Exercise $4\mathrm{.}6$ You can use the getchar$()$ function in C to get character input from

the user through the keyboard. This function stops the execution of the program and

waits for the input from the user.

The getchar$()$ function has the type int and does not require an argument. It returns

the ASCII$-$Code $($cf$.$ Appendix B$)$ of the key that has been pressed on the keyboard.

a$.$ Write a program, that asks the user to input a digit between $0$ and $9.$

b$.$ Test the input from the user and display an error message if the returned value

is not a digit. The program should then be terminated. If the test is successful,

the program should print the input value on the computer screen.