For loop says expression syntax error when initializing integer in the loop

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

In C, before C99, you need to declare your variables before your loop. In C++, and now in C99, you can declare them within the loop as you try here. The different results you are getting may be because you are sometimes compiling as C++, and sometimes compiling as C.

You could try to make sure you are always compiling your code as C++, or if you're using GCC or Clang, compile with the --std=c99 flag. C99 is unsupported by MSVC, so you will either need to use C++ or move the declaration outside of the loop if you're using MSVC.

Answer 2

What C compiler are you using?

Older versions of C prior to C99 require all variable declarations be made at the top of a code block.

Answer 3

It sounds like you have a C89 compiler (rather than C99 compiler).

In C89, you are only allowed to declare variables at the beginning of a function. You are simply not allowed to declare variables elsewhere in a function, including in the initialization part of a for statement. That's why the second syntax works and the first fails.

The second syntax is valid for C99 and C++ but not for C89.

Answer 4

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.