void insertion with realloc()

Question

I am trying to do a void insertion(), but always get a segmentation fault, see following. Same time, I referenced this link.

First, I did realloc(), then move every memory to the next space after the position, last put the insertion in the position and increase nArraySize.

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

void printArray(double array[], unsigned int size)
{
  for (unsigned int i = 0; i < size; ++i) 
  {
    printf("%.3f  ", array[i]);
  }
  printf("\n");
}

void insert(double **array,
            size_t *nArraySize,
            double insertion,
            unsigned int position)
{
  //realloc the memory space first
  *array = realloc(*array, (*nArraySize+1) * sizeof(double));

  for(unsigned int i = nArraySize[0]-1; i >= position; i--)
  {
     *array[i+1] = *array[i];
  }

  *array[position] = insertion;

  *nArraySize += 1;

}

int main()
{
  double *t = calloc(4, sizeof(double));

  t[0] = 0; 
  t[1] = 1; 
  t[2] = 2; 
  t[3] = 3;

  size_t k = 4;

  insert(&t, &k, 1, 1);

  printf("k is %zu\n", k);
  printArray(t, k);
  free(t);
}

Please help. Any suggestion is welcome.

`*array[position]` is `*(array[position])` because of precedence rules. You need to write `(*array)[position]`. — rici, Apr 26 '22 at 14:56
@12431234123412341234123, just fixed the indentation. I was a part of the function, so the original indentation was a bit odd. Sorry for the confusion. — yuw444, Apr 26 '22 at 15:09

Vlad from Moscow · Accepted Answer · 2022-04-26T15:12:19.470

3

These statements are incorrect due to the operator precedence.

*array[i+1] = *array[i];
*array[position] = insertion;

That is postfix operators have a higher precedence than unary operators.

You have to write either

( *array )[i+1] = ( *array )[i];
( *array )[position] = insertion;

or as you are already doing like

array[0][i+1] = array[0][i];
array[0][position] = insertion;

Pay attention to that your function insert is unsafe because there is no check whether the value of position is less than or equal to the current number of elements in the passed array.

Also this for loop

for(unsigned int i = nArraySize[0]-1; i >= position; i--)
{
   ( *array )[i+1] = ( *array )[i];
}

can invoke undefined behavior. First of all the variable i has always a non-negative number. That is the value of i can not be negative because the variable i has the type unsigned int. Now let's assume that *nArraySize is equal to 1 and position is equal to 0; In this case you have the loop

for(unsigned int i = 0; i >= 0; i--)
{
   ( *array )[i+1] = ( *array )[i];
}

and after the first iteration of the loop the value of i will be very big that is greater than 0.

So it will be better to rewrite the loop the following way

for(unsigned int i = *nArraySize; i > position; i--)
{
   ( *array )[i] = ( *array )[i-1];
}

edited Apr 26 '22 at 15:12

answered Apr 26 '22 at 14:57

Vlad from Moscow

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Thanks so much. The problem is resolved with your solution. And I learned something new today. As you suggested, I will add some check conditions beforehand. – yuw444 Apr 26 '22 at 15:02
@yuw444 See also my updated answer. – Vlad from Moscow Apr 26 '22 at 15:06
I didn't quite get `i` could be very big after the first iteration when using my original. Do you mean after `i--`, `i` could be very big as the `unsigned int` type? – yuw444 Apr 26 '22 at 15:17
@yuw444 Yes, i will be vey big after the operation i-- when i is equal to 0. – Vlad from Moscow Apr 26 '22 at 15:18
I see. Basically `i` will cycle back to its maximum could be defined. – yuw444 Apr 26 '22 at 15:19
Somehow, I couldn't @ you. Your user name just didn't pop up. – yuw444 Apr 26 '22 at 15:21
@yuw444 What do you mean? – Vlad from Moscow Apr 26 '22 at 15:22
Let us [continue this discussion in chat](https://chat.stackoverflow.com/rooms/244232/discussion-between-yuw444-and-vlad-from-moscow). – yuw444 Apr 26 '22 at 15:23
@yuw444 I'm sorry. I do not take part in chats. – Vlad from Moscow Apr 26 '22 at 15:24
No worries. Thanks so much. Don't worry about @username things. – yuw444 Apr 26 '22 at 15:27

void insertion with realloc()

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