In the programming introduction, we discussed three different programming structures: sequential, selection, and repetition.

• Sequential is to follow program instructions one by one (in a sequence);
• Selection allows the program to do something depending on the situation;
• Repetition enables the program to repeat something many times.

Today we discuss the selection programming structure in Python. Let's start with an example:

Here the program displays a message and asks the user to enter his or her name. To make this more personal, we can change the program to display What's your name, sir? for a male, and What's your name, madam? for a female.

So instead of asking for the name, we ask for the user's gender first:

IF... Structure

Now if one enters L and hits Enter, the variable gender will contain this value and we shall display a message concerning a lady.

In Python, we can use an if... elif... else structure to find out the condition and act accordingly.

For the problem here, we can have:

You probably notice that we have two spaces (indentation) before name=input("What is your name, madam?"). This is because this madam message is part of the if structure where the user enters L. In Python, we can use 2 (or 4) spaces as indentation to structure a program.

Also, pay attention that we have double equal signs == in the if condition. Double equals == is an equality operator, which determines whether the values on its two sides (left and right) are equal (true) or not (false).

Compare this to a single equal sign =, which is the assignment operator. For example, if you put gender="L", it will change the value of variable gender to "L", which is NOT what we wanted here.

Now, do you understand the difference between = and ==?

IF... ELIF... ELSE... Structure

Back to the program, L is not the only situation because the user may be a man and enters M, or, if she or he does not want to tell, the value of gender can be anything else. Let's expand on the above to include the other two situations:

Note that elif... stands for else if, which means, if you are NOT "L" (in the if condition), then are you ...?

When entering the gender information, the user may enter in lowercase or capital. Run your program and try entering lowercase l or m and see what happens.

We should further revise the code a bit to accommodate situations of different cases, for example:

if gender=="L" or gender=="l":

Here or is a logical OR operator -- the entire condition is true if any one of the conditions ("L" or "l") is true. In other programs, you may need the and operator, which turns true when both conditions are true.

Likewise, can you revise the code for a boy ("M" or "m")?