C Input Output (I/O)

In this tutorial, you will learn to use scanf() function to take input from the user, and printf() function to display output to the user.

C Output

In C programming, printf() is one of the main output function. The function sends formatted output to the screen. For example,


Example 1: C Output

#include <stdio.h>    
int main()
{ 
    // Displays the string inside quotations
    printf("C Programming");
    return 0;
}

Output

C Programming

How does this program work?

  • All valid C programs must contain the main() function. The code execution begins from the start of the main() function.
  • The printf() is a library function to send formatted output to the screen. The function prints the string inside quotations.
  • To use printf() in our program, we need to include stdio.h header file using #inclue <stdio.h> statement.
  • The return 0; statement inside the main() function is the "Exit status" of the program. It's optional.

Example 2: Integer Output

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

Output

Number = 5

We use %d format specifier to print int types. Here, the %d inside the quotations will be replaced by the value of testInteger.


Example 3: float and double 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

To print float, we use %f format specifier. Similarly, we use %lf to print double values.


Example 4: Print Characters

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

Output

character = a

To print char, we use %c format specifier.


C Input

In C programming, scanf() is one of the commonly used function to take input from the user. The scanf() function reads formatted input from the standard input such as keyboards.


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

Here, we have used %d format specifier inside the scanf() function to take int input from the user. When the user enters an integer, it is stored in the testInteger variable.

Notice, that we have used &testInteger inside scanf(). It is because &testInteger gets the address of testInteger, and the value entered by the user is stored in that address.


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

We use %f and %lf format specifier for float and double respectively.


Example 7: C Character I/O

#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.

When a character is entered by the user in the above program, the character itself is not stored. Instead, an integer value (ASCII value) is stored.

And when we display that value using %c text format, the entered character is displayed. If we use %d to display the character, it's ASCII value is printed.


Example 8: ASCII Value

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

    // When %c is used, a character is displayed
    printf("You entered %c.\n",chr);  

    // When %d is used, ASCII value is displayed
    printf("ASCII value is % d.", chr);  
    return 0;
}

Output

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

I/O Multiple Values

Here's how you can take multiple inputs from the user and display them.

#include <stdio.h>
int main()
{
    int a;
    float b;

    printf("Enter integer and then a float: ");
  
    // Taking multiple inputs
    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

Format Specifiers for I/O

As you can see from the above examples, we use

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

Here's a list of commonly used C data types and their format specifiers.

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