Program Flow Control

Booleans

The bool data type is used to represent boolean values (i.e. true/false values). They are useful when the program execution needs to vary based on a certain condition e.g., print 'old' if age is greater than 100.

The bool data type can hold only two values True and False (case sensitive).

>>> type(True)
<class 'bool'>

>>> type(False)
<class 'bool'>

>>> type('True')
<class 'str'>

>>> type(true)
...
NameError: name 'true' is not defined

You can use comparison operators to create expressions that evaluate to a boolean result.

Operator	Meaning
`==`	Equal to (different from the assignment operator `=`)
`!=`	Not equal to
`<`	Less than
`>`	Greater than
`<=`	Less than or equal to
`>=`	Greater than or equal to

📦 Here are some examples

print('hello' == 'Hello') # False
print('dog' != 'cat') # True
print(True == True) # True
print(True != False) # True
print(42 == 42.0) # True (they are different types of numbers, but of same value)
print(42 == '42') # False
print(42 == int('42')) # True
print(20 > 10) # True

In contrast, boolean operators (and, or and not) work only on boolean values. Here is how they work.

Expression	Result
`True` `and` `True`	`True`
`True` `and` `False`	`False`
`False` `and` `True`	`False`
`False` `and` `False`	`False`

`True` `or` `True`	`True`
`True` `or` `False`	`True`
`False` `or` `True`	`True`
`False` `or` `False`	`False`

`not` `True`	`False`
`not` `False`	`True`

You can mix both types of operators. Some examples below:

result = (2 > 5) or (10 < 20) # True
result = (2 == 2) and (3 != 3) # False

`if` Statements

Python uses the if statement to indicate that some code should only be executed if a certain condition is true.

Format:

if condition :
    statements_to_execute_if_true

📦 The code below has two if statements; one evaluates to true while the other doesn't.

price = 55
print(price)
if price > 50:
    print('Expensive')

weight = 45
print(weight)
if weight > 100:
    print('Heavy')
    
print('Done')

Output ⤵️ [Click here to visualize execution]

50
Expensive
45
Done

Note how the code to execute if the condition is true is indented (usually by 4 spaces). Python uses indentation to indicate code blocks (aka clauses) i.e., a sequence of statements that belong together. In the example below, lines 2-4 are in the same block because they are all indented by one level; if the condition is true, all three are executed; if the condition is false, all three are omitted. Line 5 goes back to the previous indentation level, indicating the end of the code block.

if name == 'Blue':          # line 1
    print("It's a color")   # line 2
    print("It's a feeling") # line 3
    print("It's a word")    # line 4
print('Done')               # line 5

Exercise : Even Numbers

Write a program to read an integer from the keyboard and print 'Even' only if it is an even number. Follow the example below:

Enter an integer:
4
Even number
Done

Enter an integer:
7
Done

if statements can be nested, using deeper indentation.

age = 13
gender = 'F'
if (age > 12) and (age < 20):          
    print("Teenager")  # indented one level - new code block starts here 
    if gender == 'F':
        print('Female')# indented two levels - this block is nested within the first block
        print('Girl')  
    if gender == 'M':
        print('Male') # another block nested within the first block
        print('Boy')
    print('Gender code is ' + gender) 
print('Age is ' + str(age))

⤵️ [visualize]

Teenager
Female
Girl
Gender code is F
Age is 13

If a situation has only two mutually exclusive possibilities e.g. the value of gender can only be M or F, we can use the else statement to deal with both conditions together.

📦 In the example below, the entire else block will be skipped if the if condition is true.

if gender == 'F':
    print('Female')
else:
    print('Not Female')

If a situation has more than two mutually exclusive possibilities, we can bring in elif (an abbreviation of else if) blocks too.

📦 The example below shows how to use an if-elif-else construct to control the flow of the execution.

if gender == 'F':
    print('Female')
elif gender == 'M':
    print('Male')
elif gender == 'O':
    print('Other')
else:
    print('Unrecognized value')

Note that in an if-elif-else construct no more than one block (the first one whose condition is true) will be executed.

