Question

While programming I have come to an unusual error. When I initialize an integer in a loop, sometimes it says that the expression is not valid, but at times it accepts it.
This is my code which gives error:

int pow(int x,int n);
int main()
{
    int x,n,result;
    printf("Enter a number:\n");
    scanf("%d",&x);
    printf("Enter its power:\n");
    scanf("%d",&n);
    result=pow(x,n);
    printf("Result is %d\n",result);
    getch();
    return 0;
}
int pow(int x,int n)
{   
    for(int i=1;i<n;i++)   //<-- here it says that declaration syntax error
    x=x*i;
    return x;
}

While when i change it like this :

int pow(int x,int n)
{   
    int i;
    for(i=1;i<n;i++)  
    x=x*i;
    return x;
}

Answer 1

C89 and earlier versions only support declaration statements at the head of a block (IOW, the only thing that can appear between an opening { and a declaration is another declaration):

/* C89 and earlier */
int main(void)
{
  int x;                      /* this is legal */
  ...
  for (x = 0; x < 10; x++)
  {
    int i;                    /* so is this */
    printf("x = %d\n", x);
    int j = 2*x;              /* ILLEGAL */
  }
  int y;                      /* ILLEGAL */
  ...
}

With C99, declaration statements can appear pretty much anywhere, including control expressions (with the caveat that something must be declared before it is used):

// C99 and later, C++
int main(void)
{
  int x;                       // same as before
  ...
  for (int i = 0; i < 10; i++) // supported as of C99
  {
    printf("i = %d\n", i);
    int j = 2 * i;             // supported as of C99
  }
  int y;                       // supported as of C99
}

Turbo C predates the C99 standard, so if you want to write code like in the second example, you will need to use a more up-to-date compiler.