Lesson 4 · 4 of 20

printf formats · C

Master printf format specifiers in C: %d, %f, %c and %s, decimals with %.2f, column widths, %% and other formats for a shop receipt.

C · printf formats in DMarlon Studio: the example's code and its output in the terminal

Every % in a printf text is a format specifier: a slot filled with the next value in the list, in order. %d is for int, %f for double, %c for char and `%s` for text. The type must match; otherwise the result is unpredictable (and -Wall warns you).

Between the % and the letter you can set the precision: %.2f prints two decimals (42.75) and %.0f rounds to a whole number (43).

A number in front is the minimum width of the column. %5d pads with spaces on the left up to 5 characters (right-aligned), %-5d aligns left and %05d pads with zeros. Put together, as in %-10s %4d %8.2f, they make the columns of the receipt.

To print a percent sign, write `%%`. There are other handy formats too: %x prints a number in hexadecimal (255 is ff) and %e in scientific notation.

Code · main.c

#include <stdio.h>int main(void) {    int items = 3;    double total = 42.75;    char size = 'M';    // %d int, %f double, %c char, %s text    printf("%d items, size %c, %s\n", items, size, "paid");    // .2 means two decimals; .0 rounds to a whole number.    printf("Total: %.2f euros\n", total);    printf("Total: %.0f euros (rounded)\n", total);    // A width pads the value: right-aligned by default, - aligns left.    printf("[%5d] [%-5d] [%05d]\n", 42, 42, 42);    // A small receipt in columns.    printf("%-10s %4s %8s\n", "Product", "Qty", "Price");    printf("%-10s %4d %8.2f\n", "T-shirt", 2, 15.0);    printf("%-10s %4d %8.2f\n", "Cap", 1, 12.5);    printf("%-10s %4d %8.2f\n", "Socks", 3, 4.99);    // %% writes a percent sign.    printf("Discount: %d%%\n", 15);    // %x shows hexadecimal, %e scientific notation.    printf("255 in hex: %x, big number: %e\n", 255, 123456.0);    return 0;}

Output · cc main.c -o main && ./main

3 items, size M, paid
Total: 42.75 euros
Total: 43 euros (rounded)
[   42] [42   ] [00042]
Product     Qty    Price
T-shirt       2    15.00
Cap           1    12.50
Socks         3     4.99
Discount: 15%
255 in hex: ff, big number: 1.234560e+05

Key points

%d int, %f double, %c char, %s text.
%.2f sets the number of decimals.
%8d takes 8 characters; %-8s aligns left.
%% prints a % sign.

Exercise

Add a total line to the receipt (quantity times price for each product, added up) lined up under the Price column.