Conditional Statements

Conditional Statements#

Note

This page has content from the Conditional_Statements notebook of an older version of the UC Berkeley data science course. See the Berkeley course section of the license file.

In many situations, actions and results depends on a specific set of conditions being satisfied. For example, individuals in randomized controlled trials receive the treatment if they have been assigned to the treatment group. A gambler makes money if she wins her bet.

In this section we will learn how to describe such situations using code. A conditional statement is a multi-line statement that allows Python to choose among different alternatives based on the truth value of an expression. While conditional statements can appear anywhere, they appear most often within the body of a function in order to express alternative behavior depending on argument values.

A conditional statement always begins with an if header, which is a single line followed by an indented body. The body is only executed if the expression directly following if (called the if expression) evaluates to a true value. If the if expression evaluates to a false value, then the body of the if is skipped.

Let us start by a conditional that sets the variable result to the string 'Positive' for a positive number:

x = 3
result = 'Not sure yet'
if x > 0:
    result = 'Positive'
result

Let us work through what Python will do when we evaluate the code above (repeated in the cell below):

x gets the value CR of int 3.
Python evaluates the assignment statement result = 'Not sure yet'. The result variable has the value: CR of “Not sure yet”.
Python evaluates the if expression x > 0. The result is the CR of True. This is a true value, so Python will execute the body of the if statement.
The body of the if statement is the single line result = 'Positive'. Python reassigns the result variable to have the value: CR of “Positive”.
result first evaluates result to get the CR of “Positive”, and, as this is an expression, Jupyter shows us the result.

Here’s the code as a code cell:

x = 3
result = 'Not sure yet'
if x > 0:
    result = 'Positive'
result

'Positive'

The cell shows 'Positive' if x is a positive number, like 3. But if x is not a positive number, such as -3, then this happens:

x has the value CR of int -3.
result gets the value: CR of “Not sure yet”. So far this is the same as for the case of x equal to 3 above.
x > 0 evaluates to False, so Python skips the body of the if statement.
result evaluates result to get the CR of “Not sure yet” (because this hasn’t changed), and Jupyter shows us the result.

x = -3
result = 'Not sure yet'
if x > 0:
    result = 'Positive'
result

'Not sure yet'

Let us refine our cell to give Not positive if x is a negative number. We can do this by adding an else clause. If the if expression evaluates to a true value, then Python executes the first clause, after the if expression, as before. If it evaluates to a false value, Python executes the clause following the else, instead.

x = -3
result = 'Not sure yet'
if x > 0:
    result = 'Positive'
else:
    result = 'Not positive'
result

'Not positive'

Python sets x to have the value CR of int -3. It checks x > 0 and finds a false value, so it skips the first clause, and executes the clause following else:. This show 'Not positive'. If you set x above to 3, you get Positive, because Python will evaluate the result = 'Positive' body, and not the result = 'Not positive body.

Notice that the if statement now deals with all possible values of x. Whatever the value of x we with either execute result = 'Positive' or we will execute result = 'Not positive'. We can now get rid of the first line in the function result = 'Not sure yet' because we will always change result from that value, in the if statement:

# Classify without the unnecessary first assignment.
x = -3
if x > 0:
    result = 'Positive'
else:
    result = 'Not positive'
result

'Not positive'

Now imagine we prefer our cell to show 'Negative for negative values and Zero if the input value is 0. It seems like we need three clauses, one each for positive, negative and 0 values.

We can do this by adding an elif clause, where elif is Python’s shorthand for the phrase “else if”.

x = -3
if x > 0:
    result = 'Positive'
elif x == 0:  # elif short for "else if"
    result = 'Zero'
else:
    result = 'Negative'
result

'Negative'

Now the cell returns the correct answer when the input is -3, 0, or 3:

x = 3
if x > 0:
    result = 'Positive'
elif x == 0:  # elif short for "else if"
    result = 'Zero'
else:
    result = 'Negative'
result

'Positive'

x = 0
if x > 0:
    result = 'Positive'
elif x == 0:  # elif short for "else if"
    result = 'Zero'
else:
    result = 'Negative'
result

'Zero'

If x is equal to 3, Python evaluates the if expression x < 0. The expression gives a True value, so Python executes the first clause, sets result to equal “Positive” and skips to the end of the if statement. Finally it evaluates result to show “Positive”.

If x equals 0, evaluates the if expression x < 0. The expression is a False value, so Python moves on to the next clause, which is elif x == 0:. This has another if expression x == 0. It is a True value, so Python executes this clause, and sets result equal to “Zero”. Then it skips to the end of the if statement, and shows result.

If x equals -3, evaluates the if expression x < 0. The expression is a false value, so Python moves on to the next clause, which is elif x == 0. This expression is also a False value so Python moves to the next clause, which is the else: clause, and executes that, setting result to be “Negative”. Now we are at the end of the if statement, and we evaluate and show result.

We can have as many elif clauses as we want. For example, imagine we want to classify the number into one of the following categories:

above 10 ('Large positive')
from (not including) 0 through 10 ('Small positive')
exactly 0 ('Zero')
from (not including) 0 through -10 ('Small negative')
below -10 ('Large negative')

x = -100
if x > 10:
    result = 'Large positive'
elif x > 0:
    result = 'Small positive'
elif x == 0:
    result = 'Zero'
elif x >= -10:  # Greater than or equal to
    result = 'Small negative'
else:
    result = 'Large negative'
result

'Large negative'

x = 0
if x > 10:
    result = 'Large positive'
elif x > 0:
    result = 'Small positive'
elif x == 0:
    result = 'Zero'
elif x >= -10:  # Greater than or equal to
    result = 'Small negative'
else:
    result = 'Large negative'
result

'Zero'

The General Form#

A conditional statement can also have multiple clauses with multiple bodies, and only one of those bodies can ever be executed. The general format of a multi-clause conditional statement appears below:

if <if expression>:
    <if body>
elif <elif expression 0>:
    <elif body 0>
elif <elif expression 1>:
    <elif body 1>
...
else:
    <else body>

There is always exactly one if clause, but there can be any number of elif clauses. Python will evaluate the if and elif expressions in the headers in order until one is found that is a true value, then execute the corresponding body. The else clause is optional. When an else header is provided, its else body is executed only if none of the header expressions of the previous clauses are true. The else clause must always come at the end (or not at all).