How to create live dynamic memory allocation in an array of numbers, while taking input?

Question

So My mission is to keep accepting numbers from users until the user input a negative number. My algorithm should be: 1)start in the size of 2. 2)double the size every time it reaches the end, free the older one. 3) stop when user hits negative number.

**I know I haven't used free and didn't check if the allocation is successful, I'm trying to keep the code as clean as possible for you guys. Please help.

#include <stdio.h>
#include <stdlib.h>
int *reallocate(int* numbers,int i,int arr[]);
int main() {
    int numbers[2];
    int *nums = numbers;
    int i = 0;
    int size = 2;
    while (i<size)
    {
        scanf("%d", (nums+i));
        if (*(nums + i) <0)
            break;
        i++;
        if (i == size) {
        nums=reallocate(nums,i,numbers);
        size = size * 2;
        }
    }
    puts("Stop");
    return 0;
}
int *reallocate(int* numbers,int i, int arr[]) {
    int newsize = 0;
    newsize =i * 2 * sizeof(int);
    numbers = (int *)realloc(arr,newsize);
    return numbers;
}

You can't call realloc on an array you allocated statically to begin with. — Turn
– Turn, Commented Dec 27, 2015 at 19:39
Well, by saying live allocation, I mean that it's "on demand". As long as the user keeps entering numbers it keeps growing. — Isan Rivkin
– Isan Rivkin, Commented Dec 27, 2015 at 19:46
@IsanRivkin you should allocate memory for array dynamically, and then reallocate it on demand. — stek29
– stek29, Commented Dec 27, 2015 at 19:50

stek29 · Accepted Answer · 2015-12-27 21:32:25Z

1

You should use malloc'ed arrays only with realloc
Here is code:

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

int main(void) {
    int *nums;
    size_t size = 2;
    nums = malloc(size * sizeof(int));
    size_t i = 0;

    while (i < size && nums != NULL) {
        scanf("%d", (nums+i));
        if (nums[i] < 0)
            break;
        i++;
        if (i == size) {
            size *= 2;
            nums = realloc(nums, size * sizeof(int));
        }
    }

    if (nums == NULL) {
        puts("Error");
        return 1;
    } else {
        free(nums);
        puts("Stop");
        return 0;
    }
}

edited Dec 27, 2015 at 21:32

answered Dec 27, 2015 at 19:46

stek29

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15 Comments

Isan Rivkin Over a year ago

It works great thanks ! I got the wrong idea of making a static array and pointing at it, thanks! *Small correction, you forgot to add sizeof(int) in the realloc, just for the people who read here, but it works perfect!

Michi Over a year ago

Why are you casting malloc? By the way your code has memory leaks.

stek29 Over a year ago

@Michi Can you please show me where? And about casting malloc, do you have any ideas how to do this in C, not C++ (I mean no vector)?

Michi Over a year ago

@stek29 I really don't understand your Question. Any way there is no need to cast malloc in C, because of its return type which is VOID*. If you ask that, then you probably don't know that. Maybe because you are used with C++? or maybe because you are using a wrong compiler

Michi Over a year ago

@stek29 Where malloc takes place, free should be present too. You should know that.

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