Mastery of logic is essential to understanding conditional
statements. It is used in pretty much any program that you
will ever write. Comparisons are the heart of logical
statements. When we write programs, we often want to compare two
pieces of information, testing to see if that comparison evaluates
to True or False. Then, we alter the way our
program operates depending on the result of that comparison.
We can make those comparisons using any of the following comparison operators, which compare two pieces of information:
Notice how you can mix variables and "raw" data and still make valid
comparisons. Unlike the assignment operator (=), it
doesn't matter what goes on the left hand or right hand side of a
You can also combine two or more comparison statements by using:
You do not have to use parentheses around a comparison statement, but it does have the benefit of making your code clearer and easier to read.
A third logical operator available to you is called not. This operates on one logical statement, flipping the truth value of that statement. So, a logical statement that is True will be flipped to False, and a logical statement that is False will be flipped to True.
Being able to make comparisons is only the first part of conditional
statements. We also need a structure to execute different code based
on the value of a comparison. There are three such structures
available: "if", "if-else", and
"if-elif-else". These structures combine with one or more
logical statements to form a conditional statement.
A basic "if" statement looks like this:
You must have an "if" statement before you use any "elif" statements or an "else" statement.
What if you want something different to happen if the logical statement is not True? To do this, just use an "else" statement right after an "if" like so:
What if there are several related logical statements you need to test? Simply use an "elif" in sequence with an "if".
Important: The very first logical statement that evaluates to True will have its associated code executed, and everything else will be skipped over. This means that only one of the code statments will be executed.