Day 5: Controlling the Flow of Codes – Crash Course on Programming

• The conditional statements: if, elif, else.
• How to create loops.
• How to select between the for loops and the while loops.

In “Python” programming, you can control the flow of your code using the following concepts:

• Conditional statements like if, elif, and else
• Loops using while or for with keywords such as break and continue.

Conditional statements allow you to execute certain blocks of code based on specific conditions. Loops let you repeat a block of code multiple times. The break keyword is used to exit a loop prematurely, while continue skips the rest of the code inside a loop and starts the next iteration. Understanding and mastering these tools will help you write a more dynamic and efficient “Python” code.

1 Conditional Statements

Conditional statements are used to perform different actions based on conditions. There are three main types of conditional statements in “Python” language: if, elif, and else.

1.1 The if Statement

The if statement is used to check if a condition is satisfied or not. If the condition is True, the code inside the if block will be executed. If the condition is False, the code inside the if block will be skipped.

# Example of if statement
x = 10
if x > 5:
    print("x is greater than 5") 

x is greater than 5

2 The elif Statement

The elif statement is short for “else if” and is used after the if statement. If the condition in the if statement is False, the condition in the elif statement will be checked. If that condition is True, the code inside the elif block will be executed.

# Example of elif statement
y = 0
if y < 0:
    print("y is negative")
elif y == 0:
    print("y is zero") 

y is zero

3 The else Statement

The else statement is used to execute a block of code when none of the conditions in the if and elif statements are True.

In the above example code of the elif statement, you can not handle the case when y is positive. To include the case, I added the else statement.

# Example of else statement
y = 3
if y < 0:
    print("y is negative")
elif y == 0:
    print("y is zero")
else:
    print("y is positive") 

y is positive

Since y is a positive value, the block in the else statement will be executed.

You can add multiple elif statements.

# Get the percentage from the user
percentage = 72

# Determine the grade based on the percentage
if percentage >= 90:
    grade = "A"
elif percentage >= 80:
    grade = "B"
elif percentage >= 70:
    grade = "C"
elif percentage >= 60:
    grade = "D"
else:
    grade = "F"

# Output the grade to the user
print(f"Your grade is: {grade}") 

Your grade is: C

4 Loops

Loops are used to execute a block of code repeatedly until a certain condition is met. There are two main types of loops in “Python” language: for loops and while loops.

4.1 The for Loops

You can use the for loop to iterate over a sequence (e.g., list, tuple, string).

The for loop executes a block of code for each item in the sequence until there are no more items left.

# Example of for loop
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit) 

apple
banana
cherry

Let me explain this code.

fruits = ["apple", "banana", "cherry"]: This line initializes a list called fruits containing three strings: "apple", "banana", and "cherry".

for fruit in fruits:: This line introduces a for loop that iterates over each item in the fruits list. In each iteration, the current item is assigned to the variable fruit.

print(fruit): Inside the loop, this line prints the value of fruit. During the first iteration, fruit will be "apple", then "banana" in the second iteration, and "cherry" in the third iteration.

When the loop finishes iterating over all items in the fruits list, the loop exits.

Therefore, when you run this code, it will output each fruit in the list fruits on a new line.

If you want to get the index of each item in the loop, enumerate() is useful.

fruits = ["apple", "banana", "cherry"]
for i, fruit in enumerate(fruits):
    print(f"Fruit index {i}: {fruit}") 

Fruit index 0: apple
Fruit index 1: banana
Fruit index 2: cherry

The enumerate() function is used along with a for loop to iterate over the fruits list while keeping track of the index of each element. During each iteration, enumerate(fruits) returns a pair of the index (i) and the value (fruit) of the current element from the list.

If you want to run loops based on indices, you can use range() function:

for i in range(0, 10):
    print(i) 

0
1
2
3
4
5
6
7
8
9

The first line for i in range(0, 10): starts a for loop. The range(0, 10) generates a sequence of numbers starting from 0 up to (but not including) 10. So, the loop will iterate over the numbers 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9. During each iteration, the loop assigns the current number from the sequence to the variable i.

Then, print(i) prints the value of i to the console. For the first iteration, i will be 0, and for subsequent iterations, i will take on the values 1, 2, 3, …, and 9.

You can also iterate through items in a dictionary.

stocks = {"apple": 5, "banana": 3, "orange":8}
for fruit, num in stocks.items():
    print(f"{fruit}: {num}") 

apple: 5
banana: 3
orange: 8

This for loop iterates over the items of the stocks dictionary using the items() method, which returns each key-value pair in the dictionary. The loop unpacks each key-value pair from the dictionary into the variables fruit and num, where fruit holds the fruit name (key) and num holds the quantity (value).

4.2 The while loops

The while loop is used to execute a block of code as long as a specified condition is True. It repeats the code inside the loop until the condition becomes False.

# Example of while loop
count = 0
while count < 5:
    print(f"Count is: {count}")
    count += 1 

Count is: 0
Count is: 1
Count is: 2
Count is: 3
Count is: 4

Let’s break this code down into snippets:

count = 0: This line initializes a variable count with a value of 0. This variable will track the current count within the while loop.

while count < 5:: The while statement is a type of loop that continues executing as long as the condition specified after the while keyword is True. Here, the loop will continue as long as the value of count is less than 5.

print(f"Count is: {count}"): Inside the while loop, this line prints the current value of the count variable using an f-string to format the output dynamically.

count += 1: This line increments the value of the count variable by 1 in each iteration of the loop. It’s important to update this variable within the loop to ensure that the loop condition (count < 5) will eventually become False to exit the loop.

When the value of count reaches 5, the condition count < 5 becomes False, and the loop exits, ending the program execution.

4.3 The break and continue statements

Using the break statement allows you to stop the loops when a certain condition is met.

# Using break to exit loop early
fruits = ["apple", "banana", "cherry", "", "elderberry"]
for fruit in fruits:
    if fruit == "":
        print("Empty data found!")
        break
    print(fruit) 

apple
banana
cherry
Empty data found!

In this example, the loop will iterate over the fruits list and print each fruit. If the loop encounters an empty data "", then the break statement is executed, causing the loop to exit.

Within loops, you can use the continue statement to skip the current iteration and proceed to the next iteration without executing the remaining code inside the loop.

# Using continue to skip specific iteration
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
for num in numbers:
    if num % 2 == 0:  # Skip even numbers
        continue
    print(num) 

1
3
5
7
9

In this example, the loop iterates over the numbers list and uses the continue statement to skip printing even numbers. When the condition in the if statement is True, continue is executed, and the loop proceeds to the next iteration without executing the remaining code inside the loop.

5 Exercises