to a pointer, so the function can assign this pointer:
void init_array(int** arrr)
{
(*arrr) = (int*)malloc(1*sizeof(int));
// ...
}
int main()
{
int* arr = NULL;
init_array(&arr); //< pass reference to arr
}
You need to pass the pointer to pointer to int to change passed pointer. Your for loop is invalid in print function. You need also to set the sentinel value yourself.
size_t find_length(const int *arrr)
{
size_t i = 0;
if(arrr)
while(arrr[i]) i++;
return i;
}
void add_element(int **arrr, int element)
{
size_t length = find_length(*arrr);
int *tmp = realloc(*arrr, (length + 2) * sizeof(**arrr));
if(tmp)
{
*arrr = tmp;
(*arrr)[length] = element;
(*arrr)[length + 1] = 0;
}
}
void print_array(const int *arrr)
{
printf("Array elements are:\n");
size_t lenght = find_length(arrr);
for(size_t i = 0; i < lenght; i++)
{
printf("arrr[%zu] = %d\n", i, arrr[i]);
}
}
int main(void) {
int *arr = NULL;
add_element(&arr, 5);
add_element(&arr, 15);
add_element(&arr, 25);
add_element(&arr, 35);
print_array(arr);
}
https://godbolt.org/z/drKej3KT5
the array on your main function is still NULL. the better way to do is just call the print_array() function after you initialize it. you just simply put print_array(arrr) inside init_array() and after the for loop statement.
The line
int lenght = find_lenght(arrr);
may invoke undefined behavior, because find_length requires its argument to be a pointer to the first element of a null-terminated int array. However, the content of the memory pointed to by arrr is indeterminate, because it has not been initialized. Therefore, it is not guaranteed to be null-terminated.
I'm trying to add new element to dynamic array in C (I know that I must free all memory. I will do it later), but I get this error every time:
But, what is strange, if I compile from terminal, like that, code works properly.
So, where is the error and how i can beat it?
Thank you!
All my code:
main.c
#include <stdio.h>
#include <stdlib.h>
typedef struct vector
{
int size;
int *array;
int alreadyIn;
}vector;
vector *vectorInit(int size)
{
vector *newVec = (vector *)malloc(sizeof(vector));
if(!newVec){printf("No memory!\n"); return NULL;}
newVec->size = size;
newVec->array = (int *)malloc(size * sizeof(int));
return newVec;
}
void allocNewMemory(vector *vect, int howMuch)
{
vect->array = (int *)realloc(vect->array ,(vect->size + howMuch) * sizeof(int));
vect->size += howMuch;
}
void pushBack(vector *vect, int number)
{
int howMuch = 5;
if(vect && vect->alreadyIn < vect->size)
{
vect->array[vect->alreadyIn] = number;
vect->alreadyIn++;
}
else
{
printf("Alloc new memory for %d elements...\n", howMuch);
allocNewMemory(vect, howMuch);
pushBack(vect, number);
}
}
void printVector(vector *vect)
{
for (int i = 0; i < vect->alreadyIn; i++)
{
printf("%d ", vect->array[i]);
}
printf("\n");
}
int main()
{
int startSize = 4;
vector * vec = vectorInit(startSize);
for (int i = 0; i < 6; i++)
{
pushBack(vec, i+1);
}
printVector(vec);
return 0;
}
You never initialize the alreadyIn member in the structure. That means its value will be indeterminate (and seemingly garbage or random).
You need to explicitly initialize it to zero:
vector *vectorInit(int size)
{
vector *newVec = malloc(sizeof(vector));
if(!newVec)
{
printf("No memory!\n");
return NULL;
}
newVec->size = size;
newVec->array = malloc(size * sizeof(int));
newVec->alreadyIn = 0; // Remember to set this to zero
return newVec;
}
This problem should have been easy to detect in the debugger.
Also note that I removed the casts from malloc. One should not cast the result of malloc, or really any function returning void *.
