Chapter 3.1 - if / elif / else statements

Chapter 3.1.1 - if statements¶

The most basic if statements are used for conditional execution of python code using a single test that generates a bool. If the bool is True the code that “belongs” to the if statement is executed. If the bool is False, the code is ignored.

The syntax is as follows:

1. Add the keyword if to your code: if
2. Add a space and then a test that is within parentheses: if (test)
3. Add a : immediately after the test: if (test):
4. Press enter and add code that starts 4 spaces from the left margin:

if (test):
    code

In this example, code will only run if test is True.

This addition changes our program type from sequential to conditional. In other words, not every line of code will be executed. What lines are or are not executed are based on the requirements of your program and the logic assocated with those requirements.

For example, say you have a version of Blackboard that sends you a message saying “Great job!” whenever you get an assignment score above 90%.

score = 85

print("Great job!")

If you wrote a simple sequential program like the one above, there would be no way to control if the message is sent or not. This would result in some students who did not score a 90% to get the message.

Instead, we want to conditionally execute lines of code using if statements.

Back to the previous example, the requirement of the program is that “Great job!” is only sent when an assignment score exceeds 90%. This is the “english” version of the requirements. To convert this into a Python version, you need to translate what “exceeds 90%” means, and note that you should probably use a > operator. Once you figure this out, you need to determine what goes on the left of the operator and what goes on the right. Typically, we put the variable or value being “tested” on the left, and the value it is being tested against on the right:

score > 90

Recalling our discussion on bool, the following examples would result in either True or False.

1. False if score is 90 or less:

score = 85

print(score > 90) #output is False, since 85 is not greater than 90

2. True if score is greater than 90:

score = 95

print(score > 90) #output is True, since 95 is greater than 90

Next we need to add the test to the if syntax as defined above and add indented code below the if statement:

score = 95

if (score > 90):
    print("Great job!")

Here is the code in action. Change score to print “Great job!”:

Notice that nothing happens if the score is not greater than 90. This is because Python ignores any code under the if statement if the condition does not result in a bool equal to True.

If we want to add code that always runs, we just add unindented code below or above the if statement:

Notice that the variable definition and the two print statements both ran even if you changed score to something less than or equal to 90.

This can become complicated if you change a variable within the if statement:

We can use print statements to follow what is happening to score.

We see that when score is greater than 90, score > 90 is True, so the indented code is executed. It is only after the indented code is executed that the score is set equal to itself minus 10. This has no influence on the if statement at that point.

Chapter 3.1.2 - if else statements¶

There are some cases where you need one of two options to be executed, but not both.

This is called mutually exclusive code because the two options cannot both occur while the program and its variables are in a particular state. Going back to the example above, perhaps you want to send a supportive message like “Get 'em next time!” to those who do not score better than a 90.

In other words, you want to send either “Great job!” or “Get 'em next time!”, but not both. What determines what message is sent is the test score > 90.

To do this, we simply add an else keyword that has the same “indentation level” as the if keyword. Under the else, you write code that will be executed if the test results in a bool equal to False. We can refer to the code belonging to the if (indented under the if) as the “if code block”, and the code belonging to the else as the “else code block”.

Change score to see both messages.

Chapter 3.1.3 - if elif else statements¶

You can extend this pattern by including as many elif keywords between the if code block and the else code block. The elif code block test should be more permissive (easier to get a True) than the if code block test or the test should be mutually exclusive.

The rules of the if elif else statement are the same as the if else, except the only code block that will be executed is the first time a test results in True. If none of the tests result in True, the else code block is executed.

What if we wanted to send “Great job!” to those with above 90, and “Good job!” to those between 81 and 90 (including 90)?

You can add a test for score > 80. You might think that this would not work, because a score of 95, for example, would be True for score > 90 and score > 80. However, as stated above, the first True will result in none of the other code blocks being executed.

Therefore, we know that elif (score > 80) will only be considered if (score > 90) was False.

Try to change score below to get every message.

3.1.4 - Indentation and whitespace¶

If you are new to programming, this might not be as hard to learn as for those who come from C++, Java, etc.

Python uses whitespace and indentation to determine what code “belongs” to what control structure (example, an if elif else statement).

The dashes are just demonstrating “whitespace” in the example below:

if (test):
----start code here
----if it belongs to
----this if statement

The actual Python code would look something like this:

score = 85

if (score >= 90):
    grade = "A"
elif (score >= 80):
    grade = "B"
elif (score >= 70):
    grade = "C"
else:
    grade = "D"
    
print("Grade =", grade)

This is in contrast to C++, Java, and even javascript that use brackets to determine to what control structure the code block belongs:

Java code:

int score = 85;
String grade;

if (score >= 90) {
grade = "A";
} else if (score >= 80) {
grade = "B";
} else if (score >= 70) {
grade = "C";
} else {
grade = "D";
}

System.out.println("Grade = " + grade);

How you can get it wrong.

if statement errors¶

Case 1

Since indentation is so important, it is crucial that you pay attention to exactly how much indentation you give your code blocks.

For example, the following results in an error in Python, but would be perfectly fine in Java as long as it is within the brackets:

Case 2

Another common error is to forget the “:” after the test. This might be the first thing you would want to check if you have an error in your if statements:

Case 3

You can also write code that is not “incorrect” from a syntax perspective, but it is incorrect from a logical perspective.

Say you wanted to only write “Good job!” if the score is better than or equal to 90.

If you do not get the indentation correct, Python will treat the code as part of the broader program and not a part of the if statement:

if else statement errors¶

Case 4

You must have a code block for both the if and the else:

If you want to debug your if code, you can temporarily use a print statement or the pass statement, but this is only temporary:

Case 5

You cannot have a “test” for the else block:

if / elif / else statement errors¶

Case 6

You must have a test for all if and elif blocks:

Although it will not give you an error, you should end if / elif / else statements with an else statement.

This is because you may miss cases that are not covered by the if and elif blocks:

Chapter 3.1.5 - Practice¶

1. Create an if/else statement to test if a score is greater than 60. Add an appropriate message.

2. Create an if/else statement to test if a score is less than 60. Add an appropriate message.

3. Create an if/else statement to test if a score is greater than or equal to 60. Add an appropriate message.

4. Create an if/else statement to test if a score is less than or equal to 60. Add an appropriate message.

5. Create an if/elif/else statement to meet the following criteria:

Water will boil into a gas at 100 C and freeze into a solid at 0 C, and will be liquid between those values. Write code blocks that say “Solid”, “Liquid”, or “Gas” based on the variable temp.

5b. Provide an example of each possibility by changing the variable ‘temp’

6. Fix all of the issues in the following if / elif / else using hints from the error messages.

In this case, we want to print the following:

1. “SEVERE” if CAPE is greater than or equal to 2000

2. “MARGINAL” if CAPE is greater than or equal to 500

3. “NO RISK” if CAPE is less than 500.

You must use an else and elif code block. Test the blocks by changing the value of CAPE.

List the problems in the following markdown cell.

Problems with the following code:

CAPE = 5000

if (CAPE < 2000)
    print("SEVERE")
elif (CAPE > 500):
    print("MARGINAL
elif (CAPE == 500):
    print("NO RISK")

Syntax problems (basic Python errors):

1. [error description here]

2. [error description here]

3. [error description here]

Logic problems (Python does not do what is required):

1. [logic problem description here]

2. [logic problem description here]

3. [logic problem description here]