Python · Loops
For Loops
The full text of this chapter, to read or revisit. To practise, do the exercises.
▶ Do the exercisesExercise 1 — Introduction to For Loops
A for loop allows you to repeat a block of code multiple times. It's like giving instructions to do something over and over with different items in a collection.
Here's the basic syntax of a for loop in Python:
for item in collection:
# Code to execute for each item
Let's see a simple example with a list of Codémon names:
codemons = ["Aquachu", "Pyrochu", "Florachu"]
for codemon in codemons:
print("I choose you,", codemon, "!")
In this example, we base the loop on the
codemons
list with 3 names. The loop goes through each name one by one, and the code inside the loop runs for each name.
Exercise 2 — For Loop Syntax
Which of the following is the correct syntax for a for loop in Python?
Options
• for item in collection:
# code block
• for (item in collection) {
# code block
}
• for item = collection:
# code block
• for each item in collection:
# code block
Hint : Remember : the correct syntax for a for loop in Python is for followed by a variable name, the keyword in, the collection, and a colon. The body of the loop must be indented.
Exercise 3 — Iterating Over Lists
One of the most common uses of for loops is to iterate over items in a list. Each time the loop runs, the loop variable takes the value of the next item in the list.
levels = [5, 10, 15, 20]
# Loop through each level
for level in levels:
print("Codémon at level", level)
Within the loop, you can basically do whatever you want, it's normal Python code. You can print the value, perform calculations, or even modify other variables.
powers = [10, 20, 30, 40]
# Calculate and print double the power for each value
for power in powers:
doubled_power = power * 2
print("Original power:", power, "Doubled:", doubled_power)
Loop Variable Scope
The loop variable (like 'codemon', 'level', or 'power' in our examples) is created when the loop starts. It continues to exist after the loop ends, containing the last value it was assigned during the loop.
Exercise 4 — Complete the For Loop
Complete the for loop to print each Codémon type with an appropriate message.
types = ["water", "fire", "grass"]
____ c_type ____ types:
print(c_type, "is a type of Codémon.")
Options: for in each from of element
Hint : The syntax for a for loop requires the 'for' keyword, followed by a variable name, then 'in', and finally the collection to iterate over.
Exercise 5 — Using range() with For Loops
The
range()
function is commonly used with for loops to repeat an action a specific number of times or to generate a sequence of numbers.
# range(stop) - Creates a sequence from 0 to stop-1
print("range(5):")
for i in range(5):
print(i)
# range(start, stop, step) - Creates a sequence with a specific step
print("range(1, 10, 2):")
for i in range(0, 10, 2):
print(i)
range()
can take one, two, or three parameters:
1. One parameter:
range(stop)
will start at
0
and stop at the
stop - 1
2. Two parameters:
range(start, stop)
will start at the
start
number and stop at
stop - 1
3. Three parameters:
range(start, stop, step)
starts at
start
, stops at
stop - 1
, and increments by
step
each time.
Exercise 6 — Match Range Expressions
Match each range expression with the sequence of numbers it generates.
Items to match
• range(5)
• range(2, 6)
• range(1, 10, 2)
• range(10, 5, -1)
Definitions
• 0, 1, 2, 3, 4
• 2, 3, 4, 5
• 1, 3, 5, 7, 9
• 10, 9, 8, 7, 6
Exercise 7 — Understanding Range Parameters
Which range expression would generate the sequence: 2, 4, 6, 8, 10?
Options
• ```range(1, 12, 2)```
• ```range(2, 10, 2)```
• ```range(2, 11, 2)```
• ```range(0, 10, 2)```
Hint : When using range(start, stop, step), remember that it includes start but excludes stop.
Exercise 8 — Tracing Loop Variable Values
Consider the following code:
items = ["potion", "revive", "codeball"]
for item in items:
print(item)
What will be the entire output of this code?
Options
• ```"potion", "revive", "codeball"```
• ```"codeball", "revive", "potion"```
• ```"item", "item", "item"```
• There is going to be an error
Hint : Remember that in 'for item in items', the variable 'item' takes on each value from the list in order.
Exercise 9 — Advanced Range Expressions
Match each range expression with the sequence it generates.