I am trying to practice with C by making a bubble sort program. The problem until now seems to be that the for loop that is giving values to the cells of the array is stuck after the condition is no longer fulfilled but it doesn't seem to be executing the commands in the loop. I don't know what is happening exactly and I have added some extra lines to see what is happening an these were my conclusions. Here is the code:
#include <stdio.h>
#include <stdlib.h>
void swap(int *x, int *y)
{
int temp = *x;
*x = *y;
*y = temp;
}
int *sort(int *array)
{
int finish = 1;
while (finish = 1)
{
finish = 0;
for (int i = 0; i <= sizeof(array); i++)
{
if ((array + i) > (array + i + 1))
{
swap(array + i, array + i + 1);
finish = 1;
}
}
}
return array;
}
int main()
{
int s, res;
printf("Give me the size of the array being sorted(larger than 1) : ");
do
{
res = scanf("%d", &s);
if (res != 1)
{
printf("Wrong Input!\n");
exit(1);
}
if (s < 2)
printf("Only numbers equal or larger than 2\n");
} while (s < 2);
int array[s];
for (int i = 0; i < s; i += 1)
{
scanf("%d", array + i);
printf("%d %d %d\n\n", *(array + i), i, i < s); // I used this to check if my values were ok
}
printf("end of reading the array"); //I added this line to see if I would exit the for loop. I am not seeing this message
sort(array);
printf("\n");
for (int i = 0; i < sizeof(array); i++)
printf("%d\n\n", array + i);
printf("Array has been sorted! Have a nice day!\n\n************************************************************");
return 0;
}
See the annotations in the code:
#include <stddef.h> // size_t 1)
#include <stdio.h>
#include <stdlib.h>
void swap(int *x, int *y)
{
int temp = *x;
*x = *y;
*y = temp;
}
int *sort(int *array, size_t size) // needs an extra parameter to know the size of the array
{
int finish = 1;
while (finish /* = 1 * you don't want assignment, you want comparison: */ == 1)
{
finish = 0;
for (int i = 0; i /* <= sizeof(array) */ < size - 1; i++) // i should be of type size_t
{
// if ((array + i) > (array + i + 1)) you are not dereferencing:
if(array[i] > array[i + 1])
{
// swap(array + i, array + i + 1); // easier to read imho:
swap(&array[i], &array[i + 1]);
finish = 1;
}
}
}
return array; // why does this function return anything? it is never used.
}
int main()
{
int s; /* , res; no need for an extra variable res */
printf("Give me the size of the array being sorted(larger than 1) : ");
do
{
// res = scanf("%d", &s);
// if (res != 1)
if (scanf("%d", &s) != 1)
{
printf("Wrong Input!\n");
// exit(1); // should be EXIT_FAILURE. Use return instead of exit() when in main().
return EXIT_FAILURE;
}
if (s < 2)
printf("Only numbers equal or larger than 2\n");
} while (s < 2);
int array[s];
for (int i = 0; i < s; /* i += 1* idiomatic: */ ++i) // size_t would be the correct type for s and i.
{
scanf("%d", /* array + i use indexes: */ &array[i]);
printf("%d %d %d\n\n", array[i], i, i < s); // again: indexes. i < s is allready ensured by the condition of the for-loop
}
printf("end of reading the array");
// sort(array); // sort will have no idea about the size of array use
sort(array, s); // instead.
printf("\n");
for (int i = 0; i < /* sizeof(array) 2) */ s; i++)
printf("%d\n\n", /* array + i * again you don't dereference */ array[i]);
printf("Array has been sorted! Have a nice day!\n\n************************************************************");
return 0;
}
1) size_t is the type that is guaranteed to be big enough to hold all sizes of objects in memory and indexes into them. The conversion specifier for scanf() is "%zu".
2) sizeof(array) in main() will yield the number of bytes in array, but you want the number of elements so you'd have to use sizeof(array) / sizeof(*array). But thats not needed since you already know its size. It is s.
This line
printf("end of reading the array");
has no line feed at the end of the string. This is a problem because printf is part of the family of functions called "buffered IO". The C library maintains a buffer of the things you want to print and only sends them to the terminal if the buffer gets full or it encounters \n in the stream of characters. You will not see, end of reading the array on your screen until after you have printed a line feed. You only do this after calling sort(). So all you know is your program is getting into an infinite loop at some point before the end of sort.
