Function declaration is not allowed

Question

When i run my code in online GCC compiler i get no errors and program gets compiled but when i try to run it in VSCODE it gives me error that function declaration is not allowed here. Code.

#include<stdio.h>
#include<stdlib.h>
#include<limits.h>
int main()
{
  int n, x, y, i, j;
  printf("How many numbers do you want to enter? ");
  scanf("%d",&n);
  int *a=(int*)malloc(n*sizeof(int));
  printf("Enter the numbers: ");
  for(int i=0; i<n; i++)
        scanf("%d",&a[i]);
  printf("Enter the value of the two distance x and y: ");
  scanf("%d %d",&x,&y);

int minDist(int arr[], int n, int x, int y)
{
    int i, j;
    int min_dist = INT_MAX;
    for (i = 0; i < n; i++) {
        for (j = i + 1; j < n; j++) {
            if ((x == arr[i] && y == arr[j]
                 || y == arr[i] && x == arr[j])
                && min_dist > abs(i - j)) {
                min_dist = abs(i - j);
            }
        }
    }
    if (min_dist > n) {
        return -1;
    }
    return min_dist;
}
  printf("\nThe Minimum Distance between %d and %d is %d.\n",x, y,minDist(a,n,x,y));
  free(a);
  return 0;
}

Answer 1

Following C standards, your program is not wrong.

You've used nested functions and that's supported in C. The reason to why your program worked just fine when compiled using gcc and not in VS Code is that

Nested functions are supported as an extension in GNU C, but are not supported by GNU C++

according to this(which was also pointed out by @luther in his answer).

VS Code's C/C++ extension uses the g++ command to compile C and C++ programs (yes, you can use both gcc and g++ to compile .c and .cpp files). g++ will compile .c and .cpp files, but will treat them as C++ files. So, now it's clear why your program does not work when compiled and run in VS Code.

Check this to look at VS Code's C/C++ extension and this to know the difference between gcc and g++.

If you really want to have your program compile and run in VS Code then you can choose to do one of these 2:

1. You can place the the minDist() before the main() function in your code like this:

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

int minDist(int arr[], int n, int x, int y)
{
    int i, j;
    int min_dist = INT_MAX;
    for (i = 0; i < n; i++) {
        for (j = i + 1; j < n; j++) {
            if ((x == arr[i] && y == arr[j]
                 || y == arr[i] && x == arr[j])
                && min_dist > abs(i - j)) {
                min_dist = abs(i - j);
            }
        }
    }
    if (min_dist > n) {
        return -1;
    }
    return min_dist;
}

int main()
{
  int n, x, y, i, j;
  printf("How many numbers do you want to enter? ");
  scanf("%d",&n);
  int *a=(int*)malloc(n*sizeof(int));
  printf("Enter the numbers: ");
  for(int i=0; i<n; i++)
        scanf("%d",&a[i]);
  printf("Enter the value of the two distance x and y: ");
  scanf("%d %d",&x,&y);
  printf("\nThe Minimum Distance between %d and %d is %d.\n",x, y,minDist(a,n,x,y));
  free(a);
  return 0;
}

2. You may declare the function before the main() function and have the function's body after the main() function. Like this:

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

int minDist(int arr[], int n, int x, int y); // declaration of the minDist() function


int main()
{
  int n, x, y, i, j;
  printf("How many numbers do you want to enter? ");
  scanf("%d",&n);
  int *a=(int*)malloc(n*sizeof(int));
  printf("Enter the numbers: ");
  for(int i=0; i<n; i++)
        scanf("%d",&a[i]);
  printf("Enter the value of the two distance x and y: ");
  scanf("%d %d",&x,&y);
  printf("\nThe Minimum Distance between %d and %d is %d.\n",x, y,minDist(a,n,x,y));
  free(a);
  return 0;
}

int minDist(int arr[], int n, int x, int y)
{
    int i, j;
    int min_dist = INT_MAX;
    for (i = 0; i < n; i++) {
        for (j = i + 1; j < n; j++) {
            if ((x == arr[i] && y == arr[j]
                 || y == arr[i] && x == arr[j])
                && min_dist > abs(i - j)) {
                min_dist = abs(i - j);
            }
        }
    }
    if (min_dist > n) {
        return -1;
    }
    return min_dist;
}

Edit: Many thanks to Erik Postpischil for pointing out that I have not completely answered the question.

Answer 2

c program is compiled in top to bottom so before using function in main you have to specify that there is a function that exist. for this write function prototype in up or write entire function body before main().

#include<stdio.h>
#include<stdlib.h>
#include<limits.h>
int minDist(int arr[], int n, int x, int y);

int main()
{
  int n, x, y, i, j;
  printf("How many numbers do you want to enter? ");
  scanf("%d",&n);
  int *a=(int*)malloc(n*sizeof(int));
  printf("Enter the numbers: ");
  for(int i=0; i<n; i++)
        scanf("%d",&a[i]);
  printf("Enter the value of the two distance x and y: ");
  scanf("%d %d",&x,&y);
  printf("\nThe Minimum Distance between %d and %d is  %d.\n",x,y,minDist(a,n,x,y));
  free(a);
  return 0;
}

int minDist(int arr[], int n, int x, int y)
{
    int i, j;
    int min_dist = INT_MAX;
    for (i = 0; i < n; i++) {
        for (j = i + 1; j < n; j++) {
            if ((x == arr[i] && y == arr[j]
                 || y == arr[i] && x == arr[j])
                && min_dist > abs(i - j)) {
                min_dist = abs(i - j);
            }
        }
    }
    if (min_dist > n) {
        return -1;
    }
    return min_dist;
}

Answer 3

Take it and move it before the main function.

Here is your code

#include<stdio.h>
#include<stdlib.h>
#include<limits.h>
int minDist(int arr[], int n, int x, int y);

int minDist(int arr[], int n, int x, int y)
{
    int i, j;
    int min_dist = INT_MAX;
    for (i = 0; i < n; i++) {
        for (j = i + 1; j < n; j++) {
            if ((x == arr[i] && y == arr[j]
                 || y == arr[i] && x == arr[j])
                && min_dist > abs(i - j)) {
                min_dist = abs(i - j);
            }
        }
    }
    if (min_dist > n) {
        return -1;
    }
    return min_dist;
}
int main()
{
  int n, x, y, i, j;
  printf("How many numbers do you want to enter? ");
  scanf("%d",&n);
  int *a=(int*)malloc(n*sizeof(int));
  printf("Enter the numbers: ");
  for(int i=0; i<n; i++)
        scanf("%d",&a[i]);
  printf("Enter the value of the two distance x and y: ");
  scanf("%d %d",&x,&y);
  printf("\nThe Minimum Distance between %d and %d is %d.\n",x, y,minDist(a,n,x,y));
  free(a);
  return 0;
}

Answer 4

You're declaring minDist inside main. Nested functions are a GNU extension. Other compilers might not allow them.

Answer 5

Why are you declaring the function inside the main function?

Take it and move it before the main function.