Loops and Conditionals

Estimated time: 25 minutes

Overview

Questions

• How can I do the same thing for many values?
• How can my program make decisions?

Objectives

• Use a for loop to repeat an operation for each element of a list.
• Use if, elif and else to run code only under certain conditions.

For loops

---

A for loop repeats an operation once for each element in a collection. The general structure of a loop is:

PYTHON

for variable in collection:
    # do things using variable, such as print

• We can name the loop variable anything we like.
• There must be a colon at the end of the line that starts the loop.
• We must indent anything that we want to run inside the loop. Unlike in many other programming languages, there is no command to signify the end of the loop body. Anything indented after the for statement belongs to the loop.

PYTHON

names = ["Lovelace", "Hopper", "Noether", "Turing"]

for person in names:
    print(person + " did awesome stuff.")

OUTPUT

Lovelace did awesome stuff.
Hopper did awesome stuff.
Noether did awesome stuff.
Turing did awesome stuff.

Loops are a robust way of dealing with collections such as lists. Even if the values in the list change, the loop will still work.

As with all variable names, it is helpful to give loop variables meaningful names that you will understand as your code grows. For example, for person in names is easier to understand than for kitten in names.

Challenge

Loop over a dictionary

Looping over a dictionary gives you its keys. Use this to print one line per person in authors_and_birth_years, e.g. Lovelace was born in 1815.

PYTHON

authors_and_birth_years = {"Lovelace": 1815,
                           "Hopper": 1906,
                           "Noether": 1882,
                           "Turing": 1912}

Show me the solution

PYTHON

for person in authors_and_birth_years:
    print(person + " was born in " + str(authors_and_birth_years[person]))

OUTPUT

Lovelace was born in 1815
Hopper was born in 1906
Noether was born in 1882
Turing was born in 1912

Conditions and if statements

---

An if statement is a conditional statement that determines whether a block of code is executed. The syntax of an if statement is similar to that of a for statement:

• The first line begins with if and ends with a colon.
• The body is indented (usually by four spaces).

PYTHON

temp = 20
if temp > 15:
    print("It's warm")

OUTPUT

It's warm

The condition itself is just a value of type bool:

PYTHON

20 > 15

OUTPUT

True

We can use an else statement following an if statement to specify an alternative code block to execute when the if branch is not taken.

PYTHON

temp = 10
if temp > 15:
    print("It's warm")
else:
    print("It's cold")

OUTPUT

It's cold

You can use elif (short for “else if”) to provide several alternative choices, each with its own test. An elif statement must always follow an if statement and come before the else statement (the catch-all).

PYTHON

temp = 15
if temp >= 25:
    print("It's hot")
elif temp >= 15:
    print("It's warm")
else:
    print("It's cold")

OUTPUT

It's warm

Conditions are tested in order and only once. Python steps through the branches of the conditional statement in order, testing each one in turn, so the order of your statements matters.

Callout

Comparison operators

• == equal to (note the two equals signs!)
• != not equal to
• > and < greater than, less than
• >= and <= greater than or equal to, less than or equal to

Challenge

Combine loops and conditionals

Print only the names of the people in authors_and_birth_years who were born before 1900.

Show me the solution

PYTHON

for person in authors_and_birth_years:
    if authors_and_birth_years[person] < 1900:
        print(person)

OUTPUT

Lovelace
Noether

Key Points

• A for loop runs its indented body once for each element of a collection.
• Use if, elif and else to choose which code to run.
• Conditions are tested in order; the first one that is True wins.
• Indentation defines which lines belong to a loop or an if statement.