Chapter 2 - Compound Conditions

Compound Conditions with AND

Let’s examine the && (AND) and || (OR) operators in more detail to figure out how they work. In computer programming, the definition is a bit more strict than the way those words are used in English. Consider these statements about the following diagram:

This shows that a compound condition with && is true only when both parts of the condition are true. If even one part is false, the whole combination is false.

Compound Conditions with OR

Now let’s look at the same diagram with ||.

This shows that a compound condition with || is true if either part of the condition is true. If even one part is true, the whole combination is true. The only way an || compound turns out false is when everything is false.

When you have three or more conditions joined with &&, then the compound is true only when all the parts are true. When you have three or more conditions joined with ||, the compound is true when any of the parts is true.

Early Exit

Let’s say I tell you that Cathy owns both a cat and a dog. You want to find out if that’s true or not, so you ask Cathy, “Do you own a cat?”

If Cathy says “no,” you don’t even have to ask her about the dog. You already know that I wasn’t correct. When the first part of an and condition is false, the whole thing is false, no matter whether the second part is true or not. Why waste time asking the second question?

Similarly, let’s say I tell you that Cathy owns either a dog or a horse, and you want to find out if that’s true or not. You start by asking Cathy, “Do you own a dog?”

If Cathy says “yes,” you don’t have to go any further. As soon as the first part of an or condition is true, the whole thing is true, no matter whether the second part is true or not.

In a similar way, C++ stops evaluating a condition as soon as it knows whether the answer will be true or false. Let’s say I have a variable called nPlayers (number of players) and a totalScore and I want to test if the average score is greater than 75. Because I can’t divide by zero, I have to test that the number of players is greater than zero first:

if (nPlayers > 0 && totalScore / nPlayers > 75)
{
   cout << "Good game, people!" << endl;
}
else
{
   cout << "Better luck next time!" << endl;
}

This will work exactly as we want. If nPlayers is greater than zero, the program must test the second half to see if the entire condition is true or false, so the program will do the division. If nPlayers is equal to zero, it’s not greater than zero, so the first part of the condition comes back false. That means the whole condition is false, so C++ does not do the division.

Writing Compound Conditions

When you have a compound condition, you must write out all its parts completely. If you want to test if score is greater than 100 and less than 200, you must write the condition like this:

score > 100 && score < 200

You must never write it in either of the following ways. The first one will generate a syntax error, and the second way will always come back as true.

score > 100 && < 200
100 < score < 200

Similarly, to test if a number n is equal to 4 or 5, you must write it this way:

n == 4 || n == 5

And never this way, which doesn’t give you a syntax error, but always returns true.

n == 4 || 5