How to let user determine array size of global variables?

Question

I want to try to create a C program which lets the user determine the array size of global variables (to make my program more dynamic). But there's a catch because you can't use scanf outside the scope. Can you guys help me with this?

I've tried using scanf outside the scope and it's not possible.

This is the code that works (the user can't determine the array size of the global variables):

#include <stdio.h>

int arr[100];
int tempArr[100];

void merge(int left, int middle, int right){
    int leftIndex, rightIndex, index;
    for(leftIndex = left, rightIndex = middle + 1, index = left;leftIndex<=middle && rightIndex<=right;index++){<br>
        if(arr[leftIndex]<arr[rightIndex]){
            tempArr[index] = arr[leftIndex++];
        }else{
            tempArr[index] = arr[rightIndex++];
        }
    }
    while(leftIndex<=middle){
        tempArr[index++]=arr[leftIndex++];
    }
    while(rightIndex<=right){
        tempArr[index++]=arr[rightIndex++];
    }
    for(int j = left; j<=right; j++){
        arr[j]=tempArr[j];
    }
}

void sort(int left, int right){
    int middle =0;
    if(left<right){
        middle=(left+right)/2;
        sort(left, middle);
        sort(middle+1, right);
        merge(left,middle,right);
    }
}

int binarySearch(int arr[], int left, int right, int search){
    int middle = 0;
    if(left<right){
        middle = (left+right)/2;
        if(arr[middle]==search){
            return middle;
        }else if(arr[middle]>search){
            return binarySearch(arr,left,middle,search);
        }else{
            return binarySearch(arr,middle+1,right,search);
        }
    }
    return -1;
}

int main(){
    int search;
    int index = 0;
    int arraySize = 0;
    scanf("%d", &arraySize); getchar();
    for(int i=0;i<arraySize; i++){
        int number;
        scanf("%d", &number); getchar();
        arr[index] = number;
        index++;
    }

    printf("Before sorting\n");
    int size=sizeof(arr)/sizeof(arr[0])-1;

    for(int i =0;i<=arraySize;i++){
        if(arr[i]!=0){
            printf("%d %d\n",arr[i],i);
        }
    }

    sort(0, arraySize-1);

    printf("\nAfter sorting\n");
    for(int i =0;i<=arraySize;i++){
        if(arr[i]!=0){
            printf("%d %d\n",arr[i],i);
        }
    }   

    printf("\nSearch a number: ");
    scanf("%d",&search);
    int result = 0;
    result = binarySearch(arr,0,arraySize,search);
    printf("Result: %d",result);
    return 0;
}

And I tried these few lines below and they didn't work. I'm not sure if there is another way I can think of.

#include <stdio.h>

int slot;
scanf("%d",&slot);
int arr[slot];
int tempArr[slot];

Answer 1

How to let user determine array size of global variables?

Users don't determine sizes of variables, programmers do. It might for example make perfect sense to set a static, maximum allowed size and then one valid use-case could be to only use part of the array.

In case you need to set the size of an array in run-time, you use dynamic allocation with malloc. Study dynamic allocation. And that's it.

Answer 2

The original code is:

#include <stdio.h>

int slot;
scanf("%d",&slot);
int arr[slot];
int tempArr[slot];

The modified code is:

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

int slot;
scanf("%d",&slot);
int *arr=malloc(sizeof *arr *slot);
int *tempArr=malloc(sizeof *tempArr *slot);

Here is an example about "let user determine array size of global variables".

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

// [1]. use "pointer" instead of "array" 
int *arr;
int size=0;

void init(void)
{
    int i;
    for(i=0;i<size;i++)
    {
        arr[i]=i;
    }
}

void display(void)
{
    int i;
    for(i=0;i<size;i++)
    {
        printf("%d, ",arr[i]);
    }
    printf("\n");
}

int main(void)
{
    if(scanf("%d",&size)==-1)
    {
        return 0;
    }

    //[2]. use "malloc" to request variable size of memory
    //     "malloc" require <stdlib.h>
    arr=malloc(sizeof *arr *size);
    if(arr==NULL)
    {
        return 0;
    }

    //init and display
    init();
    display();

    //[3]. releasing the memory space is a good habit.
    free(arr);

    return 0;
}

Answer 3

Arrays declared "globally" (at file scope) cannot have their sizes set at runtime. Their sizes must be specified with a constant expression that can be evaluated at compile time (such as a numeric literal or a sizeof operation on another object).

Objects with static storage duration (globals and anything declared with the static keyword) are loaded into memory as soon as the program starts, well before you prompt the user for any input, and their sizes have to be known immediately.

So, if you want a "global" array whose size isn't known until runtime, you'll have to use malloc or calloc:

int *arr = NULL;
int *tmpArr = NULL;
...
int main( void )
{
  int arrsize = 0;
  ...
  scanf( "%d", &arrsize ); // ideally want to check that scanf succeeded.

  arr = malloc( sizeof *arr * arrsize );
  if ( !arr )
  {
    // malloc failed, probably want to exit here
  }
  tempArr = malloc( sizeof *tempArr * arrsize );
  if ( !tempArr )
  {
    // same as above
  }

  ...

When you're done, you release the memory with free before exiting the program:

  ...

  free( arr );
  free( tempArr );

  exit( EXIT_SUCCESS );
}