Printing a string in C using a function

Question

I'm brand new to C and am trying to learn how to take a string and print it using a function. I see examples everywhere using while(ch = getchar(), ch >= 0), but as soon as I put it into a function (instead of main()), it ceases to work. Right now, it's stuck in an endless loop... why is that?

// from main():
// printString("hello");

void printString(char *ch)
{
    while (*ch = getchar(), *ch >= 0)
    putchar(*ch);
}

Answer 1

getchar() reads user input from stdin. If you want to print the string being passed in then there's no need for getchar().

Let's take it step by step. The loop you have reads one character a time from stdin until it reaches end-of-file. That's what the ch >= 0 test checks for: keep reading as long as we're getting valid characters. For printing the characters of a string, the condition changes. Now a valid character is anything that's not NUL ('\0'). So we'll change the loop condition to:

while (*ch != '\0')

Next is figuring out the loop body. putchar(*ch) is fine; we'll leave that there. But without getchar() we have to figure out what the equivalent statement to "get the next character" is.

That would be ch++. This advances the ch pointer to the next character in the string. If we put that at the end of the loop then we'll print a character, advance one space, and then check if the next character is non-NUL. If it is then we print it, advance, and check.

while (*ch != '\0') {
    putchar(*ch);
    ch++;
}