Length and iteration methods in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the len() function do in Python?

The len() function returns the number of items in an object like a list, string, tuple, or dictionary.

beginner

How can you loop through each item in a list named fruits?

Use a for loop:
for fruit in fruits:
  print(fruit) This goes through each item one by one.

intermediate

What is the difference between for and while loops?

for loops run a set number of times over a sequence. while loops run as long as a condition is true.

beginner

How do you find the length of a string name = 'Alice'?

Use len(name). It will return 5 because 'Alice' has 5 characters.

intermediate

What happens if you try to use len() on an integer like len(5)?

You get a TypeError because integers don’t have a length. len() works only on collections like strings, lists, or tuples.

What does len([1, 2, 3, 4]) return?

A 4

B 3

C 1

D Error

len() counts the number of items in the list, which is 4.

Which loop is best to use when you know exactly how many times to repeat?

A while loop

B try-except block

C if statement

D for loop

for loops are ideal for repeating a fixed number of times over a sequence.

What will this code print?
for i in range(3):
  print(i)

A 1 2 3

B 0 1 2

C 3 2 1

D Error

range(3) generates numbers 0, 1, 2. The loop prints each number.

What does len('hello') return?

A 6

B 4

C 5

D Error

The string 'hello' has 5 characters, so len() returns 5.

Which of these can you use len() on?

A List

B Integer

C Float

D Boolean

len() works on collections like lists, strings, tuples, but not on single numbers or booleans.

Explain how to find the length of a list and how to loop through its items.

Think about counting items and visiting each one.

Describe the difference between a for loop and a while loop with examples.

One repeats over a sequence, the other repeats while a condition is true.

Practice

1. What does the len() function do when used on a list in Python?

easy

A. It returns the number of items in the list.

B. It returns the last item in the list.

C. It adds all the items in the list.

D. It removes the first item from the list.

Solution

1. Step 1: Understand the purpose of len()

   The len() function counts how many items are inside a collection like a list.

2. Step 2: Apply to a list

   When used on a list, it returns the total number of elements present in that list.

3. Final Answer:

   It returns the number of items in the list. -> Option A

4. Quick Check:

   len(list) = number of items [OK]

Hint: Remember: len() counts items, it doesn't change them. [OK]

Common Mistakes:

• Thinking len() returns the last item
• Confusing len() with sum()
• Assuming len() removes items

2. Which of the following is the correct syntax to loop through all items in a list named fruits?

easy

A. for fruit in fruits:

B. for fruits in fruit:

C. loop fruit in fruits:

D. foreach fruit in fruits:

Solution

1. Step 1: Identify correct for-loop syntax in Python

   Python uses for variable in collection: to loop through items.

2. Step 2: Match variable and collection names

   The variable should be singular (fruit) and collection plural (fruits) for clarity and correctness.

3. Final Answer:

   for fruit in fruits: -> Option A

4. Quick Check:

   for item in list: is correct syntax [OK]

Hint: Use 'for item in collection:' to loop in Python. [OK]

Common Mistakes:

• Swapping variable and collection names
• Using 'foreach' which is not Python syntax
• Writing 'loop' instead of 'for'

3. What will be the output of this code?

items = ['a', 'b', 'c']
count = 0
for item in items:
    count += 1
print(count)

medium

A. 0

B. 3

C. ['a', 'b', 'c']

D. Error

Solution

1. Step 1: Understand the loop iteration

   The loop goes through each item in the list items, which has 3 elements.

2. Step 2: Track the count variable

   Each time the loop runs, count increases by 1. After 3 iterations, count becomes 3.

3. Final Answer:

   3 -> Option B

4. Quick Check:

   Loop runs 3 times, count = 3 [OK]

Hint: Count increments once per item; total equals list length. [OK]

Common Mistakes:

• Thinking count stays 0
• Confusing count with list itself
• Expecting a list output instead of a number

4. Find the error in this code snippet:

numbers = [1, 2, 3]
for i in numbers
    print(i)

medium

A. print() cannot be used inside a for loop.

B. Variable 'i' should be 'numbers'.

C. List 'numbers' should be a tuple.

D. Missing colon ':' after the for loop statement.

Solution

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

   Python requires a colon ':' at the end of the for loop line to start the block.

2. Step 2: Identify the missing colon

   The code line for i in numbers is missing the colon, causing a syntax error.

3. Final Answer:

   Missing colon ':' after the for loop statement. -> Option D

4. Quick Check:

   for loop line must end with ':' [OK]

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

Common Mistakes:

• Forgetting the colon ':'
• Changing variable names unnecessarily
• Thinking print() can't be inside loops

5. Given a list data = [3, 0, 5, '', None, 7], which code correctly counts only the items that are considered 'truthy' in Python?

hard

A. count = len(data)

B. count = sum(data)

C. count = sum(1 for x in data if x)

D. count = len([x for x in data if x == True])

Solution

1. Step 1: Understand 'truthy' values in Python

   Truthy values are those that evaluate to True in conditions; 0, '', and None are falsy.

2. Step 2: Analyze each option

   count = len(data) counts all items, ignoring truthiness. count = sum(1 for x in data if x) sums 1 for each truthy item, correctly counting them. count = sum(data) sums values, not counts. count = len([x for x in data if x == True]) checks for exact True, missing other truthy values.

3. Final Answer:

   count = sum(1 for x in data if x) -> Option C

4. Quick Check:

   Sum 1 for truthy items counts them correctly [OK]

Hint: Use sum with condition to count truthy items. [OK]

Common Mistakes:

• Using len() counts all items, not just truthy
• Summing values instead of counting
• Checking equality to True instead of truthiness