Exercise : Grade Analyzer

Write a program to read project score and exam score from the keyboard and print a bar chart of the scores and a grade. Follow the example below:

Enter project score:
30

Enter exam score:
40

Total: 70

Project : ======
Exam    : ========
Total   : ==============
Grade   : A

Note how the colons are aligned.
Note the extra blank lines in the expected output.
The bar chart has one = for each 5 marks 24 marks should be shown as ====.
Assume all inputs are positive integers.

Grading rules:

A: at least 25 marks for each component and at least 60 in total
B: at least one component has 25 or more marks and at least 50 in total
C: at least 40 in total
D: otherwise

Exercise : Grade Analyzer (Extended)

Extend the code you wrote for the exercise Grade Analyzer with the following additional feature.

If the grade is determined to be D ask the user if the grade should be withheld. If the user agrees, show the grade as NA instead of D.

Enter project score
10

Enter exam score
20

Total: 30

Grade is low. Withhold? y/n?
y
 
Project : ==
Exam    : ====
Total   : ======
Grade   : NA

`while` Statements

Python uses the while statement to repeat a code block until a certain condition is true. Such an execution path is also known as a loop and each execution of the code block is called an iteration.

Format:

while condition :
    statements_to_execute_if_condition_is_true

📦 the code below prints 'Hello' 3 times (i.e., the loop is executed for 3 iterations), followed by 'Done'.

counter = 0
while counter < 3:
    print('Hello')
    counter = counter + 1
print('Done')

→

Hello
Hello
Hello
Done

[Visualize execution]

❗️ Infinite Loops: Sometimes programming mistakes can result in infinite loops i.e., loops that never terminate. In the example below, the condition counter < 3 always evaluates to True (because the statement to increment counter has been left out by mistake)

counter = 0
while counter < 3:
    print('Hello')

💡 When using IDLE, if a bug in your code caused it to go into an infinite loop, you can use Ctrl + C to force it to stop executing.

Exercise : Vending Machine - Add Loop

The code below simulates a vending machine that asks for a coin and gives out the product and the balance.

price = 100
coin_value = 0

print('Price:', price)

coin_value = int(input('Enter a coin:'))
print('You have entered:', coin_value)

print('Here is the product')
print('Your balance:', coin_value - price)

💡 Note how the above code uses input function with a string argument e.g., input('Enter a coin:') to show the string to the user and read the input from the user at the same time.

`name = input('Input a name:')`	→	`Input a name: John`
`print('Input a name:') name = input()`	→	`Input a name: John`

💡 Also note how the above code uses print() with two parameters, one an integer and one a string. The print() method can take multiple parameters and the parameters can be of different types. Note that the output will have a space between each parameter.

print('ABC', 3, 4.50)
print('ABC' + str(3) + str(4.50))
print('ABC' + ' ' + str(3) + ' ' + str(4.50))

→


ABC 3 4.50
ABC34.50
ABC 3 4.50

Example output from the code:

Price: 100
Enter a coin: 200
You have entered: 200
Here is the product
Your balance: 100

As you can see, this code does not consider the case where the inserted coin is less than the price of the product, resulting in the following incorrect behavior.

Price: 100
Enter a coin: 50
You have entered: 50
Here is the product
Your balance: -50

Modify the code to repeat these two lines until the coin entered is enough to pay for the product. Note that you don't need to add up all the coins entered. Instead, assume the vending machine can accept only one coin for a purchase and keeps rejecting coins until a big-enough coin is entered.

coin_value = int(input('Enter a coin:'))
print('You have entered:', coin_value)

The output should be something like this:

Price: 100
Enter a coin: 50
You have entered: 50
Enter a coin: 100
You have entered: 100
Here is the product
Your balance: 0

Exercise : Vending Machine - Accept Multiple Coins

The vending machine you wrote in the previous exercise (an example solution given below) can accept only one coin as the payment. If the coin is not enough, it rejects the coin and request the user to insert a new coin.

price = 100
coin_value = 0

print('Price:', price)

while price > coin_value:
  coin_value = int(input('Enter a coin:'))
  print('You have entered:', coin_value)

print('Here is the product')
print('Your balance:', coin_value - price)

Modify the code so that it can accumulate coins until there is enough to pay for the product. An example output is given below:

Price: 100
Enter a coin: 20
You have entered: 20
Enter a coin: 50
You have entered: 70
Enter a coin: 50
You have entered: 120
Here is the product
Your balance: 20

...
coin_sum = 0
...
  coin_sum = coin_sum + coin_value
...

