How to change the size of an array and insert another element?

Question

I have an array 0 1 2 4 5 6 7 8 9. I want to insert 3 between 2 and 4.

Length stays the same after calling the function, even though I have added an value, why?

printf("%d,", feld[9]); gives me the right value, code works - but I get a warning.

How can I insert a value even though I initialize with int feld[9] = {0,1,2,4,5,6,7,8,9}; or int feld[] = {0,1,2,4,5,6,7,8,9}; ?


nt insertArray(int* array, int length, int value, int pos) 
{
    int i;

    if (pos < length)
    {
        for (i = length; i > pos; i--)
        {
            array[i] = array[i - 1];
        }
        array[i] = value;
        length++;
    }

    else if (pos == length)
    {
        array[pos] = value;
        length++;
    }
    return length;
}


int main()
{
    int feld[9] = {0,1,2,4,5,6,7,8,9};

    size_t length = sizeof(feld) / sizeof(int);

    insertArray(feld, length, 3, 3);

    length = sizeof(feld) / sizeof(int);

    for (int i = 0; i < length; i++)
    {
        printf("%d,", feld[i]);
    }
    printf("\n");

    printf("%d,", feld[9]);

    return 0;
}

You want to add an additional element in between, right? Not just swap the already existing one? If that's the case you can not do that in C because arrays are of fixed-size, hence you can't just add elements arbitrarily. — Anton9988, May 06 '20 at 08:26
@Tilo The program has undefined behavior. You may not enlarge an array. — Vlad from Moscow, May 06 '20 at 08:29
You can't extend the length of the fixed array, maybe you can reserve more memory or go for dynamic memory, in both case you will have to remember the actual number of items yourself. — kiran Biradar, May 06 '20 at 08:30
Good question @ Jabberwocky :-) : warning: array index 9 is past the end of the array (which contains 9 elements) [-Warray-bounds] — Tilo, May 06 '20 at 14:11

RobertS supports Monica Cellio · Accepted Answer · 2020-05-06T18:23:32.800

Per C syntax, It isn´t allowed to modify the length of an static allocated array after its definition. Any attempt to do so invokes undefined behavior.

Instead, Allocate dynamic memory with malloc(), use pointer offsets to access certain pseudo-elements and use realloc() to resize the memory.

Copy the content of the elements 4 to 9 to the elements 5 to 10. Now you can store 3 in the 4th element.

One demonstrative example:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define SIZE 9
#define ELEM_TO_CH 4

int main(void)
{
    int* feld_ptr = malloc(sizeof(int) * SIZE);
    if(!feld_ptr)
    {
       fprintf(stderr,"Memory could not be allocated for feld_ptr!");
       return 1;
    }

    for(int i = 0; i < SIZE; i++)
    {
       if (i > (ELEM_TO_CH - 2))
          feld_ptr[i] = i + 1;
       else
          feld_ptr[i] = i;
    }

    printf("Before:\n\n");
    for(int i = 0; i < SIZE; i++)
    {
       printf("feld_ptr[%d] = %d\n", i, feld_ptr[i]);
    }

    printf("\n\n");

    feld_ptr = realloc(feld_ptr, SIZE + 1);
    if(!feld_ptr)
    {
       fprintf(stderr,"Error at resizing memory pointed by feld_ptr!");
       return 1;
    }

    memcpy(&feld_ptr[ELEM_TO_CH], &feld_ptr[ELEM_TO_CH-1], sizeof(int) * ((SIZE + 1) - ELEM_TO_CH));

    feld_ptr[ELEM_TO_CH-1] = 3;

    printf("After:\n\n");
    for(int i = 0; i < (SIZE + 1); i++)
    {
        printf("feld_ptr[%d] = %d\n", i, feld_ptr[i]);
    }

    free(feld_ptr);

    return 0;
}

Output:

Before:

feld_ptr[0] = 0
feld_ptr[1] = 1
feld_ptr[2] = 2
feld_ptr[3] = 4
feld_ptr[4] = 5
feld_ptr[5] = 6
feld_ptr[6] = 7
feld_ptr[7] = 8
feld_ptr[8] = 9

After:

feld_ptr[0] = 0
feld_ptr[1] = 1
feld_ptr[2] = 2
feld_ptr[3] = 3
feld_ptr[4] = 4
feld_ptr[5] = 5
feld_ptr[6] = 6
feld_ptr[7] = 7
feld_ptr[8] = 8
feld_ptr[9] = 9

score 0 · Answer 2 · answered May 06 '20 at 08:51

You need to use the heap instead of the stack for your task.

As said in the comments below your post, in case you declare the array it will have a fixed size. Of course, you can edit it and add another value but remember you are allocating some additional space in the memory that does not correspond to your array, thus destroying some of the other variables in the stack (maybe) which is not safe.

You can use malloc() to reserve a required amount of memory in heap then realloc() function to change the initial size. Remember to free() the memory at the end of your program.

score 0 · Answer 3 · answered May 06 '20 at 08:53

As the comments above, you can not extend the size of array. In this case, i propose two solutions:

Using the pointer in the insert function. In this case you have to allocate for this pointer with size = length of old array + 1. The insert function becomes as:

int * insertArray(int* arr, int length, int value, int pos) 
{
    int * array = malloc(sizeof(int) * (length + 1));
    if(!array) {
       return NULL;
    }
    for(int i = 0; i < length; i++) {
        array[i] = arr[i];
    }
    int i;

    if (pos < length)
    {
        for (i = length; i > pos; i--)
        {
            array[i] = array[i - 1];
        }
        array[i] = value;
        length++;
    }

    else if (pos == length)
    {
        array[pos] = value;
        length++;
    }
    return array;
}

Then in main function:

int main()
{
    int feld[9] = {0,1,2,4,5,6,7,8,9};

    size_t length = sizeof(feld) / sizeof(int);

    int * array = insertArray(feld, length, 3, 3);

    for (int i = 0; i <= length; i++)
    {
        printf("%d,", array[i]);
    }
    free(array);
    return 0;
}

Using pointer in main function instead of array, then reallocate pointer in the insert function.

void insertArray(int* array, int length, int value, int pos) 
{
    array = realloc(array, sizeof(int) * (length + 1));
    if (!array)
       return;
    ...
}

In the main function:

int main()
{
    int *feld = malloc(sizeof(int) * 9);
    if(!feld)
      return -1;

    for(int i = 0; i < 9; i++) {
        if(i<3)
          feld[i] = i;
        else
          feld[i] = i+1;
    }

    insertArray(feld, 9, 3, 3);

    for (int i = 0; i <= 9; i++)
    {
        printf("%d,", feld[i]);
    }
    free(feld);
    return 0;
}

How to change the size of an array and insert another element?

3 Answers3