Iterating over lists in Python - Step-by-Step Execution

Concept Flow - Iterating over lists

Start with list

↓

Set index i = 0

↓

Check: i < length of list?

No→Exit loop

Yes↓

Access list[i

↓

Execute loop body

↓

Increment i by 1

↩Back to Check

This flow shows how we start from the first item in the list, check if we still have items left, process the current item, then move to the next until all items are done.

Execution Sample

Python

fruits = ['apple', 'banana', 'cherry']
for fruit in fruits:
    print(fruit)

This code goes through each fruit in the list and prints its name one by one.

Execution Table

Iteration	Variable 'fruit'	Action	Output
1	'apple'	Access fruits[0]	apple
2	'banana'	Access fruits[1]	banana
3	'cherry'	Access fruits[2]	cherry
-	-	No more items, loop ends	-

💡 After 3 iterations, all items are processed, loop stops.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
fruit	undefined	'apple'	'banana'	'cherry'	'cherry'

Key Moments - 3 Insights

Why does the loop stop after the last item?

The loop checks if there are more items by comparing the current index with the list length. When the index reaches the list length (3), the condition fails and the loop ends, as shown in the last row of the execution_table.

Is the variable 'fruit' still available after the loop?

After the loop ends, 'fruit' holds the last item value ('cherry') until overwritten or the program ends, as shown in variable_tracker final column.

What if the list is empty?

If the list is empty, the loop body never runs because the condition to start the loop is false immediately. So no output is produced and 'fruit' is never assigned.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'fruit' during iteration 2?

A 'cherry'

B 'banana'

C 'apple'

D undefined

💡 Hint

Check the 'Variable fruit' column in the execution_table row for iteration 2.

At which iteration does the loop stop according to the execution_table?

A After iteration 2

B After iteration 1

C After iteration 3

D It never stops

💡 Hint

Look at the exit_note and the last row of the execution_table.

If the list was empty, what would the output be?

A No output

B Error message

C Prints 'undefined'

D Prints the last item

💡 Hint

Refer to the key_moments explanation about empty lists.

Concept Snapshot

Iterating over lists in Python:
Use 'for item in list:' to go through each element.
The loop runs once per item, from first to last.
Inside the loop, 'item' holds the current element.
Loop ends when all items are processed.
If list is empty, loop body does not run.

Full Transcript

This lesson shows how to go through each item in a list using a for loop in Python. We start with the first item, assign it to a variable, run the loop body, then move to the next item until all are done. The execution table tracks each step, showing the variable's value and output. Key points include understanding when the loop stops, what happens if the list is empty, and the variable's state after the loop. The quiz tests your understanding by asking about variable values during iterations and loop behavior with empty lists.

Practice

1. What does the following code do?

for item in [1, 2, 3]:
    print(item)

easy

A. Prints the whole list once

B. Prints numbers 1 to 3 without spaces

C. Prints each number 1, 2, and 3 on a new line

D. Causes an error because of the loop

Solution

1. Step 1: Understand the for loop over a list

   The loop goes through each element in the list [1, 2, 3] one by one.

2. Step 2: Print each element during iteration

   Each element (1, then 2, then 3) is printed on its own line because of the print statement.

3. Final Answer:

   Prints each number 1, 2, and 3 on a new line -> Option C

4. Quick Check:

   Loop prints items one by one [OK]

Hint: For loops print each list item separately [OK]

Common Mistakes:

• Thinking it prints the whole list at once
• Expecting numbers to print on the same line
• Confusing loop variable with list itself

2. Which of these is the correct syntax to loop over a list named fruits?

easy

A. for fruit in fruits:
print(fruit)

B. for fruits in fruit:
print(fruits)

C. loop fruit in fruits:
print(fruit)

D. for fruit to fruits:
print(fruit)

Solution

1. Step 1: Identify correct for loop syntax

   The correct Python syntax uses 'for variable in list:' to loop over items.

2. Step 2: Check each option

   for fruit in fruits: print(fruit) uses 'for fruit in fruits:', which is correct. Others use invalid keywords or reversed names.

3. Final Answer:

   for fruit in fruits: print(fruit) -> Option A

4. Quick Check:

   Use 'for item in list:' syntax [OK]

Hint: Remember: for variable in list: [OK]

Common Mistakes:

• Swapping variable and list names
• Using wrong keywords like 'loop' or 'to'
• Missing colon after for statement

3. What is the output of this code?

numbers = [2, 4, 6]
sum = 0
for n in numbers:
    sum += n
print(sum)

medium

A. 12

B. 0

C. 246

D. Error

Solution

1. Step 1: Understand the loop adding numbers

   The loop adds each number in the list [2, 4, 6] to sum, starting from 0.

2. Step 2: Calculate the total sum

   sum = 0 + 2 + 4 + 6 = 12

3. Final Answer:

   12 -> Option A

4. Quick Check:

   Sum of list items = 12 [OK]

Hint: Add each item inside the loop to get total [OK]

Common Mistakes:

• Printing sum before loop
• Concatenating numbers as strings
• Forgetting to initialize sum to 0

4. Find the error in this code:

items = [10, 20, 30]
for i in items
    print(i)

medium

A. List name is incorrect

B. Wrong variable name in loop

C. Indentation error in print

D. Missing colon after for statement

Solution

1. Step 1: Check for syntax errors in for loop

   The for loop must end with a colon ':' to be valid syntax.

2. Step 2: Identify missing colon

   The code misses ':' after 'for i in items', causing a syntax error.

3. Final Answer:

   Missing colon after for statement -> Option D

4. Quick Check:

   For loops need ':' at end [OK]

Hint: Always put ':' after for loop header [OK]

Common Mistakes:

• Forgetting colon after for statement
• Using wrong variable names
• Incorrect indentation of print

5. You have a list of lists: data = [[1, 2], [3, 4], [5, 6]]. How do you print all numbers one by one using iteration?

hard

A. print(data[0][0], data[1][1], data[2][2])

B. for sublist in data:
for num in sublist:
print(num)

C. for i in range(len(data)):
print(data[i])

D. for num in data:
print(num)

Solution

1. Step 1: Understand nested lists and iteration

   Each item in data is a list itself, so we need two loops: one for each sublist, one for numbers inside.

2. Step 2: Use nested for loops to access all numbers

   The outer loop goes through each sublist, the inner loop prints each number inside that sublist.

3. Final Answer:

   for sublist in data: for num in sublist: print(num) -> Option B

4. Quick Check:

   Nested loops print all inner items [OK]

Hint: Use nested loops for lists inside lists [OK]

Common Mistakes:

• Trying to print sublists directly
• Using single loop only
• Accessing wrong indexes causing errors