You can use a break statement to break out of a loop.

📦 The code below uses the break statement to break out of the loop when the password given is abcd. Without the break statement, the loop will repeat forever because the condition in while True: is always True.

while True:
  password = input('What is the password?')
  if password == 'abcd':
    break # exit the loop
  else:
    print('Password incorrect. Try again.')
print('Password correct. You may proceed.')

Exercise : Vending Machine - Allow Abort

The vending machine you wrote in the previous exercise (an example solution given below) does not allow the user to abort the purchase and get the money back.

price = 100
coin_sum = 0

print('Price:', price)

while price > coin_sum:
  coin_value = int(input('Enter a coin:'))
  coin_sum = coin_sum + coin_value
  print('You have entered:', coin_sum)

print('Here is the product')
print('Your balance:', coin_sum - price)

Modify the code so that the user can abort the purchase by entering 0 as the coin value:

Price: 100
Enter a coin: 20
You have entered: 20
Enter a coin: 0
Your balance: 20

...
is_aborting = False
...
  if coin_value == 0:
    is_aborting = True
    break
... 
if is_aborting:
  print('Your balance:', coin_sum)

You can use a continue statement to skip the remainder of the current iteration and go back to the while condition.

📦 The code below is for reading three words from the user and printing all three at the end. It uses the continue statement to skip the remainder of the iteration if the word entered is too short (i.e., shorter than 4 letters).

accepted_words = ''
count = 0
while count < 3:
  word = input('Enter a word (with 4 letters or more):')
  if len(word) < 4:
    print('Too short. Ignored.')
    continue # skip the remainder of the iteration
  accepted_words = accepted_words + ' ' + word
  count = count + 1
print('Accepted words: ' + accepted_words)

Exercise : Vending Machine - Legit Coins

The vending machine you wrote in the previous exercise (an example solution given below) accepts any value as the coin value. However, coins come in only certain values only.

price = 100
coin_sum = 0
is_aborting = False

print('Price:', price)

while price > coin_sum:
  coin_value = int(input('Enter a coin:'))
  if coin_value == 0:
    is_aborting = True
    break
  coin_sum = coin_sum + coin_value
  print('You have entered:', coin_sum)

if is_aborting:
  print('Your balance:', coin_sum)
else:
  print('Here is the product')
  print('Your balance:', coin_sum - price)

Modify the code so that it accepts coins of value 10, 20, 50, 100 only:

Price: 100
Enter a coin: 30
Coin not recognized
Enter a coin: 100
You have entered: 100
Here is the product
Your balance: 0

...
if coin_value != 10 and ... :
    ...
    continue
...

`for` Statements

You can use a for statement, together with the range() function, to repeat a code block a pre-determined number of times.

Format:

for variable_used_as_index in range(number_of_times_to_repeat) :
    statements_to_repeat

📦 the code below use a for loop to iterate three times. Note how the variable i is used as an indexing variable and how i in range(3) causes i to take values 0, 1, 2 over the three iterations.

for i in range(3):
  print(i, 'Knock knock, Penny!')

→

0 Knock knock, Penny!
1 Knock knock, Penny!
2 Knock knock, Penny!

💡 Note how the above for loop is equivalent to the following while loop but more concise.

i = 0
while i < 3:
  print(i, 'Knock knock, Penny!')
  i = i + 1

Exercise : Multiplication Table

Write a program to read in an integer and generate the multiplication table (for 1 to 10) for that number. An example is given below:

Enter a number: 7
7 x 1 = 7
7 x 2 = 14
7 x 3 = 21
7 x 4 = 28
7 x 5 = 35
7 x 6 = 42
7 x 7 = 49
7 x 8 = 56
7 x 9 = 63
7 x 10 = 70

for i in range(10) iterates from 0 to 9. You can use range(1, 11) to iterate from 1 to 10.

You can use break and continue in for loops as well, with similar effects as in while loops.

