An Introduction to Control Flow

This session is the second in a series of programming fundamentals. We recognise that this content might be a bit more dry and abstract, but it is important background to know when you start to actually use Python in your day to day work.

Much as the flow of a stream describes how it goes from its source to its mouth, control flow describes the logical path a program is expected to take when you run it. Just as you can divert the flow of a stream with structures like dams and bridges, you can change the direction a program flows by the use of control and repetition structures. The below slides aim to provide an introduction to these concepts and the way we can use them.

Slides

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What is Control Flow?

Control flow is the way your code will flow in runtime. In general this will follow a sequential order completing each statement from top to bottom, however, there are ways to control the flow of your code, even repeating through sections.

Control (or Decision) Structures

Like a case statement in SQL, control structures can be used to select different options and actions based on the input variable. These follow the structure:

if <this boolean condition is true>:
  <do this>
elif <this boolean condition is true>:
  <do that>
else:
  <do something else>

In its most basic form, only an if clause is required. The else clause allows the bucketing of all circumstances not handled previously so that code can be applied in any given circumstance.

Repetition Structures (or Loops)

Repetition structures allow for sections of code to be repeated until a condition is met. for loops repeat code over a set number of iterations based on an iterable condition. while loops repeat code until a predetermined condition is met.

for Loops

Below are two examples of for code loops. The first loops through a list called ‘providers’ and prints each item. The second loops through a range of numbers and prints each.

Logical Structure

for <i> in <iterable>:
    <code_to_iterate>

for <i> in range(<a> - <b>):
  print(<i>)

Python

for provider in providers:
  print(provider)

for num in range(0-6):
  print(num)

with Loops

while loops check the state of a boolean condition. In this case the loop runs until a declared variable is over 5 printing each incremental value.

Logical Structure

while <boolean is true>:
    <code_to_iterate>

Python

var = 0
while var <= 5:
  print(var)

Exercises

1. Write a function that prints whether a number is negative, zero, or positive.

Solution

def classify(x):
    if x < 0:
        print("Negative")
    elif x == 0:
        print("Zero")
    else:
        print("Positive")

2. Loop through a list of ages and print if each person is a Child (<13), Teenager (13–17), Adult (18–64), or Senior (65+).

Solution

ages = [10, 15, 30, 70]
for age in ages:
    if age < 13:
        print("Child")
    elif age < 18:
        print("Teenager")
    elif age < 65:
        print("Adult")
    else:
        print("Senior")

Child
Teenager
Adult
Senior

3. Use a while loop to count down from 10 to 0.

Solution

x = 10
while x >= 0:
    print(x)
    x -= 1

10
9
8
7
6
5
4
3
2
1
0

4. Loop from 1 to 20 and print Fizz for multiples of 3, Buzz for 5, FizzBuzz for both.

Solution

for i in range(1, 21):
    if i % 3 == 0 and i % 5 == 0:
        print("FizzBuzz")
    elif i % 3 == 0:
        print("Fizz")
    elif i % 5 == 0:
        print("Buzz")
    else:
        print(i)

1
2
Fizz
4
Buzz
Fizz
7
8
Fizz
Buzz
11
Fizz
13
14
FizzBuzz
16
17
Fizz
19
Buzz

5. Use random.randint to simulate rolling a die until you get a 6.

Solution

import random
rolls = 0
while True:
    rolls += 1
    if random.randint(1, 6) == 6:
        break
print("Rolled a 6 in", rolls, "tries")

Rolled a 6 in 13 tries

6. Loop through job titles and print if they contain “analyst” or “manager.”

Solution

titles = ["Data Analyst", "HR Manager", "Intern"]
for title in titles:
    t = title.lower()
    if "analyst" in t:
        print("Analyst role")
    elif "manager" in t:
        print("Manager role")
    else:
        print("Other")

Analyst role
Manager role
Other