String length and membership test in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the len() function do when used with a string?

The len() function returns the number of characters in the string, including spaces and special characters.

beginner

How can you check if a character or substring exists inside a string in Python?

You can use the in keyword to test membership. For example, 'a' in 'apple' returns True.

beginner

What will len('Hello World') return?

It will return 11 because there are 11 characters including the space.

beginner

What is the result of 'x' in 'example'?

It returns True because the character 'x' is present in the string 'example'.

beginner

Can in be used to check for substrings longer than one character?

Yes, in works for any substring. For example, 'cat' in 'concatenate' returns True.

What does len('Python') return?

A 6

B 5

C 7

D 0

len() counts all characters. 'Python' has 6 letters.

Which expression checks if 'a' is in the string 'data'?

A 'data' in 'a'

B 'a' not in 'data'

C 'a' in 'data'

D 'data' == 'a'

The in keyword checks membership. 'a' in 'data' is True.

What will 'z' in 'pizza' return?

A False

B True

C Error

D None

'z' is present in 'pizza', so the result is True.

What is the length of the string '' (empty string)?

A None

B 1

C Error

D 0

An empty string has zero characters, so length is 0.

Which of these is True?

A 'cat' in 'concatenate'

B 'dog' in 'concatenate'

C 'bat' in 'concatenate'

D 'rat' in 'concatenate'

'cat' is a substring of 'concatenate', so it returns True.

Explain how to find the number of characters in a string and how to check if a character is inside that string.

Think about counting characters and searching inside text.

Describe what happens when you use in with a substring longer than one character.

Consider if 'cat' is in 'concatenate'.

Practice

1.

What does the expression len('hello') return in Python?

easy

A. 5

B. 'hello'

C. 4

D. Error

Solution

1. Step 1: Understand the len() function

   The len() function counts how many characters are in a string.

2. Step 2: Count characters in 'hello'

   The word 'hello' has 5 letters: h, e, l, l, o.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   len('hello') = 5 [OK]

Hint: Count letters to find length quickly [OK]

Common Mistakes:

• Counting letters incorrectly
• Confusing len() with printing string
• Expecting len() to return string itself

2.

Which of the following is the correct syntax to check if the letter 'a' is in the string word?

easy

A. word.contains('a')

B. word in 'a'

C. in 'a' word

D. 'a' in word

Solution

1. Step 1: Recall Python membership syntax

   To check if a substring is inside a string, use substring in string.

2. Step 2: Apply to letter 'a' and variable word

   We write 'a' in word to check if 'a' is inside word.

3. Final Answer:

   'a' in word -> Option D

4. Quick Check:

   Use 'in' as: substring in string [OK]

Hint: Use 'in' with substring first, then string [OK]

Common Mistakes:

• Reversing order of 'in'
• Using non-Python syntax like contains()
• Syntax errors with 'in' placement

3.

What is the output of this code?

word = 'python'
print(len(word) > 5 and 'y' in word)

medium

A. False

B. 6

C. True

D. Error

Solution

1. Step 1: Calculate len(word)

   The string 'python' has 6 characters, so len(word) is 6.

2. Step 2: Evaluate conditions

   Check if 6 > 5 (True) and if 'y' is in 'python' (True). Both are True, so the whole expression is True.

3. Final Answer:

   True -> Option C

4. Quick Check:

   len(word)>5 and 'y' in word = True [OK]

Hint: Check length first, then membership with 'and' [OK]

Common Mistakes:

• Confusing > with >= operator
• Forgetting 'and' logic
• Miscounting string length

4.

Find the error in this code snippet:

text = 'apple'
if 'p' not in text:
    print('No p found')
else
    print('p is present')

medium

A. Missing colon ':' after else

B. Wrong use of 'not in' operator

C. Variable 'text' is not defined

D. Indentation error on print statements

Solution

1. Step 1: Check syntax of if-else

   Python requires a colon ':' after both if and else statements.

2. Step 2: Identify missing colon

   The else line is missing a colon at the end, causing a syntax error.

3. Final Answer:

   Missing colon ':' after else -> Option A

4. Quick Check:

   else: needs colon [OK]

Hint: Always put ':' after else and if lines [OK]

Common Mistakes:

• Omitting colon after else
• Misusing 'not in' operator
• Incorrect indentation

5.

Given a list of words, write a Python expression to create a new list containing only words longer than 4 characters and containing the letter 'e'.

Which of these is correct?

hard

A. [w for w in words if len(w) > 4 or 'e' in w]

B. [w for w in words if len(w) > 4 and 'e' in w]

C. [w for w in words if len(w) >= 4 and 'e' not in w]

D. [w for w in words if len(w) < 4 or 'e' not in w]

Solution

1. Step 1: Understand the conditions

   We want words longer than 4 characters and that contain 'e'. This means length > 4 and 'e' in word.

2. Step 2: Check list comprehension syntax

   The correct syntax is [w for w in words if len(w) > 4 and 'e' in w] which filters words by both conditions.

3. Final Answer:

   [w for w in words if len(w) > 4 and 'e' in w] -> Option B

4. Quick Check:

   Filter with len()>4 and 'e' in word [OK]

Hint: Use 'and' to combine length and membership tests [OK]

Common Mistakes:

• Using 'or' instead of 'and'
• Wrong comparison operators
• Negating membership incorrectly