C Programming Absolute Beginner's Guide (3rd Edition)

if (isdigit(inChar))

{

printf("A number\n");

}

Note

Do you see why these are called character-testing functions? Both isalpha() and

isdigit() test character content and return the relational result of the test.

Is the Case Correct?

The isupper() and islower() functions let you know whether a variable contains an upper- or
lowercase value. Using isupper() keeps you from having to write long if statements like this:

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if ((inLetter >= 'A') && (inLetter <= 'Z'))

{

printf("Letter is uppercase\n");

}

Instead, use isupper() in place of the logical comparison:

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if (isupper(inLetter))

{

printf("Letter is uppercase\n");

}

Note

islower() tests for lowercase values in the same way as isupper() tests for

uppercase values.

You might want to use isupper() to ensure that your user enters an initial-uppercase letter when
entering names.

Here’s a quick little program that gets a username and password and then uses the functions described
in this chapter to check whether the password has an uppercase letter, a lowercase letter, and a
number in it. If a user has all three, the program congratulates him or her for selecting a password
with enough variety to make it harder to crack. If the password entered does not have all three
categories, the program suggests that the user consider a stronger password.

Click here to view code image

// Example program #1 from Chapter 19 of Absolute Beginner's Guide

// to C, 3rd Edition