String slicing behavior in Python

Introduction

String slicing helps you get parts of a word or sentence easily, like cutting a piece from a cake.

You want to get the first few letters of a name.

You need to remove the last character from a word.

You want to reverse a string to check if it reads the same backwards.

You want to get every second letter from a sentence.

Syntax

Python

string[start:stop:step]

start is where slicing begins (included). If empty, starts from the beginning.

stop is where slicing ends (excluded). If empty, goes to the end.

step is how many characters to jump each time. Default is 1.

Examples

Gets letters from position 1 up to 3 (4 is excluded), so prints 'ell'.

Python

text = "hello"
print(text[1:4])

Gets letters from start up to position 2, prints 'wor'.

Python

text = "world"
print(text[:3])

Gets letters from position 2 to the end, prints 'thon'.

Python

text = "python"
print(text[2:])

Gets every second letter from start to end, prints 'ace'.

Python

text = "abcdef"
print(text[::2])

Sample Program

This program shows different ways to slice a string: from the start, middle, end, and reversed.

Python

word = "programming"

# Get first 4 letters
first_part = word[:4]

# Get letters from position 3 to 7
middle_part = word[3:8]

# Get last 3 letters
last_part = word[-3:]

# Reverse the word
reversed_word = word[::-1]

print(f"First 4 letters: {first_part}")
print(f"Letters 3 to 7: {middle_part}")
print(f"Last 3 letters: {last_part}")
print(f"Reversed word: {reversed_word}")

Important Notes

If start or stop is negative, counting starts from the end of the string.

If step is negative, slicing goes backwards.

Slicing never changes the original string; it creates a new one.

Summary

String slicing extracts parts of a string using start, stop, and step positions.

Start is included, stop is excluded in the slice.

You can use negative numbers to count from the end or reverse the string.

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