Mastering C Input and Output (I/O): scanf() and printf() Explained

C Input and Output (I/O) in C

This tutorial walks you through using scanf() to capture user input and printf() to present output, with clear examples and best practices.

Video: Getting User Input in C Programming

Watch the accompanying video to see live demonstrations of scanf() in action.

C Output

The printf() function is the primary tool for sending formatted text to the console. Below are illustrative examples.

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Example 1: Basic Text Output

#include <stdio.h>
int main()
{
    printf("C Programming");
    return 0;
}

Output

C Programming

This program prints a simple string. Key points:

• The main() function is mandatory; program execution starts here.
• Include stdio.h to access printf().
• The optional return 0; signals successful completion.

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Example 2: Integer Output

#include <stdio.h>
int main()
{
    int testInteger = 5;
    printf("Number = %d", testInteger);
    return 0;
}

Output

Number = 5

The %d specifier injects the integer value into the output string.

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Example 3: Floating‑Point Output

#include <stdio.h>
int main()
{
    float number1 = 13.5;
    double number2 = 12.4;
    printf("number1 = %f\n", number1);
    printf("number2 = %lf", number2);
    return 0;
}

Output

number1 = 13.500000
number2 = 12.400000

Use %f for float and %lf for double values.

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Example 4: Character Output

#include <stdio.h>
int main()
{
    char chr = 'a';
    printf("character = %c", chr);
    return 0;
}

Output

character = a

Print a single character with %c.

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C Input

The scanf() function reads formatted data from standard input, typically the keyboard. Below are practical examples.

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Example 5: Integer Input/Output

#include <stdio.h>
int main()
{
    int testInteger;
    printf("Enter an integer: ");
    scanf("%d", &testInteger);
    printf("Number = %d", testInteger);
    return 0;
}

Output

Enter an integer: 4
Number = 4

The address operator & supplies scanf() with the memory location to store the user’s input.

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Example 6: Float and Double Input/Output

#include <stdio.h>
int main()
{
    float num1;
    double num2;
    printf("Enter a number: ");
    scanf("%f", &num1);
    printf("Enter another number: ");
    scanf("%lf", &num2);
    printf("num1 = %f\n", num1);
    printf("num2 = %lf", num2);
    return 0;
}

Output

Enter a number: 12.523
Enter another number: 10.2
num1 = 12.523000
num2 = 10.200000

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Example 7: Character Input/Output

#include <stdio.h>
int main()
{
    char chr;
    printf("Enter a character: ");
    scanf("%c", &chr);
    printf("You entered %c.", chr);
    return 0;
}

Output

Enter a character: g
You entered g.

A character is internally represented by its ASCII integer value; printing it with %c restores the original symbol.

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Example 8: Displaying ASCII Value

#include <stdio.h>
int main()
{
    char chr;
    printf("Enter a character: ");
    scanf("%c", &chr);
    printf("You entered %c.\n", chr);
    printf("ASCII value is %d.", chr);
    return 0;
}

Output

Enter a character: g
You entered g.
ASCII value is 103.

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Handling Multiple Inputs

Collecting several values in a single scanf() call is efficient and concise.

#include <stdio.h>
int main()
{
    int a;
    float b;
    printf("Enter an integer and then a float: ");
    scanf("%d%f", &a, &b);
    printf("You entered %d and %f", a, b);
    return 0;
}

Output

Enter integer and then a float: -3
3.4
You entered -3 and 3.400000

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Format Specifiers for I/O

Below is a concise reference for common C data types and their printf() / scanf() specifiers.

• %d – int
• %f – float
• %lf – double
• %c – char

Data Type	Format Specifier
int	%d
char	%c
float	%f
double	%lf
short int	%hd
unsigned int	%u
long int	%li
long long int	%lli
unsigned long int	%lu
unsigned long long int	%llu
signed char	%c
unsigned char	%c
long double	%Lf