String slicing behavior in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of negative step slicing

What is the output of this Python code?

text = 'abcdefg'
result = text[5:2:-1]
print(result)

Python

text = 'abcdefg'
result = text[5:2:-1]
print(result)

A 'cde'

B 'def'

C 'fed'

D 'edc'

💡 Hint

Remember that slicing with a negative step goes backwards and excludes the stop index.

The slice text[5:2:-1] starts at index 5 ('f'), goes backwards up to but not including index 2 ('c'), so it includes indices 5,4,3 which are 'f','e','d'.

❓ Predict Output

intermediate

Slicing with omitted start and stop

What does this code print?

text = 'python'
result = text[::-2]
print(result)

Python

text = 'python'
result = text[::-2]
print(result)

A 'nhy'

B 'nohty'

C 'nhot'

D 'nht'

💡 Hint

When start and stop are omitted with a negative step, slicing goes from end to start.

text[::-2] means start from the end and take every second character backwards: indices 5('n'),3('h'),1('y').

❓ Predict Output

advanced

Effect of out-of-range indices in slicing

What is the output of this code?

text = 'hello world'
result = text[3:100]
print(result)

Python

text = 'hello world'
result = text[3:100]
print(result)

A 'lo world!'

B 'lo world'

C IndexError

D 'lo worl'

💡 Hint

Slicing does not raise errors if stop index is beyond string length.

Slicing stops at the string end if the stop index is too large. text[3:] is from index 3 to end: 'lo world'.

❓ Predict Output

advanced

Slicing with step zero

What happens when you run this code?

text = 'example'
result = text[::0]
print(result)

Python

text = 'example'
result = text[::0]
print(result)

A ValueError

B '' (empty string)

C 'example'

D TypeError

💡 Hint

Step cannot be zero in slicing.

Using step=0 in slicing raises ValueError: slice step cannot be zero.

🧠 Conceptual

expert

Understanding slice indices with negative start and stop

Given the string s = 'abcdefgh', what is the output of s[-3:-7:-1]?

Python

s = 'abcdefgh'
print(s[-3:-7:-1])

A 'defg'

B 'efgh'

C 'dcba'

D 'fedc'

💡 Hint

Negative indices count from the end. The slice goes backwards from -3 to -7 (excluded).

Index -3 is 'f' (index 5), -7 is 'b' (index 1). The slice goes backwards from index 5 down to 2 inclusive, so 'f','e','d','c'.

Practice

1. What does the slice s[2:5] do on the string s = 'Python'?

easy

A. Extracts characters from index 5 to index 2

B. Extracts characters from index 2 to index 5 including both

C. Extracts characters from the start to index 5

D. Extracts characters from index 2 up to but not including index 5

Solution

1. Step 1: Understand slice notation s[start:stop]

   The slice includes the start index but excludes the stop index.

2. Step 2: Apply to s = 'Python' with s[2:5]

   Characters at indices 2, 3, 4 are 't', 'h', 'o'. Index 5 is excluded.

3. Final Answer:

   Extracts characters from index 2 up to but not including index 5 -> Option D

4. Quick Check:

   Slice excludes stop index [OK]

Hint: Remember stop index is excluded in slicing [OK]

Common Mistakes:

• Including the stop index character
• Confusing start and stop positions
• Thinking slicing reverses string by default

2. Which of the following is the correct syntax to get every second character from string s starting from the first character?

easy

A. s[:2:2]

B. s[2::]

C. s[::2]

D. s[1:2]

Solution

1. Step 1: Understand slice syntax s[start:stop:step]

   The step value controls the interval between characters taken.

2. Step 2: Identify correct syntax for every second character from start

   s[::2] means start at 0, go to end, step by 2, so every second character.

3. Final Answer:

   s[::2] -> Option C

4. Quick Check:

   Step controls interval [OK]

Hint: Use empty start and stop with step for intervals [OK]

Common Mistakes:

• Using wrong start index
• Confusing step with stop
• Using slice with only two colons incorrectly

3. What is the output of the following code?

s = 'abcdefg'
print(s[5:2:-1])

medium

A. 'cde'

B. 'fed'

C. 'edc'

D. 'gfed'

Solution

1. Step 1: Understand slicing with negative step

   Negative step means slicing backwards. Start at index 5, stop before index 2.

2. Step 2: Extract characters from s = 'abcdefg'

   Index 5 is 'f', then 'e' at 4, then 'd' at 3. Stop before 2 means exclude index 2 ('c'). So result is 'fed'.

3. Final Answer:

   'fed' -> Option B

4. Quick Check:

   Negative step slices backwards excluding stop [OK]

Hint: Negative step slices backward excluding stop index [OK]

Common Mistakes:

• Including stop index when stepping backwards
• Confusing forward and backward slicing
• Misreading indices for negative step

4. Find the error in this code snippet:

s = 'hello'
print(s[4:1])

medium

A. No error, prints empty string

B. No error, prints 'llo'

C. SyntaxError due to invalid slice

D. RuntimeError due to wrong indices

Solution

1. Step 1: Analyze slice s[4:1]

   Start index 4, stop index 1, default step is +1, so slice moves forward.

2. Step 2: Understand slice behavior when start > stop with positive step

   Since start is after stop and step is positive, slice returns empty string without error.

3. Final Answer:

   No error, prints empty string -> Option A

4. Quick Check:

   Forward slice with start > stop = empty string [OK]

Hint: Forward slice with start > stop returns empty string [OK]

Common Mistakes:

• Expecting an error for invalid slice
• Expecting reversed substring without negative step
• Confusing slice indices order

5. Given s = 'abcdefgh', which slice extracts the string 'hgfed'?

hard

A. s[7:2:-1]

B. s[7:3:-1]

C. s[7:1:-1]

D. s[7:0:-1]

Solution

1. Step 1: Identify characters in 'hgfed'

   These are characters at indices 7 ('h'), 6 ('g'), 5 ('f'), 4 ('e'), 3 ('d').

2. Step 2: Check slice s[7:2:-1]

   Start at index 7 ('h'), move backward to index 3 (stop before 2), so includes indices 7 to 3. This matches 'hgfed'.

3. Final Answer:

   s[7:2:-1] -> Option A

4. Quick Check:

   Negative step excludes stop index [OK]

Hint: Negative step excludes stop index, count backward carefully [OK]

Common Mistakes:

• Including stop index in negative step slice
• Choosing wrong stop index
• Confusing slice direction with step sign