C++ Programming

Chapter 4: Booleans and Conditions

Boolean values

Boolean values are either true or false. For example, the following code defines a boolean variable (type bool) and sets it to true:

bool x = true;

A Boolean value also be interpreted as a number, with true being 1 and false is 0. On the other hand, any numeric value can be interpreted as a boolean: the number 0 is false and all other numbers are true.

Boolean expressions

A boolean expression often results from a comparison. In C++, standard comparison operators are used ==, !=, <, <=, > and >=. For example, the expression a+b == c is true, if a+b and c are equal.

Ordinary logical operators are && (and) and || (or). For example, the expression x >= 3 && x <= 8 is true, if x is between 3...8. Also, the operator ! in front of the boolean expression reverses its truth value. For example, x > 5 and !(x <= 5) have the same meaning.

We can clarify the interpretation of a boolean expression using parentheses. For example, an expression x%4 == 0 && (x%100 != 0 || x%400 == 0) is true when x is a leap year. The expression requires that x be divisible by 4 and if x is divisible by 100, it is also divisible by 400.

Parts of the boolean expression go from left to right, and if the result is already clear, the remaining parts will not be checked. For example, the expression b != 0 && a/b == 2 a/b == 2 is not checked if b == 0, because the result is definitely false. So it doesn't hurt, even though in the section a/b == 2 we would be dividing by zero.

Conditional statements

In C++, the basic structure of a conditional statement is:

if (condition) {
    code
}

Here, the condition is some boolean expression. If the condition holds, the code in the block is executed, and otherwise nothing will happen. We can also add another branch:

if (condition) {
    code1
} else {
    code2
}

Now if the condition holds, execute code1, and otherwise execute code2.

More generally, conditions can be concatenated as follows:

if (condition1) {
    code1
} else if (condition2) {
    code2
} else if (condition3) {
    code3
...
}

This code goes through the conditions and executes the first code for which the preceding condition is true.

Example

The following program asks the user's age and prints a message based on it:

int age;
cout << "How old are you? ";
cin >> age;
if (age < 0 || age > 120) {
    cout << "Unlikely";
} else if (ika < 18) {
    cout << "You're a minor";
} else {
    cout << "You're an adult";
}

If age is below 0 or above 120, the message is "Unlikely". Otherwise, the program will print "You're a minor", if you are under 18, or "You're an adult", if you are at least 18 years old.

Program execution may look like this:

How old are you? 10
You're a minor

= and ==

There is an important difference between = and ==: = sets the value of a variable, whereas == compares values.

A common programming error is to write = in a condition instead of ==:

if (x = 5) {
    cout << "monkey";
}

The condition here is the expression x = 5 that is, the setting of a variable. The value of the expression is 5, which is true, because all numbers except 0 are true. So the condition is always true, that is, the code always prints "monkey".

One statement blocks

If there is only one statement inside a block, no block is needed and the statement can be written directly without the { and } characters. For example, the following codes work the same way:

if (x == 5) {
    cout << "monkey";
}

if (x == 5) cout << "monkey";

The following code does not work as you might imagine:

if (x == 5)
    cout << "monkey";
    cout << "banana";

Because there is no block, the condition only affects the following statement that is, the code matches this:

if (x == 5) {
    cout << "monkey";
}
cout << "banana";

Conditional expressions

The conditional expression a ? b : c gets b, if a applies, and otherwise c. Here's how to use a conditional expression:

int s = x%2 == 0 ? 5 : 8;

This corresponds to the following code:

int s;
if (x%2 == 0) {
    s = 5;
} else {
    s = 8;
}