Test Your Knowledge (Ch 4)

The scanf function is used to read formatted input from the console. It allows you to specify the format of the input and store it in the appropriate variables.

The scanf function is defined in the <stdio.h> header file, which stands for standard input/output. This header file provides necessary functions to perform input and output operations in C.

The scanf function needs a minimum of two arguments:
1. the format string: “%d”
2. a variable

For example, the following program is capable of receiving a single integer from the console and storing it in the variable named x.

#include <stdio.h>

int main() {
    int x;
    scanf("%d", &x);
    printf("The value of x is %d", x);
    return 0;
}

Output:

10
The value of x is 10

The scanf function always return the number of items matched and assigned successfully. This means it returns the number of characters read successfully.

For example, consider the following program:

#include <stdio.h> 
int main() {
    int x, y, z;
    printf("The number of items read are %d", scanf("%d%d%d", &x, &y, &z));
    return 0;
}

Output:

10 20 30
The number of items read are 3

If only two integers are provided, then the output will be 2.

10 20
The number of items read are 2

By using ‘*’ before the format specifier, we can tell scanf to skip whitespace characters.

For example, consider the following program:

#include <stdio.h>
int main()
{
    int n;
    char c;
    scanf("%d%c", &n, &c); 
    printf("n = %d and c = %c", n, c); return 0;
}

Let’s say the inputs are “10 A” (without quotes). Then, the output will be

10 A
n = 10 and c = 

The variable c is not empty; it is holding the whitespace character.
Now, to skip reading the whitespace characters, we can include the format specifier %*c before the format specifier %c as follows:

#include <stdio.h>
int main()
{
    int n;
    char c;
    scanf("%d%*c%c", &n, &c); 
    printf("n = %d and c = %c", n, c); return 0;
}

Output:

10 A
n = 10 and c = A

Alternatively, we can add a whitespace between %d and %c as follows:

#include <stdio.h>
int main()
{
    int n;
    char c;
    scanf("%d %c", &n, &c); 
    printf("n = %d and c = %c", n, c); return 0;
}

Output:

10 A
n = 10 and c = A

Option (d) is clearly the correct option. For verification, consider the following program:

#include <stdio.h>
int main() {
    int x;
    float y;
    scanf("%d %f", &x, &y);
    printf("x = %d and y = %f", x, y);
    return 0;
 }

Output:

10 3.14159
x = 10 and y = 3.141590

The format specifier mentioned in the printf function is %c, and therefore, the program will print the ASCII character corresponding to the number entered by the user.

In the given program, the address-of operator (&) is used in front of the variable “num” inside the printf call. Therefore, the address of the variable “num” will be displayed.

As the user input is “23Hi” (without quotes), the scanf function will read the integers 2 and 3, and store them as 23 in variable i. The remaining characters (Hi) are left to be read by the next call of scanf.

Try running the following code:

#include <stdio.h>
int main()
{
    float x, z;
    int y;
    scanf("%f%d%f", &x, &y, &z); 
    printf("x = %f, y = %d and z = %f", x, y, z); 
    return 0;
}

Output:

Hence, verified.