Conditional statements are, as the introduction to this chapter said, statements consisting of a test and one or more actions, whereby the actions only get executed if the test evaluates to True. Conditional statements are also called “if-statements,” as they are written using the special keyword if.

Here is an example:

    x = 5
    if x == 5:
        print( "x equals 5" )

The syntax of the if statement is as follows:

if <boolean expression>:
    <statements>

Note the colon (:) after the boolean expression, and the fact that <statements> is indented.

Code blocks

In the syntactic description of the if statement above, you see that the <statements> are “indented,” i.e., they are placed one tabulation to the right. This is intentional and necessary. Python considers statements that are following each other and that are at the same level of indentation part of a code block. The code block underneath the first line of the if statement is considered to be the list of actions that are executed when the boolean expression evaluates to True. For example:

x = 7
if x < 10:
    print( "This line is only executed if x < 10." )
    print( "And the same holds for this line." )
print( "This line, however, is always executed." )

Change the value of x to see how it affects the outcome of the code.

Thus, all the statements under the if that are indented, belong to the code block that is executed when the boolean expression of the if statement evaluates to True. This code block is skipped if the boolean expression evaluates to False. Statements which follow the if construction which are not indented (as deep as the code block under the if), are executed, regardless of whether the boolean expression evaluates to True or False.

Naturally, you are not restricted to having just a single if statement in your code. You can have as many as you like.

x = 5
if x == 5: 
    print( "x equals 5" )
if x > 4: 
    print( "x is greater than 4" )
if  x >= 5:
    print( "x is greater than or equal to 5" )
if x < 6: 
    print( "x is less than 6" ) 
if x <= 5:
    print( "x is less than or equal to 5" )
if x != 6 :
    print( "x does not equal 6" )

Again, try changing the value of x and see how it affects the outcome.

Indentation

In Python, correct indenting is of the utmost importance! Without correct indentation, Python will not be able to recognize which statements belong together as one code block, and therefore cannot execute your code correctly.¹¹

Note that you can indent using the Tab key, or indent using spaces. Most editors will auto-indent for you, i.e., if, for instance, you write the first line of an if statement, once you press Enter to go to the next line, it will automatically “jump in” one level of indentation (if it does not, it is very likely that you forgot the colon at the end of the conditional expression). Also, when you have indented one line to a certain level of indentation, the next line will use the same level. You can get rid of indentations using the Backspace key.

For Python programs, a normal level of indentation is four spaces, i.e., one press of the Tab key should “jump in” four spaces. As long as you are in one editor, you can in such a case either use the Tab key, or press the spacebar four times, to go up one indentation level. So far so good. You may get into problems, however, if you port your code to another editor, which might have a different setting for the Tab key. If you edit your code in a such a different editor, even though it might look okay, Python may see that there are indentation conflicts (a mix of tabulations and space-indentations) and may report a syntax error when you try to run your code. Most editors therefore offer the option to automatically replace tabulations with spaces, so that such problems do not arise. If you use a text editor to write Python code, check if it contains such an option, and if so, ensure that tabulations are set to 4 and are automatically replaced by spaces.

The following code contains multiple indentation errors. Fix them all.

# This code contains indentation errors!
x = 3
y = 4
if x == 3 and y == 4:
    print( "x is 3" )
   print( "y is 4" )
if x > 2 and y < 5:
print( "x > 2" )
print( "y < 5" )
if x < 4 and y > 3:
    print( "x < 4" )
        print( "y > 3" )

Two-way decisions

Often a decision branches, e.g., if a certain condition arises, you want to take a particular action, but if it does not arise, you want to take another action. This is supported by Python in the form of an expansion to the if statement that adds an else branch.

x = 4
if x > 2:
    print( "x is bigger than 2" )
else:
    print( "x is smaller than or equal to 2" ) 

The syntax is as follows:

if <boolean expression>:
    <statements>
else:
    <statements>

Note the colon (:) after both the boolean expression and the else.

It is important that the word else is aligned with the word if that it belongs to. If you do not align them correctly, this results in an indentation error.

A consequence of adding an else branch to an if statement is that always exactly one of the two code blocks will be executed. If the boolean expression of the if statement evaluates to True, the code block directly under the if will be executed, and the code block directly under the else will be skipped. If it evaluates to False, the code block directly under the if will be skipped, while the code block directly under the else will be executed.

You can test whether an integer is odd or even using the modulo operator. Specifically, when x % 2 equals zero, then x is even, else it is odd. Write some code that asks for an integer and then reports whether it is even or odd (you can use the getInteger() function from pcinput to ask for an integer).

Note: As far as indentation is concerned, it is not absolutely necessary to have the code block under the else branch use the same number of spaces in indentation as the code block under the if branch, as long as the indentation is consistent within the code block. However, accomplished programmers use consistent indentation throughout their programs, which makes it easier to see what the whole if-else statement encompasses. For example, in the code below the indentation in the else branch uses less spaces than the indentation in the if branch. While syntactically correct, this makes the code less readable.

