5.8. Bool Functions¶

Functions can return bool values just like any other type, which is often convenient for hiding complicated tests inside functions. For example:

bool isSingleDigit (int x) {
if (x >= 0 && x < 10) {
return true;
}
else {
return false;
}
}


The name of this function is isSingleDigit. It is common to give boolean functions names that sound like yes/no questions. The return type is bool, which means that every return statement has to provide a bool expression.

The code itself is straightforward, although it is a bit longer than it needs to be. Remember that the expression ‘’x >= 0 && x < 10’’ has type bool, so there is nothing wrong with returning it directly, and avoiding the if statement altogether:

bool isSingleDigit (int x) {
return (x >= 0 && x < 10);
}


In main you can call this function in the usual ways:

cout << isSingleDigit (2) << endl;
bool bigFlag = !isSingleDigit (17);


The first line outputs the value true because 2 is a single-digit number. Unfortunately, when C++ outputs bools, it does not display the words true and false, but rather the integers 1 and 0.

The second line assigns the value true to bigFlag only if 17 is not a single-digit number.

The most common use of bool functions is inside conditional statements

if (isSingleDigit (x)) {
cout << "x is little" << endl;
}
else {
cout << "x is big" << endl;
}


Construct a block of code that first checks if a number is positive, then checks if it’s even, and then prints out a message to classify the number. It prints “both” if the number is both positive and even, “even” if the number is only even, and finally “positive” if the number is only positive.