Length and iteration methods in Python - Step-by-Step Execution

Concept Flow - Length and iteration methods

Start with a collection

↓

Call len() function

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Return number of items

↓

Use for loop to iterate

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Access each item one by one

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End after last item

First, we find how many items are in a collection using len(). Then, we use a for loop to go through each item one by one until all are processed.

Execution Sample

Python

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

This code prints the number of fruits, then prints each fruit on its own line.

Execution Table

Step	Action	Expression Evaluated	Result/Output
1	Create list 'fruits'	fruits = ['apple', 'banana', 'cherry']	fruits = ['apple', 'banana', 'cherry']
2	Call len() on fruits	len(fruits)	3
3	Print length	print(3)	3
4	Start for loop, first item	fruit = fruits[0]	'apple'
5	Print first item	print('apple')	apple
6	Next loop iteration	fruit = fruits[1]	'banana'
7	Print second item	print('banana')	banana
8	Next loop iteration	fruit = fruits[2]	'cherry'
9	Print third item	print('cherry')	cherry
10	Loop ends after last item	No more items	Exit loop

💡 Loop ends because all items in the list have been processed.

Variable Tracker

Variable	Start	After Step 4	After Step 6	After Step 8	Final
fruits	['apple', 'banana', 'cherry']	['apple', 'banana', 'cherry']	['apple', 'banana', 'cherry']	['apple', 'banana', 'cherry']	['apple', 'banana', 'cherry']
fruit	undefined	'apple'	'banana'	'cherry'	'cherry'

Key Moments - 3 Insights

Why does the loop stop after printing 'cherry'?

The loop stops because it has gone through all items in the list. This is shown in execution_table row 10 where it says 'No more items' and the loop exits.

What does len(fruits) return and why?

len(fruits) returns 3 because there are three items in the list. This is shown in execution_table row 2 where len(fruits) evaluates to 3.

Why does the variable 'fruit' change each loop?

In each loop iteration, 'fruit' is assigned the next item from the list. This is shown in execution_table rows 4, 6, and 8 where 'fruit' takes values 'apple', 'banana', and 'cherry' respectively.

Visual Quiz - 3 Questions

Look at the execution_table at Step 2. What value does len(fruits) return?

A 3

B 2

C 0

D Error

💡 Hint

Check the 'Result/Output' column at Step 2 in the execution_table.

At which step does the loop assign 'banana' to the variable 'fruit'?

A Step 4

B Step 6

C Step 8

D Step 10

💡 Hint

Look at the 'Expression Evaluated' column for the assignment of 'fruit' in the execution_table.

If the list had 4 items instead of 3, how would the execution_table change?

A The loop would stop earlier.

B The length printed would still be 3.

C There would be one more loop iteration with a new fruit value.

D The variable 'fruit' would not change.

💡 Hint

Refer to how the loop runs once per item in the list shown in the execution_table.

Concept Snapshot

len(collection) returns the number of items in a collection.
A for loop goes through each item one by one.
Inside the loop, a variable holds the current item.
Loop ends after the last item is processed.
Use print() to show values during iteration.

Full Transcript

This example shows how to find the length of a list using len() and then use a for loop to go through each item. First, the list 'fruits' is created with three items. Calling len(fruits) returns 3, which is printed. Then the for loop starts, assigning each fruit to the variable 'fruit' in turn. Each fruit is printed. The loop stops after the last fruit 'cherry' is printed because there are no more items. The variable 'fruit' changes each iteration to hold the current item. This process helps us count and access items in a collection easily.