For loop controller using pointers

**Pat Wanjau** · 01-04-2015

I'm familiar with the concept of int in the for loop. However, I've seen a case where the condition for the for loop is a pointer. How is the pointer used or interpreted in the for loop condition in C or C++. As an example:

Code:

void printer(char *fmt, ...) {
    va_list args;
    char *p, *sval;
    int ival;
    double dval;

    va_start(args, fmt);
    for(p = fmt; *p; p++) {
        if(*p != '%') {
            putchar(*p);
            continue;
        }
        switch(*++p) {
            case 'd':
                ival = va_arg(args, int);
                printf("%d", ival);
                break;
            case 'f':
                dval = va_arg(args, double);
                printf("%.2f", dval);
                break;
            case 's':
                for(sval = va_arg(args, char*); *sval; sval++)
                    putchar(*sval);
                break;
            default:
                putchar(*p);
        }
    }
    va_end(args); // cleanup when done
}

This example is derived from the book:
The C programming Language
By Brian W. Kernighan and Dennis M. Ritchie.
Published by Prentice-Hall in 1988
ISBN 0-13-110362-8 (paperback)
ISBN 0-13-110370-9

**Aslaville** · 01-04-2015

Woah, Database Error is becoming some irritating thing.

Anyways, as you increment the pointer, it points to a string one character less, and eventually points to nothing hence the condition-expression eventually evaluates to false in this way.

Same as you could do:

Code:

char *string = NULL;

if(string) { //evaluates to false
}

char *name = "Something";

if(name) { //evaluates to true

}

/* dereferencing a pointer to a string gives you the first character in the string  */

if (*name) { //evaluates to true
 }

if (*string) { //evalutes to false 
}

**Salem** · 01-04-2015

> for(p = fmt; *p; p++)
Is read as
for(p = fmt; *p != '\0' ; p++)

**Pat Wanjau** · 01-05-2015

Thank you so much Aslaville &Salem. That helped so much, and I now understand.

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