Lambda with sorted() in Python - Step-by-Step Execution

Concept Flow - Lambda with sorted()

Start with list

↓

Call sorted()

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Use lambda as key

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Apply lambda to each item

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Compare results

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Return sorted list

The sorted() function takes a list and a lambda function as a key to decide the order, then returns a new sorted list.

Execution Sample

Python

numbers = [5, 2, 9, 1]
sorted_numbers = sorted(numbers, key=lambda x: -x)
print(sorted_numbers)

Sorts the list numbers in descending order using a lambda function as the key.

Execution Table

Step	Action	Lambda Input (x)	Lambda Output (-x)	Comparison	Sorted List State
1	Apply lambda to 5	5	-5	-	[5]
2	Apply lambda to 2	2	-2	Compare -2 > -5	[5, 2]
3	Apply lambda to 9	9	-9	Compare -9 < -5 and -2	[5, 2, 9]
4	Apply lambda to 1	1	-1	Compare -1 > -2	[5, 2, 9, 1]
5	Sort by lambda output	-	-	Order by ascending lambda output	[9, 5, 2, 1]
6	Return sorted list	-	-	-	[9, 5, 2, 1]

💡 All items processed and sorted by lambda key, sorted list returned.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
numbers	[5, 2, 9, 1]	[5, 2, 9, 1]	[5, 2, 9, 1]	[5, 2, 9, 1]	[5, 2, 9, 1]	[5, 2, 9, 1]
sorted_numbers	[]	[]	[]	[]	[]	[9, 5, 2, 1]

Key Moments - 2 Insights

Why does the lambda use '-x' instead of just 'x'?

Using '-x' reverses the order because sorted() sorts ascending by default; the lambda output is used to compare items, so negating x sorts descending (see execution_table steps 1-5).

Does sorted() change the original list?

No, sorted() returns a new list and leaves the original list unchanged, as shown in variable_tracker where 'numbers' stays the same.

Visual Quiz - 3 Questions

Look at the execution_table at Step 3, what is the lambda output for input 9?

A 9

B -9

C -5

D 5

💡 Hint

Check the 'Lambda Output (-x)' column at Step 3 in execution_table.

At which step does the sorted list first show the item 9 at the front?

A Step 2

B Step 4

C Step 5

D Step 6

💡 Hint

Look at the 'Sorted List State' column in execution_table to see when 9 moves to front.

If the lambda was changed to 'lambda x: x', how would the final sorted list change?

A [1, 2, 5, 9]

B [9, 5, 2, 1]

C [5, 2, 9, 1]

D No change

💡 Hint

Using 'lambda x: x' sorts ascending, check variable_tracker final sorted_numbers for comparison.

Concept Snapshot

sorted(iterable, key=lambda x: ...)
- sorted() returns a new sorted list
- lambda defines sorting key for each item
- Default sort is ascending
- Use '-x' in lambda to sort descending
- Original list stays unchanged

Full Transcript

This example shows how sorted() uses a lambda function as a key to sort a list. The lambda takes each item and returns its negative, so sorted() sorts the list in descending order. The original list remains unchanged. The execution table traces each step applying the lambda and sorting. The variable tracker shows the original and sorted lists over time.