overview/0x03·Foundations

Input & Output

Talking to the terminal with printf and scanf, and their format specifiers.

C has no built-in print statement — output is handled by ordinary library functions, the most common being printf and scanf, both declared in <stdio.h>. Both rely on a small template language called format specifiers to know how to interpret your data.

printf and format specifiers

printf.c

1#include <stdio.h>
2 
3int main(void) {
4    int age = 19;
5    float gpa = 3.8f;
6    char initial = 'N';
7 
8    printf("Age: %d\n", age);
9    printf("GPA: %.2f\n", gpa);
10    printf("Initial: %c\n", initial);
11    printf("%s is %d years old\n", "Nour", age);
12    return 0;
13}

%d — a signed integer.
%f — a floating-point number; %.2f rounds to 2 decimal places.
%c — a single character.
%s — a string (a char*, covered in 0x08).
%% — a literal percent sign.

warning: Mismatched specifiers are undefined behavior

Passing a double where printf expects an %d, or vice versa, compiles but produces garbage output — the compiler doesn't check this for you by default. Modern gcc will warn about it with -Wall, which is worth always enabling.

Reading input with scanf

scanf reads formatted input and needs the address of each variable it fills in, using the & operator (more on addresses in 0x09):

scanf.c

1#include <stdio.h>
2 
3int main(void) {
4    int age;
5    printf("Enter your age: ");
6    scanf("%d", &age);
7    printf("You are %d years old.\n", age);
8    return 0;
9}

note: Strings are the exception

A plain array already refers to its own address, so reading a string with scanf("%s", name) doesn't use &. This is explained fully once arrays and pointers connect in 0x0A.

Escape sequences

Inside string literals, a backslash introduces a special character:

escapes.c

1"\n"   // newline
2"\t"   // tab
3"\\"   // a literal backslash
4"\""   // a literal double quote

← 0x02Operators & Expressions 0x04 →Conditionals