015 Control in Python: `if`

Introduction

Purpose

In this section we will learn how to add conditional control to our codes. We will cover the conditional statement: if.

Prerequisites

You will need some understanding of the following:

Comparison Operators and `if`

Comparison Operators

A comparison operator 'compares' two terms (e.g. the contents of variables) and returns a boolean data type (True or False).

For example, to see if the value of some variable a has 'the same value as' ('equivalent to') the value of some variable b, we use the equivalence operator (==). To test for non equivalence, we use the not equivalent operator != (read the ! as 'not'):

a = 100
b = 10
#
# These are *not* the same, so we expect 
# a == b : False

# Note the use of \n and \t in here
# from 010 for formatting
print (f'a is {a} and b is {b}')
print (f'a is equivalent to b? {a == b}')

a is 100 and b is 10
a is equivalent to b? False

Exercise 1

insert a new cell below here. Use f-strings in forming strings.
copy the code above
add a print statement to your code that tests for non equivalence of a and b
repeat this in a new cell, but now change the values (or type) of the variables a and b to float or bool

A fuller set of comparison operators allows greater or less than tests:

symbol	meaning
==	is equivalent to
!=	is not equivalent to
>	greater than
>=	greater than or equal to
<	less than
<=	less than or equal to

so that, for example:

# Comparison examples

# is one plus one list equal to two list?
print (f'1 + 1 == 2    : {1 + 1 == 2}')

# is one less than or equal to 0.999?
print (f'1 <= 0.999    : {1 <= 0.999}')

# is one plus one not equal to two?
print (f'1 + 1 != 2    : {1 + 1 != 2}')

# "is 100 less than 2?"
print (f'100 < 2       : {100 < 2}')

1 + 1 == 2    : True
1 <= 0.999    : False
1 + 1 != 2    : False
100 < 2       : False

Exercise 2

insert a new cell below here
create variables a and b and set them to types and values of your choice
create a variable called gt_test and set it to the result of a > b
print a statement of what you have used, and the value of gt_test
explain why you get the result you do

Conditional test: `if ... elif ... else ...`

A common use of comparisons is for program control, using an if statement of the form:

if condition1:
    # do this 1
    doit1()
    ...
elif condition2:
    # do this 2
    doit2()
    ...
else:
    # do this 3
    doit3()
    ...

Implicit in these statements is that the conditions return True to pass the tests, i.e. we could more fully write:

if condition1 == True:
    # do this 1
    ...

This form of conditional statement allows us to run blocks of code only under a particular condition (or set of conditions).

In Python, the statement(s) we run on condition are indented.

The indent can be one or more spaces or a <tab> character, the choice is up to the programmer. However, it must be consistent. It is generally best to use spaces rather than tab characters, it is all too easy to mistake one for the other.

Pay attention to indentation in conditional statements. Getting it wrong is one of the more common errors new Python coders make.

test = 3
print('test result is',test)

# initialise retval
retval = None

# conduct some tests, and set the 
# variable retval to True if we pass
# any test

if test >= 1:
    retval = True
    print('passed test 1: "if test >= 1"')
elif test == 0:
    retval = True
    print('passed test 2: "if test == 0"')
else:
    retval = False
    print('failed both tests')

print('retval is',retval)

test result is 3
passed test 1: "if test >= 1"
retval is True

Exercise 3

insert a new cell below here
set a variable doy to represent the day of year and initialise it to some integer between 1 and 365 inclusive
set a variable month to be 'January'
set a variable year to be '2020'
Write a series of conditional statements that set the variable month to the correct month for the value of doy
Print the month for the given doy
Test that you get the right result for several doy values

You should assume that doy value 1 represents January 1st.

You might find a DOY calendar helpful here.

DOY calendar

We will see later that this is not the best way to do calculations of this sort. First, it is all too easy to make mistakes in typing in both the doy boundaries and the month names. Second, it is not very flexible: for instance, consider how would you need to change it for leap or non-leap years. Third, it is not at all pythonic, i.e. doesn't make use of the features of Python that could make it clear and concise.

That said, it is an easily-understandable exercise to try out using conditional statements.

Summary

We should know know how to use if statements in Python to control program flow. We can make choices as to what happens in the code, depending on whether or not one or more tests are passed. This is a common feature of all coding languages, but it is important here that you get used to doing this in Python.

We know that conditions inside if statements use indentation in Python, and we know to be careful in our use of this.

There are additional notes in docs.python.org you can follow up to deepen your understanding of these topics.

Summary of material in this notebook:

Comparison operators:

symbol	meaning
==	is equivalent to
!=	is not equivalent to
>	greater than
>=	greater than or equal to
<	less than
<=	less than or equal to

If ... elif ... else:

    if condition1:
        # do this 1
        doit1()
        ...
    elif condition2:
        # do this 2
        doit2()
        ...
    else:
        # do this 3
        doit3()
        ...

Last update: October 8, 2020

015 Control in Python: if