Write2 of 14 · about 5 min

Printing values, not just text

A format string can contain placeholders. %d means "put a whole number here", %c a single character, %s a string, and %f a number with a decimal point. The values come after the format string, in order.

printf is not told what you passed. If you write %d and hand it a string, it prints whatever it finds — there is no check. Matching the specifier to the type is your job, and it is the source of a good share of C bugs.

This practice runtime tells you when they do not match. A real compiler warns only when it can see the format string at compile time.

Example

#include <stdio.h>

int main(void) {
    int bits = 32;
    char unit = 'b';
    printf("A word is %d %cits wide.\n", bits, unit);
    return 0;
}

prints: "A word is 32 bits wide.\n"

Your task

Declare an int called year set to 1972, and a char called initial set to D. Print exactly: C appeared in 1972, and D stands for Dennis.

·Use the year variable rather than typing 1972 into the text
·Use the initial variable rather than typing D into the text

Stuck?

Explain it

Why does %s fail here when initial holds a character?

Not graded — this is for you.

Where this goes

Project 1 has you print details about the machine you are compiling on. Every one of those is a value with a matching specifier.

#include <stdio.h>

int main(void) {
    int year = 1972;
    char initial = 'D';

    /* Print the sentence using year and initial. */

    return 0;
}

Ctrl+Enter checks

This is a teaching runtime for a subset of C, running in your browser on a 32-bit model machine. It reports mistakes a real compiler lets through — reading uninitialised memory, running off an array, signed overflow — and it is not the compiler your project uses.

Press Run to see what your program does, or Check when you think it is right. Everything runs here in your browser.