Question

Write a program using elif which uses the conditional operator

Answer 1

In the last chapter, you were introduced to the concept of flow of control: the sequence of statements that the computer executes. In procedurally written code, the computer usually executes instructions in the order that they appear. However, this is not always the case. One of the ways in which programmers can change the flow of control is the use of selection control statements.

In this chapter we will learn about selection statements, which allow a program to choose when to execute certain instructions. For example, a program might choose how to proceed on the basis of the user’s input. As you will be able to see, such statements make a program more versatile.

We will also look at different kinds of programming errors and discuss strategies for finding and correcting them.

Selection: if statement

People make decisions on a daily basis. What should I have for lunch? What should I do this weekend? Every time you make a decision you base it on some criterion. For example, you might decide what to have for lunch based on your mood at the time, or whether you are on some kind of diet. After making this decision, you act on it. Thus decision-making is a two step process – first deciding what to do based on a criterion, and secondly taking an action.

Decision-making by a computer is based on the same two-step process. In Python, decisions are made with the if statement, also known as the selection statement. When processing an if statement, the computer first evaluates some criterion or condition. If it is met, the specified action is performed. Here is the syntax for the if statement:

if condition:
if_body
When it reaches an if statement, the computer only executes the body of the statement only if the condition is true. Here is an example in Python, with a corresponding flowchart:

if age < 18:
print("Cannot vote")


As we can see from the flowchart, the instructions in the if body are only executed if the condition is met (i.e. if it is true). If the condition is not met (i.e. false), the instructions in the if body are skipped.

Relational operators

Many if statements compare two values in order to make a decision. In the last example, we compared the variable age to the integer 18 to test if age less than 18. We used the operator < for the comparison. This operator is one of the relational operators that can be used in Python. The table below shows Python’s relational operators.