What does this for loop's condition do?

Question

This is a program for converting letters to upper case.

Could anyone explain me what the condition of the for loop in the below program does?

#include<stdio.h>
#include<string.h>
#include<ctype.h>

int main()
{
    int i;
    char a[50];
    gets(a);

    for(i=0;a[i];i++)
      a[i]=toupper(a[i]);

    puts(a);
    return 0;
}

Answer 1

gets() will return a 0-terminated string, as all C strings should be. So if you entered "four", the actual contents added to a will be those four letters followed by a 0 byte.

 // as if you'd declared
 char a[] = { 'f', 'o', 'u', 'r', 0 };

The loop tests each character to see that it's not 0 . When a[i] is 0 , the condition fails, and the loop ends. It's simply looping over all of the actual characters in the string.

Answer 2

a[i] evaluates to non-zero (true) for all the characters except the terminating null character. Hence, the loop breaks when a[i] is the terminating null character.

Answer 3

By convention, strings in C have a zero byte at the end to indicate the end of the string. The for checks for this byte and stops executing when it is reached because all non-zero bytes in the string evaluate to true.