📦 The code below totals the numbers entered by the user. It uses range(5) to limit the number of entries to 5. It uses a break to exit the loop if user hits Enter without entering a value. It uses a continue statement to skip negative numbers entered by the user.

total = 0
for n in range (5):
  number_as_string = input('Enter a number (press enter to exit):')
  
  # exit if user input is empty
  if number_as_string == '':
    print('Exiting as per user request...')
    break
    
  # convert the input into an integer
  number_as_int = int(number_as_string)
  
  # skip if the number is negative
  if number_as_int < 0 :
    print('Negative number, skipped')
    continue 
    
  # update the total and print the running total
  total = total + number_as_int
  print('Running total:' + str(total))

print('Grand total of non-negative numbers:', str(total))

Exercise : Multiplication Table (Extended)

Modify the code a program to,

print multiplication values of up to x 20 (not up to x 10 as before)
omit squares (i.e. n*n). Note how 8 x 8 has been omitted in the example below. Recommended: use a continue statement to achieve this.
not print rows that has a value higher than 100. Note how the printing stops after 8 x 12 = 96 in the example below. Recommended: use a break statement for this.

Enter a number: 8
8 x 1 = 8
8 x 2 = 16
8 x 3 = 24
8 x 4 = 32
8 x 5 = 40
8 x 6 = 48
8 x 7 = 56
8 x 9 = 72
8 x 10 = 80
8 x 11 = 88
8 x 12 = 96

Loops can be nested.

📦 The code below use two nested for loops to print multiplication tables for 2, 3, and 4.

for i in range(2, 5):
  print('Multiplication table for', i)
  for j in range(1, 11):
    print(i, 'x', j, '=', i*j)

Exercise : Three Dice Throws

Write a program to generate all possible results you can get by throwing a 6-sided dice three times. The expected result (partial) is given below.

_{source: wikipedia}

Use three nested for loops.

Importing Modules

In addition to built-in functions such as print(), input(), and len(), Python has a set of functions called the standard library. Functions in the standard library are divided in to modules. A module is a python program that contains a related group of functions. You can think of it as a file containing some python functions.

📦 Some example modules in the Python standard library:

Name	What it contains
`math`	mathematics-related functions
`random`	random number–related functions
`sys`	provides ways to interact with the Python interpreter

Unlike built-in functions which you can use simply use in your code, to use a function in the standard library you need to import the corresponding module first.

Format: import module_name(s)

📦 Some examples:

import math (imports the math module)
import math, sys, random (imports all three modules in one statement)

Furthermore, to use a function from an imported module, you should use the module_name.function_name() format.

📦 the code below imports the random module and uses its randint() function to generate a number between 1 and 10.

import random

print('Going to print 5 random numbers between 1 and 10')
for i in range(5):
    print(random.randint(1, 10))

💡 It is common practice to put the import statements at the top of the program code.

Exercise : Circle Area

Write a program to read the radius of a circle (a float number) from the input and print the area of the circle, rounded to five decimal places.

💡 some useful hints:

Area = PI * radius²
You can use math.pi as the value of PI.
You can use the built-in function round to round off a number to a specified number of decimal places. e.g., round(1.12345, 2) returns 1.12

Enter radius: 4.5
Area: 63.61725

Early Termination

At times you may need to terminate a program execution early i.e., without reaching the end of the code. You can use the exit() function in the system module to terminate a program early.

📦 The code below prints out the factorials of 1 to n where n is specified by the user. If n is 0, it uses sys.exit() to terminate the program immediately.

import math, sys

n = int(input('Number of factorials (0 to exit):'))

if n == 0:
  print('Terminating as requested...')
  sys.exit()
  
for i in range(n):
    print(math.factorial(i+1)) # use i+1 because i starts from 0

Program Flow Control

Booleans

if Statements

Exercise : Even Numbers

Exercise : Grade Analyzer

Exercise : Grade Analyzer (Extended)

while Statements

Exercise : Vending Machine - Add Loop

Exercise : Vending Machine - Accept Multiple Coins

Exercise : Vending Machine - Allow Abort

Exercise : Vending Machine - Legit Coins

for Statements

Exercise : Multiplication Table

Exercise : Multiplication Table (Extended)

Exercise : Three Dice Throws

Importing Modules

Exercise : Circle Area

Early Termination

`if` Statements

`while` Statements

`for` Statements