So there are actually three loops that could be infinite: the for loop you identified, the while loop in sort and the for loop inside the while loop. As the other answers point out, you have made the classic mistake of using assignment in the while conditional
while (finish = 1)
// ^ not enough equals signs
Unless your C compiler is really old, it is probably outputting a warning on that line. You should heed warnings.
Also, you should learn to use a debugger sooner rather than later. Believe me, it will save you a lot of time finding bugs.
In the sort function sizeof(array) returns the size of the pointer. (you can check it by yourself using printf("%d", sizeof(array).
The solution is to change your function to:
int sort(int* array, size_t size) { ... }
and call it with the correct array size:
sort(array, s);
As a homework, I'm supposed to create 2 functions that enable you to push and pop elements to an array that acts as a queue. We're supposed to do this dynamically allocating memory. My program is almost working, but sometimes when adding and removing too many elements, I get errors like "realloc(): invalid next size", double free (when I've only called the free function once) and some of the elements in the beginning of the queue are set to 0. For instance, if I first add 100 elements, then remove 90 and try to add another 20, I get "free(): invalid next size (fast): 0x0000000001ea6010".
What am I doing wrong here?
According to suggestions below I changed my functions to take a double pointer as an input for the array. That, however, now gives me a Segmentation fault - which means now I don't know what to look for at all...
#include <stdio.h>
#include <stdlib.h>
void enqueue(int **arr, int* lastElementIdx, size_t* totalElements, int element) {
if (*lastElementIdx >= *totalElements) { // check if memorry is sufficient, otherwise double
*totalElements *= 2;
int* temp = realloc(arr, (*totalElements * sizeof(int)));
if (temp == NULL) { // just in case realloc fails
printf("Allocation error\n");
} else {
*arr = temp;
}
}
if (*lastElementIdx <= *totalElements) {
*lastElementIdx += 1; // once everything is done: add element
*arr[*lastElementIdx] = element;
}
}
int dequeue(int **arr, int* lastElementIdx, size_t* totalElements) {
if (*lastElementIdx > -1) { // if queue is not empty...
int deleted = *arr[0]; // save deleted value first (in case it's still needed)
for (int i = 0; i <= *lastElementIdx; i++) { // shift all elements
*arr[i] = *arr[i + 1];
}
*lastElementIdx -= 1; // index is now decreased by 1
if (((*totalElements / 2) >= 10) && ((*lastElementIdx + 1) < (*totalElements / 2))) { // cut memory in half if not needed
*totalElements /= 2;
*arr = realloc(arr, (*totalElements * sizeof(int)));
int* temp = realloc(arr, (*totalElements * sizeof(int)));
if (temp == NULL) { // in case realloc fails
printf("Allocation error\n");
return 0;
} else {
*arr = temp;
}
}
return deleted;
} else { // if queue is empty, print that there's nothing to dequeue
printf("There are no elements inside the queue\n");
return 0;
}
}
void printQueue(int arr[], int lastElementIdx) {
for (int i = 0; i <= lastElementIdx; i++) { // print entire queue
printf("[%d] = %d\n", i, arr[i]);
}
printf("\n");
}
int main (void) {
size_t totalElements = 10; // number of needed elements at the time
int lastElementIdx = -1; // index of last element in queue at the time
int *arr = calloc(totalElements, sizeof(int));
int **arrpointer = &arr;
for (int i = 1; i < 101; i++) {
enqueue(arrpointer, &lastElementIdx, &totalElements, i);
}
printQueue(arr, lastElementIdx);
for (int i = 0; i < 90; i++) {
dequeue(arrpointer, &lastElementIdx, &totalElements);
}
printQueue(arr, lastElementIdx);
for (int i = 1; i < 21; i++) {
enqueue(arrpointer, &lastElementIdx, &totalElements, i);
}
printQueue(arr, lastElementIdx);
free(arr);
return EXIT_SUCCESS;
}
When you expand or contract the storage for your queue, you need to provide a pointer to the storage back to the caller. This is because realloc() does not necessarily resize a memory block in-place -- it may create a new, differently sized block elsewhere. It is permitted to do so even when it resizes to a smaller block, not only when it resizes to a larger one.