Items to match
• range(10, 0, -2)
• range(5, 20, 5)
• range(3, 4)
• range(0, -5, -1)
Definitions
• 10, 8, 6, 4, 2
• 5, 10, 15
• 3
• 0, -1, -2, -3, -4
Exercise 10 — Accumulating Values in Loops
A common pattern with loops is to accumulate or calculate a result based on all items in a collection. For example, summing numbers.
Example: Summing values in a list
powers = [45, 60, 35, 50]
# Initialize the total to 0
total_power = 0
# Add each power to the total
for power in powers:
total_power = total_power + power
print("Total power:", total_power)
Accumulation Pattern
The accumulation pattern generally follows these steps:
1. Initialize an accumulator (like total_power = 0 or an empty list)
2. Loop through the collection
3. Update the accumulator with each item
4. After the loop, the accumulator contains the final result
Exercise 11 — What Will This Loop Print?
What will be the output of the following code?
elements = ["water", "fire", "grass"]
result = ""
for element in elements:
result = result + element[0]
print(result)
Options
• ```wfg```
• ```WFG```
• ```waterfiregrass```
• ```WATERFIREGRASS```
Hint : Looping over a list using a ```in``` statement allows you to access each element directly.
Exercise 12 — Loops with Indices
Another way to access indices AND values of the list is using range() with the length of the list:
codemons = ["Aquachu", "Pyrochu", "Florachu"]
for i in range(len(codemons)):
print("Position", i, "contains", codemons[i])
When to Use Indices
Use indices when you need to:
- Know the position of each item
- Access multiple items in relation to the current item (like the next or previous item)
- Modify the original list while iterating
Exercise 13 — Working with Indices
Complete the code to print each Codémon's name along with its power level using indices.
names = ["Aquachu", "Pyrochu", "Florachu"]
powers = [75, 80, 70]
# Use range and len to loop through indices
for i in ____(____(names)):
print(names[____], "has a power level of", powers[____])
Options: range len i names powers enumerate list i
Hint : When using indices, range(len(list)) creates a sequence of indices from 0 to the length of the list minus 1. You can then use these indices to access corresponding elements in multiple lists.
Exercise 14 — Analyzing Loop Behavior
What will be the final value of result after this code executes?
numbers = [5, 2, 8, 1, 7]
result = 0
for i in range(len(numbers)):
if i % 2 == 0: # If index is even
result += numbers[i]
Hint : In this exercise we loop through the whole list but a condition kind of filter what we add to the result.
Exercise 15 — Three Ways to Use For Loops
You've learned three different techniques for using for loops in Python. Each technique is useful in different situations. Let's review them:
Technique 1: Looping Directly Over a List
# Technique 1: Direct iteration over list
codemons = ["Aquachu", "Pyrochu", "Florachu"]
for codemon in codemons:
print(codemon)
Technique 2: Looping with range()
# Technique 2: Using range() to repeat N times
# Repeat 5 times
for i in range(5):
print("Training session", i + 1)
Technique 3: Looping with range(len())
# Technique 3: Using range(len()) to access indices
names = ["Aquachu", "Pyrochu", "Florachu"]
powers = [75, 80, 70]
for i in range(len(names)):
print("Position", i, ":", names[i], "with power", powers[i])
Quick Decision Guide
- Need only values? → Use
for item in list
- Need to repeat N times? → Use
for i in range(N)
- Need indices or multiple lists? → Use
for i in range(len(list))
Exercise 16 — Codémon Challenge - Training Session Analyzer
Welcome to the Training Session Analyzer! You need to analyze the performance of your Codémon team during training.
Your tasks:
1. Loop through each Codémon name and simulate 3 training rounds for each one. Display which Codémon is training and which round it is. (Hint: like nested if statements, you can nest loops inside other loops!)
2. Loop through both the names and experience_gained lists using indices to display how much experience each Codémon gained
Starter code
print("=== TRAINING SESSION ANALYZER ===")
codemon_names = ["Aquachu", "Pyrochu", "Florachu"]
experience_gained = [150, 200, 175]
# Task 1: Loop through each Codémon and simulate 3 training rounds
print("\n--- Training Sessions ---")
# Task 2: Display experience gained by each Codémon using indices
print("\n--- Experience Report ---")