String indexing (positive and negative) in Python

Introduction

String indexing helps you pick out single letters from a word or sentence. You can count from the start or the end.

When you want to get the first letter of a name.

When you need the last character of a password to check it.

When you want to read a specific letter in a sentence.

When you want to reverse a string by using negative indexes.

When you want to slice parts of a string using positions.

Syntax

Python

string[index]

# index can be positive (start at 0) or negative (start at -1 from end)

Positive index starts at 0 for the first character.

Negative index starts at -1 for the last character.

Examples

Positive indexes: 0 is first letter, 4 is last letter in 'hello'.

Python

word = "hello"
print(word[0])  # prints 'h'
print(word[4])  # prints 'o'

Negative indexes: -1 is last letter, -5 is first letter in 'hello'.

Python

word = "hello"
print(word[-1])  # prints 'o'
print(word[-5])  # prints 'h'

Mixing positive and negative indexes to get letters from middle.

Python

sentence = "Python"
print(sentence[2])   # prints 't'
print(sentence[-3])  # prints 'h'

Sample Program

This program shows how to get letters from a word using positive and negative indexes.

Python

word = "friend"

first_letter = word[0]
last_letter = word[-1]
third_letter = word[2]
second_last_letter = word[-2]

print(f"First letter: {first_letter}")
print(f"Last letter: {last_letter}")
print(f"Third letter: {third_letter}")
print(f"Second last letter: {second_last_letter}")

Important Notes

Trying to use an index outside the string length will cause an error.

Strings are like a row of boxes, each box has one letter and a number (index).

Summary

Use positive indexes to count letters from the start (0, 1, 2...).

Use negative indexes to count letters from the end (-1, -2, -3...).

String indexing helps you pick any single letter easily.

Practice

1.

What does the index 0 represent in the string "hello"?

easy

A. The last letter 'o'

B. The middle letter 'l'

C. The first letter 'h'

D. An error because indexing starts at 1

Solution

1. Step 1: Understand positive indexing

   In Python, string indexes start at 0 for the first character.

2. Step 2: Apply to the string "hello"

   The character at index 0 is the first letter 'h'.

3. Final Answer:

   The first letter 'h' -> Option C

4. Quick Check:

   Index 0 = first letter 'h' [OK]

Hint: Index 0 always means the first letter [OK]

Common Mistakes:

• Thinking indexing starts at 1
• Confusing index 0 with last letter
• Assuming negative indexes start at 0

2.

Which of the following is the correct way to get the last character of a string s in Python?

easy

A. s[1]

B. s[-1]

C. s[0]

D. s[len(s)]

Solution

1. Step 1: Recall negative indexing

   Negative index -1 refers to the last character in a string.

2. Step 2: Check each option

   s[1] is second character, s[0] is first, s[len(s)] causes error (index out of range).

3. Final Answer:

   s[-1] -> Option B

4. Quick Check:

   Last char = s[-1] [OK]

Hint: Use -1 to get last character quickly [OK]

Common Mistakes:

• Using s[len(s)] causes IndexError
• Confusing s[1] as last character
• Forgetting negative indexes start at -1

3.

What is the output of this code?

word = "python"
print(word[2], word[-3])

medium

A. t h

B. t o

C. t t

D. h n

Solution

1. Step 1: Find character at index 2

   Index 0='p', 1='y', 2='t'. So word[2] = 't'.

2. Step 2: Find character at index -3

   Negative indexes count from end: -1='n', -2='o', -3='h'. So word[-3] = 'h'.

3. Final Answer:

   t h -> Option A

4. Quick Check:

   word[2] = 't', word[-3] = 'h' [OK]

Hint: Count from start with positive, from end with negative [OK]

Common Mistakes:

• Mixing positive and negative indexes
• Counting negative indexes starting at 0
• Confusing letters at indexes 2 and -3

4.

Find the error in this code snippet:

text = "example"
print(text[-0])

medium

A. No error, prints 'e' (same as text[0])

B. SyntaxError due to invalid index

C. IndexError because -0 is out of range

D. IndexError: -0 is invalid

Solution

1. Step 1: Understand -0 as index

   In Python, -0 is the same as 0, so text[-0] equals text[0].

2. Step 2: Check output

   text[0] is 'e', the first letter, so it prints 'e' without error.

3. Final Answer:

   No error, prints 'e' (same as text[0]) -> Option A

4. Quick Check:

   -0 = 0 index, prints first letter 'e' [OK]

Hint: -0 equals 0, so no error and prints first letter [OK]

Common Mistakes:

• Thinking -0 is invalid index
• Expecting IndexError for -0
• Confusing negative zero with positive zero

5.

Given the string s = "programming", which expression returns the substring "ing" using only negative indexes?

hard

A. s[-3:-0]

B. s[-4:-1]

C. s[-3:-1]

D. s[-3:]

Solution

1. Step 1: Understand slicing with negative indexes

   s[-3:] means start at third last character to end.

2. Step 2: Apply to "programming"

   Third last character is 'i', so s[-3:] = 'ing'. Other options exclude last character or cause empty slice.

3. Final Answer:

   s[-3:] -> Option D

4. Quick Check:

   Negative slice from -3 to end = 'ing' [OK]

Hint: Use s[-3:] to get last 3 letters easily [OK]

Common Mistakes:

• Using s[-3:-1] misses last letter
• Using s[-4:-1] gives 'min' not 'ing'
• Using s[-3:-0] is invalid slice