String length and membership test in Python

Introduction

We use string length to know how many characters are in a word or sentence. Membership test helps us check if a letter or word is inside another word or sentence.

Checking if a password is long enough before accepting it.

Finding out if a message contains a certain word.

Counting how many letters are in a name.

Seeing if a letter is part of a word.

Syntax

Python

len(string)

substring in string

len(string) gives the number of characters in the string.

substring in string checks if substring is inside string and returns True or False.

Examples

This counts the letters in "hello" and gives 5.

Python

len("hello")  # returns 5

Checks if letter "a" is in the word "apple". It is, so it returns True.

Python

"a" in "apple"  # returns True

Checks if letter "z" is in "apple". It is not, so it returns False.

Python

"z" in "apple"  # returns False

Empty string has zero characters.

Python

len("")  # returns 0

Sample Program

This program counts letters in the word "banana" and checks if letters "a" and "z" are inside it.

Python

word = "banana"
print(f"The word '{word}' has {len(word)} letters.")

letter = "a"
if letter in word:
    print(f"The letter '{letter}' is in the word.")
else:
    print(f"The letter '{letter}' is not in the word.")

letter = "z"
if letter in word:
    print(f"The letter '{letter}' is in the word.")
else:
    print(f"The letter '{letter}' is not in the word.")

Important Notes

Spaces and punctuation count as characters in length.

Membership test is case sensitive: 'A' and 'a' are different.

Summary

Use len() to find how many characters are in a string.

Use in to check if a smaller string or letter is inside a bigger string.

These tools help us understand and check text easily.

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