String concatenation and repetition in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is string concatenation?

String concatenation is joining two or more strings together to make one longer string.

beginner

How do you concatenate strings in Python?

Use the + operator to join strings. For example, 'Hello' + ' ' + 'World' results in 'Hello World'.

beginner

What does string repetition mean?

String repetition means repeating the same string multiple times in a row.

beginner

How do you repeat a string in Python?

Use the * operator with an integer. For example, 'ha' * 3 results in 'hahaha'.

intermediate

What is the output of this code: 'Hi' + '!' * 3?

The output is 'Hi!!!' because '!' is repeated 3 times and then concatenated with 'Hi'.

Which operator is used to join two strings in Python?

A /

B *

C -

D +

The + operator joins (concatenates) two strings.

What does 'abc' * 2 produce?

A 'abcabc'

B 'abc2'

C 'abc abc'

D 'aabbcc'

The * operator repeats the string twice, so 'abcabc'.

What is the result of 'Hello' + ' ' + 'World'?

A 'HelloWorld'

B 'Hello World'

C 'Hello World'

D 'Hello+World'

The strings are joined with a space in between, resulting in 'Hello World'.

Which of these is NOT a valid way to concatenate strings in Python?

A 'a' + 'b'

B 'a' * 3

C 'a' - 'b'

D 'a'.join(['b', 'c'])

The - operator cannot be used with strings.

What will '!' * 0 return?

A '' (empty string)

B '!'

C Error

D '0'

Repeating a string zero times returns an empty string.

Explain how to join two strings and repeat a string in Python with examples.

Think about using + to add strings and * to repeat them.

What happens if you multiply a string by zero or a negative number in Python?

Consider how repetition count affects the output.

Practice

1. What does the + operator do when used with strings in Python?

easy

A. Joins two strings together into one

B. Repeats the string multiple times

C. Converts the string to uppercase

D. Splits the string into a list

Solution

1. Step 1: Understand the + operator with strings

   In Python, + combines two strings by joining them end to end.

2. Step 2: Compare with other options

   Repeating strings uses *, uppercase uses .upper(), splitting uses .split().

3. Final Answer:

   Joins two strings together into one -> Option A

4. Quick Check:

   String + String = Joined String [OK]

Hint: Remember: + joins strings, * repeats strings [OK]

Common Mistakes:

• Confusing + with * for repetition
• Thinking + changes string case
• Assuming + splits strings

2. Which of the following is the correct syntax to repeat the string 'hi' 3 times in Python?

easy

A. 'hi' + 3

B. 'hi' * 3

C. 3 'hi'

D. 'hi' ** 3

Solution

1. Step 1: Identify the repetition operator

   In Python, * repeats a string when multiplied by an integer.

2. Step 2: Check other options

   'hi' + 3 causes TypeError (can't add string and int). 3 'hi' is SyntaxError (missing * operator). 'hi' ** 3 causes TypeError (** is for exponents, not string repetition).

3. Final Answer:

   'hi' * 3 -> Option B

4. Quick Check:

   String * Number = Repeated String [OK]

Hint: Use * between string and number to repeat [OK]

Common Mistakes:

• Using + instead of * for repetition
• Trying to use ** which is invalid for strings
• Placing number before string without *

3. What is the output of the following code?

result = 'ab' + 'cd' * 2
print(result)

medium

A. abcdabcd

B. ababcd

C. abcdcdcd

D. abcdcd

Solution

1. Step 1: Evaluate the repetition part

   'cd' * 2 repeats 'cd' twice, resulting in 'cdcd'.

2. Step 2: Concatenate strings

   'ab' + 'cdcd' joins to form 'abcdcd'.

3. Final Answer:

   abcdcd -> Option D

4. Quick Check:

   'ab' + ('cd' * 2) = 'abcdcd' [OK]

Hint: Calculate repetition first, then join strings [OK]

Common Mistakes:

• Adding strings before repeating
• Repeating the whole concatenated string
• Miscounting repeated parts

4. Find the error in this code snippet:

text = 'go' * '3'
print(text)

medium

A. TypeError because string cannot be multiplied by string

B. SyntaxError due to missing parentheses

C. NameError because 'text' is undefined

D. No error, prints 'gogogo'

Solution

1. Step 1: Identify the operands of the * operator

   The code tries to multiply a string 'go' by another string '3'.

2. Step 2: Understand Python's type rules for * operator

   Python allows string * integer but not string * string, causing a TypeError.

3. Final Answer:

   TypeError because string cannot be multiplied by string -> Option A

4. Quick Check:

   String * String = TypeError [OK]

Hint: Multiply string only by integer, not by string [OK]

Common Mistakes:

• Using quotes around number for repetition
• Assuming implicit conversion to int
• Ignoring error messages

5. You want to create a string that repeats the word 'yes' 4 times, separated by a dash -. Which code produces 'yes-yes-yes-yes'?

hard

A. 'yes' + '-' * 3

B. 'yes-' * 4

C. ('yes-' * 4)[:-1]

D. '-'.join('yes' * 4)

Solution

1. Step 1: Understand the desired output

   The string should be 'yes-yes-yes-yes' with 3 dashes separating 4 'yes'.

2. Step 2: Analyze each option

   ('yes-' * 4)[:-1] repeats 'yes-' 4 times: 'yes-yes-yes-yes-' then removes last dash with [:-1]. 'yes-' * 4 leaves trailing dash. '-'.join('yes' * 4) joins characters, not words. 'yes' + '-' * 3 adds only one dash.

3. Final Answer:

   ('yes-' * 4)[:-1] -> Option C

4. Quick Check:

   Repeat with dash, then remove last dash [OK]

Hint: Repeat with dash, then remove last dash [OK]

Common Mistakes:

• Leaving extra dash at end
• Joining characters instead of words
• Adding fewer dashes than needed