4.3 formatted Input/Output Functions

4.3 formatted Input/Output Functions

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Formatted Input/Output functions in C programming are used to read user input from standard input devices and write program output to standard output devices in a user -friendly way. The most common formatted input/output function in C, defined in header file “stdio.h” are:

1. printf(): writes to standard output(stdout)
2. scanf(): reads from standard input(stdin)

The syntax of these functions are as follows:

printf("control string", arg1, arg2, ...., argn);

scanf("control string", &arg1, &arg2, ...., &argn);

4.3.1. Control String (flags, field width, precision, and specifier)

A control string in the context of C programming typically refers to the format string used with functions like printf and scanf(). It is a character string that contains format specifiers and optional additional characters, which control the conversion and formatting of input and output.

The format specifiers in a control string indicate the type and format of the data being processed. They begin with the percent sign (%) and are followed by a character that represents the type of conversion to be performed. For example, %d is a format specifier for integers, %f for floating-point numbers, %s for strings, etc.The most commonly used conversion characters are:

 

The following letters may be used as prefix for certain conversion character:

h for short integer

I for long integer or double

L for long double

C program for control string with a format specifier and additional characters:

#include<stdio.h>
int main()
{
    int number = 42;
    printf("The number is: %05d\n", number);
    return 0;
}

Output:

The number is: 00042

In this example, the control string is "The number is: %05d\n". It includes the format specifier %d for integers and the additional characters '05', where '0' is a flag for zero-padding and '5' is the field width, indicating that the number should be printed with a minimum width of 5 characters, zero-padded if necessary.

4.3.2. Formatted I/O (scanf(), printf())

scanf() function

To read data in from standard input (keyboard), we call the scanf function. The basic form of a call to scanf is:

scanf("control string", &arg1, &arg2, ...., &argn);

• The control string is used as defined in section “control string” section
• Since we need space to store incoming data, hence the list of variable addresses are used.

C program to understand basic usage of scanf() function:

#include<stdio.h>
int main()
{
    int num;
    float fnum;
    char ch;
    printf("\nEnter a number: ");
    //read integer using %d
    scanf("%d", &num);
    printf("\nEnter a floating point number: ");
    //read float value using %f
    scanf("%f", &fnum);
    printf("\nEnter a character: ");
    //read character using %c
    scanf(" %c", &ch);

    printf("\n you entered following values; ");
    printf("\n%d\t%f\t%c\n", num, fnum, ch);
    return 0;
}

Output:

Enter a number: 10

Enter a floating point number: 2.4

Enter a character: a

 you entered following values; 
10	2.400000	a

Now, let us understand the control string used in scanf() function:

1. Flags:

Flags modify the behavior of conversion. In the case of scanf, the most common flag is *, which indicates that the corresponding input should be read but ignored.

#include <stdio.h>

int main() {
    int x, y;

    // Ignore the first integer
    scanf("%*d %d", &y);

    printf("In y: %d\n", y);

    return 0;
}

2. Field Width:

Field width specifies the maximum number of characters to be read for the input. It prevents overflows and limits the number of characters to be read.

#include <stdio.h>

int main() {
    char name[10];

    // Read up to 9 characters for the name
    scanf("%9s", name);

    printf("Name: %s\n", name);

    return 0;
}

3. Precision:

Precision in scanf is not as commonly used as in printf. However, for string inputs, it specifies the maximum number of characters to be read.

#include <stdio.h>

int main() {
    char greeting[5];

    // Read up to 4 characters for the greeting
    scanf("%4s", greeting);

    printf("Greeting: %s\n", greeting);

    return 0;
}

4. Specifiers:

Specifiers in scanf are used to indicate the type of input to be read.

#include <stdio.h>

int main() {
    int num;
    float price;

    // Read an integer followed by a floating-point number
    scanf("%d %f", &num, &price);

    printf("Integer: %d\n", num);
    printf("Float: %.2f\n", price);

    return 0;
}

printf() function 

To write program data to the standard output device (terminal) we call printf() function. The basic form of printf() function is as follows:

printf("control string", list of variables);

where:

• Control string is the control string discussed in above section
• list_of_variables is a comma-separated list of variables or expressions yielding results to be inserted into the output.

C program to understand basic usage of printf() function: 

#include<stdio.h>
int main()
{
    int a = 10;
    float b = 2.5;
    char c = 'x';
    printf("\nHello world\n\n");
    printf("\na= %d\tb=%f\tc=%c\n", a, b, c);
    printf("\"Hello World\"\n");
    return 0;
}

Output:

Hello world


a= 10	b=2.500000	c=x
"Hello World"

Various control string used in printf() function to control output display:

Flags:

Flags modify the output format by changing the alignment or padding. Two common flags are '-' for left-justification and '0' for zero-padding.

#include <stdio.h>

int main() {
    int integerNumber = 42;

    // Left-justified with a field width of 10
    printf("Left-justified: |%-10d|\n", integerNumber);

    // Zero-padded with a field width of 10
    printf("Zero-padded:    |%010d|\n", integerNumber);

    return 0;
}

Field Width:

Field width specifies the minimum width of the output field. It can be used to ensure a minimum number of characters are printed.

#include <stdio.h>

int main() {
    int integerNumber = 42;

    // Minimum field width of 5
    printf("Minimum width of 5: |%5d|\n", integerNumber);

    // Minimum field width of 10 for a string
    printf("Minimum width of 10: |%10s|\n", "Hello");

    return 0;
}

Precision:

Precision is used with floating-point numbers and specifies the number of digits after the decimal point.

#include <stdio.h>

int main() {
    float floatingNumber = 3.14159;

    // Display two digits after the decimal point
    printf("Two digits after decimal: |%.2f|\n", floatingNumber);

    return 0;
}

Specifiers:

Specifiers define the type of the variable to be printed. Common specifiers include %c for characters and %s for strings.

#include <stdio.h>

int main() {
    char character = 'A';

    // Display as a character
    printf("Character: |%c|\n", character);

    // Display as a string
    printf("String:    |%s|\n", "Hello, World!");

    return 0;
}