Unit 3: Input and Output

I/O operation in C

C does not have built-in I/O statements. Instead, it provides input and output through standard library functions declared in the header file <stdio.h>.

I/O functions in C are divided into two categories:

• Formatted I/O — printf(), scanf().
• Unformatted I/O — getchar(), putchar(), gets(), puts().

Conversion specifications

A conversion specification (or format specifier) tells printf() and scanf() the type of data being read or written. It begins with % followed by a character.

Specifier	Type
%d	Signed integer
%u	Unsigned integer
%f	Float
%lf	Double
%c	Character
%s	String
%x	Hexadecimal integer
%o	Octal integer
%e	Scientific notation
%%	Prints a literal % sign

Width and precision can be specified, for example %5d, %.2f, %-10s.

Formatted I/O

printf()

printf() writes formatted output to the standard output (screen).

#include <stdio.h>

int main(void)
{
    int age = 20;
    float height = 5.75f;
    printf("Age = %d, Height = %.2f\n", age, height);
    return 0;
}

scanf()

scanf() reads formatted input from the standard input (keyboard). Variables are passed by their address using the & operator.

#include <stdio.h>

int main(void)
{
    int n;
    float f;

    printf("Enter an integer and a float: ");
    scanf("%d %f", &n, &f);

    printf("You entered %d and %.2f\n", n, f);
    return 0;
}

Unformatted I/O

Unformatted I/O functions deal with one character or one string at a time without conversion specifications.

getchar() and putchar()

• getchar() reads a single character from the keyboard.
• putchar() writes a single character to the screen.

#include <stdio.h>

int main(void)
{
    char ch;
    printf("Enter a character: ");
    ch = getchar();
    printf("You entered: ");
    putchar(ch);
    putchar('\n');
    return 0;
}

gets() and puts()

• gets() reads a string (until newline) from the keyboard.
• puts() writes a string to the screen followed by a newline.

#include <stdio.h>

int main(void)
{
    char name[50];
    printf("Enter your name: ");
    gets(name);
    puts("Hello,");
    puts(name);
    return 0;
}