Text and Numbers

printf can print numbers too, and it can even calculate with them. The f at the end of its name stands for formatted. The text in quotes is a format string, and it can contain format specifiers: placeholders that start with %. The specifier %d means "a whole number goes here". You write the number itself after the text, separated by a comma:

printf("%d\n", 42);

This prints:

42

The letter d stands for decimal: the number is printed in the decimal system, as usual. Write numbers in code without spaces and commas: 1250, not 1 250 or 1,250.

Instead of a number, you can write a calculation after the comma. C works it out first and prints the result. A text in quotes, however, is printed exactly as it is:

printf("%d\n", 5 + 3);
printf("5 + 3\n");

This prints:

8
5 + 3

You add with +, subtract with - and multiply with a star * (not with the letter x). As in math, multiplication goes before addition, and parentheses change the order:

printf("%d\n", 2 + 3 * 4);
printf("%d\n", (2 + 3) * 4);

This prints:

14
20

Division / has a surprise in store in C, so it waits for the next lesson.

The format string can contain text around %d. The value is printed in place of %d, and the rest of the text stays as it is:

printf("The game has %d levels.\n", 12 + 8);

This prints:

The game has 20 levels.

The spaces around %d are part of the text, so you have to write them yourself: "The game has%dlevels.\n" would print The game has20levels.

Common mistakes:

• A forgotten value after the comma: for printf("The game has %d levels.\n");, the compiler doesn't run the program and reports the error more '%' conversions than data arguments (more % specifiers than values).
• A calculation written into the text: printf("The game has 12 + 8 levels.\n"); prints The game has 12 + 8 levels. C doesn't calculate inside quotes.
• A missing comma: printf("%d\n" 42); ends with the error expected ')'.