printf and scanf

2 min read 5 runnable examples Quiz included

More printf formatting

Format specifiers can include a width and a precision:

#include <stdio.h>

int main() {
    printf("[%5d]\n", 42);      // right-aligned in 5 characters
    printf("[%-5d]\n", 42);     // left-aligned
    printf("[%05d]\n", 42);     // padded with zeros
    printf("[%.3f]\n", 3.14159);
    printf("[%8.2f]\n", 3.14159);
    return 0;
}

To print a percent sign itself, write %% in the format string:

printf("Battery at 100%%\n");    // prints: Battery at 100%

Widths line up columns:

#include <stdio.h>

int main() {
    printf("%-10s %6s\n", "Item", "Price");
    printf("%-10s %6.2f\n", "Coffee", 3.5);
    printf("%-10s %6.2f\n", "Sandwich", 6.25);
    return 0;
}

%s prints a string.

Reading input with scanf

scanf reads from the keyboard. It needs the address of each variable to store into, which is what the & means:

#include <stdio.h>

int main() {
    int age;
    printf("How old are you? ");
    scanf("%d", &age);
    printf("Next year you'll be %d.\n", age + 1);
    return 0;
}

Several values at once

#include <stdio.h>

int main() {
    int a, b;
    scanf("%d %d", &a, &b);
    printf("%d + %d = %d\n", a, b, a + b);
    return 0;
}

Decimals and words

#include <stdio.h>

int main() {
    float weight;
    char name[30];
    scanf("%f %s", &weight, name);
    printf("%s weighs %.1f kg\n", name, weight);
    return 0;
}

A word goes into a character array (char name[30] holds up to 29 characters plus an end marker). Arrays are already addresses, so name needs no &. %s reads one word, stopping at a space.

Specifier	Reads
`%d`	an `int`
`%f`	a `float`
`%c`	a single `char`
`%s`	one word into a `char` array

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