# Example of syntactically correct but ugly indenting.
x = 1
if x > 2:
    print( "x is bigger than 2" )
else:
  print( "x is smaller than or equal to 2" )

Flow charts

In the early days of programming, programmers often expressed the algorithms they wanted to implement by means of so-called “flow charts.” Nowadays, flow charts are seldom used anymore. However, I found that it often helps students understanding how exactly conditions (and, for the next chapter, iterations) work if they are shown a flow chart of the process.

A flow chart is a diagram that shows different kinds of blocks with arrows in between. The blocks are of three kinds (at least, that is all I need for this book). Rectangular blocks contain statements that are executed. Diamond-shaped blocks contain a condition that evaluates to either True or False. Rectangular blocks with rounded corners indicate either the start (using the text “Start”) or end (using the text “Stop”) of the algorithm.

To interpret a flow chart, you begin at the “Start” block, and follow the arrows, executing all statements that you encounter. When you get to a diamond-shaped block, you evaluate the condition in it, and then either follow the arrow marked True in case it evaluates to True, or the arrow marked False when it evaluates to False. When you encounter the “Stop” block, you are finished.

For example, the code shown above, that checks if a number is greater than 2 or smaller than or equal to 2, and prints some text that states as much, is equivalent to the flow chart shown in previous figure.

Multi-branch decisions

Occasionally, you encounter multi-branch decisions, where one of multiple blocks of commands has to be executed, but never more than one block. Such multi-branch decisions can be implemented using a further expansion of the if statement, namely in the form of one or more elif statements (elif stands for “else if”).

age = 21
if age < 12:
    print( "You're still a child!" )
elif age < 18:
    print( "You are a teenager!" )
elif age < 30:
    print( "You're pretty young!" )
elif age < 50:
    print( "Wisening up, are we?" )
else:
    print( "Aren't the years weighing heavy?" )

This code is equivalent to the algorithm expressed by the flow chart in the following figure.

In the code block above, change age to different values and observe the results.

The syntax is as follows:

if <boolean expression>:
    <statements>
elif <boolean expression>:
    <statements>
else:
    <statements>

The syntax above shows only one elif, but you can have multiple. The different tests in an if-elif-else construct are executed in order. The first boolean expression that evaluates to True will cause the code block that belongs to that expression to be executed. None of the other code blocks of the construct will be executed.

So, first the boolean expression next to the if will be evaluated. If it evaluates to True, the code block underneath the if will be executed. Otherwise, the boolean expression for the first elif will be evaluated. If that turns out to be True, the code block underneath it will be executed. Otherwise, the boolean expression for the next elif will be evaluated. Etcetera. Only when the boolean expressions for the if and all of the elifs evaluate to False, the code block underneath the else will be executed.

Therefore, in the code above, you do not need to test age >= 12 and age < 18 for the first elif. Just testing age < 18 suffices, because if age was smaller than \(12\), already the boolean expression for the if would have evaluated to True, and the boolean expression for the first elif would not even have been encountered by Python.

Note that the inclusion of the else branch is always optional. However, in most cases where I need elifs I include it anyway, if only for error checking.

Write a program that defines a variable weight. If weight is greater than \(20\) (kilo’s), print: “There is a $$25 surcharge for luggage that is too heavy.” If weight is smaller than \(20\), print: “Have a safe flight!” If weight is exactly \(20\), print: “Pfew! The weight is just right!” Make sure that you change the value of weight a couple of times to check whether your code works.

Nested conditions

Given the rules of the if-elif-else statements and identation, it is perfectly possible to use an if statement within another if statement. This second if statement is only executed if the condition for the first if statement evaluates to True, as it belongs to the code block of the first if statement. This is called “nesting.”

x = 41
if x%7 == 0:
    # --- Here starts a nested block of code ---
    if x%11 == 0:
        print( x, "is dividable by both 7 and 11." )
    else:
        print( x, "is dividable by 7, but not by 11." )
    # --- Here ends the nested block of code ---
elif x%11 == 0:
    print( x, "is dividable by 11, but not by 7." )
else:
    print( x, "is dividable by neither 7 nor 11." )

This code is equivalent to the algorithm expressed in the following figure.

Change the value of x and observe the results.

Note that the example above is just to illustrate nesting; you probably already know different ways of getting the same results that are a bit more readable. In particular, nesting of if statements can often be avoided by judicious use of elifs. To give an example, here is a “nested” example of the age code given in the beginning of this paragraph:

age = 21
if age < 12:
    print( "You're still a child!" )
else:
    if age < 18:
        print( "You are a teenager!" )
    else:
        if age < 30:
            print( "You're pretty young!" )
        else:
            if age < 50:
                print( "Wisening up, are we?" )
            else:
                print( "Aren't the years weighing heavy?" )

I assume you agree that the version with the elifs looks better.