Your usage of variable temp gives the appearance that you are aware of this issue, but as #DerkHermann first observed, you mishandle the resulting pointer. Perhaps you meant to write something along the lines of
arr = temp;
instead. Even that is not sufficient, however. C has only pass-by-value, so if you modify the value of function parameter arr, that modification is visible only in the function (which receives in arr a copy of the value the caller passes). In the event that realloc() allocates a new block, that leaves the caller with an invalid pointer.
If you want your enqueue() and dequeue() functions to be able to resize the storage for the queue, then you must pass the pointer to that storage indirectly. The most straightforward way of doing that, given where you are now, would be to pass a double pointer, so that you can modify its referrent:
void enqueue(int **arr, int* lastElementIdx, size_t* totalElements, int element) {
/* ... */
*arr = temp;
/* ... */
}
Observe, however, that you are passing three separate pointers that among them represent the state of the queue. It would be cleaner to create a struct type that combines those details in one package, and to pass a pointer to an object of that type:
struct queue {
int *arr;
size_t capacity;
size_t last_element_index;
};
void enqueue(struct queue *queue, int element) {
/* ... */
queue->arr = temp;
/* ... */
}
Maybe it's not the only problem, but at least the following line does not what you seem to expect:
*temp = *arr;
It looks as if you want to replace arr with the result of the realloc, delivering it back to the calling function (as with your other inout arguments). But, arr is not an inout argument: It is an array of integers, not a pointer to an array of integers. What you are actually doing with your above line of code is to copy the first element of arr to the newly allocated memory range. That newly allocated memory range temp, then, is nevertheless forgotten, creating a memory leak.
Adding a double pointer to reallocate the space in the right places, changing the comparing function with size_t totalElements and fixing a few additional mistakes finally did the trick.
#include <stdio.h>
#include <stdlib.h>
void enqueue(int **arr, int* lastElementIdx, size_t* totalElements, int element) {
if (*lastElementIdx + 1 >= (int)(*totalElements) - 1) { // check if memorry is sufficient, otherwise double
*totalElements *= 2;
int* temp = realloc(*arr, (*totalElements * sizeof(int)));
if (temp == NULL) { // just in case realloc fails
printf("Allocation error\n");
} else {
*arr = temp;
}
}
if (*lastElementIdx + 1 <= (int)(*totalElements) - 1) {
*lastElementIdx += 1; // once everything is done and if there is now enough space: add element
(*arr)[*lastElementIdx] = element;
}
}
int dequeue(int **arr, int* lastElementIdx, size_t* totalElements) {
if (*lastElementIdx > -1) { // if queue is not empty...
int deleted = (*arr)[0]; // save deleted value first (in case it's still needed)
for (int i = 0; i <= *lastElementIdx; i++) { // shift all elements
(*arr)[i] = (*arr)[i + 1];
}
*lastElementIdx -= 1; // index is now decreased by 1
if (((*totalElements / 2) >= 10) && ((*lastElementIdx + 1) < (*totalElements / 2))) { // cut memory in half if not needed
*totalElements /= 2;
int* temp = realloc(*arr, (*totalElements * sizeof(int)));
if (temp == NULL) { // in case realloc fails
printf("Allocation error\n");
return 0;
} else {
*arr = temp;
}
}
return deleted;
} else { // if queue is empty, print that there's nothing to dequeue
printf("There are no elements inside the queue\n");
return 0;
}
}
void printQueue(int arr[], int lastElementIdx) {
for (int i = 0; i <= lastElementIdx; i++) { // print entire queue
printf("[%d] = %d\n", i, arr[i]);
}
printf("\n");
}
int main (void) {
size_t totalElements = 10; // number of needed elements at the time
int lastElementIdx = -1; // index of last element in queue at the time
int *arr = calloc(totalElements, sizeof(int));
int **arrpointer = &arr;
for (int i = 1; i < 101; i++) {
enqueue(arrpointer, &lastElementIdx, &totalElements, i);
}
printQueue(arr, lastElementIdx);
for (int i = 0; i < 102; i++) {
dequeue(arrpointer, &lastElementIdx, &totalElements);
}
printQueue(arr, lastElementIdx);
for (int i = 1; i < 21; i++) {
enqueue(arrpointer, &lastElementIdx, &totalElements, i);
}
printQueue(arr, lastElementIdx);
free(arr);
return EXIT_SUCCESS;
}
I am using realloc to allocated memory at runtime in dynamic array. Firstly, I allocated a memory with calloc with sizeof a random integer a. In my program, I have taken a=2. After that I want to store some 14 random values generated, so I have to resize the memory using realloc. I am doing the same in a for loop. FOr 1 iteration, realloc works but after that size doesnt increase and a error occurs "corruption in heap". I am not able to understand the problem. Pls help me if you can, in understanding where the problem is occuring and how to solve it.
Thanks a lot.
Below is my code:
j=j*a; //a=3
numbers = (int*) calloc(b, j); //b=14, no of elements I want to store
printf("Address:%p\n",numbers);
if (numbers == NULL)
{
printf("No Memory Allocated\n");
}
else
{
printf("Initial array size: %d elements\n", a);
printf("Adding %d elements\n", b);
}
srand( (unsigned) time( NULL ) );
for(count = 1; count <= b ; count++)
{
if(i <= j)
{
numbers[count] = rand() % 100 + 1;
printf( "Adding Value:%3d Address%p\n", numbers[count],numbers[count] );
i++;
}
if (i > j)
{
printf("Increasing array size from %d bytes to %d bytes\n",j,j*a);
j=j*a;
numbers = (int*) realloc(numbers,j);
printf("Address:%p\n",numbers);
if(numbers == NULL)
{
printf("No Memory allocated\n");
}
}
}
free(numbers);
return 0;
}
The initial array length (length and size are not the same) is b, not a.
Adding b elements? I don't think you are.
Arrays are zero-based in C. You loop should be for(count=0; count<b ; count++).
count is a terrible name for a loop variable. count should hold the number of elements and not be a loop variable.
It's hard to imagine what j could be. Since you use it as the element size in your call to calloc it ought be at least be a multiple of 4, the size of in int. What is it?!
The realloc doesn't seem to bear any relation to the calloc.
I'm sure there are lots of other problems. If you want more help then a clear statement of what your goal is would be required.
EDIT
It sounds like you want something like this:
int capacity = 10;
int count = 40;
int i;
int* array = (int*)malloc(capacity*sizeof(int));
for (i=0; i<count; i++)
{
if (i==capacity)
{
capacity *= 2;
array = (int*)realloc(array, capacity*sizeof(int));
}
array[i] = RandomIntInRange(1, 100);
}
free(array);
Notes:
No error checking. In production code you would check that the allocations succeeded, and the realloc done this way would leak if it failed. But there's no point confusing the message with error checking when you are still at this level of understanding.
No reading input - you can do that.
No writing output - you can do that.
The integer "j" is not initialized in your code, resulting in a = 0 * 3, meaning a will be zero and no memory will be allocated. The segfault is due to you not handling that numbers is NULL. Change to and set j to something meaningful
#include <stdlib.h>
#include <stdio.h>
void
main (int argc, char *argv[])
{
int a = 3;
int j = 1 * a; //a=3
int b = 14;
int *numbers = calloc (b, j); //b=14, no of elements I want to store
int count = 0, i = 0;
printf ("Address:%p\n", numbers);
if (numbers == NULL)
{
printf ("No Memory Allocated\n");
return;
}
else
{
printf ("Initial array size: %d elements\n", a);
printf ("Adding %d elements\n", b);
}
srand ((unsigned) time (NULL));
for (count = 1; count <= b; count++)
{
if (i <= j)
{
numbers[count] = rand () % 100 + 1;
printf ("Adding Value:%3d Address%p\n", numbers[count],
&(numbers[count]));
i++;
}
if (i > j)
{
printf ("Increasing array size from %d bytes to %d bytes\n", j,
j * a);
j = j * a;
numbers = (int *) realloc (numbers, j);
printf ("Address:%p\n", numbers);
if (numbers == NULL)
{
printf ("No Memory allocated\n");
}
}
}
free (numbers);
}