String indexing (positive and negative) in Python - Step-by-Step Execution

Concept Flow - String indexing (positive and negative)

Start with string

↓

Index with positive number

↓

Get character at that position

↓

Index with negative number

↓

Get character from end

↓

Use character or error if out of range

We start with a string, then use positive or negative numbers to get characters by position. Positive counts from start, negative from end.

Execution Sample

Python

s = "hello"
print(s[1])
print(s[-1])

This code prints the second character and the last character of the string "hello".

Execution Table

Step	Expression	Index Used	Character Retrieved	Explanation
1	s = "hello"	-	-	String 'hello' is stored in variable s
2	s[1]	1	e	Index 1 gets second character 'e' (counting from 0)
3	s[-1]	-1	o	Index -1 gets last character 'o' (counting from end)

💡 All indexes are valid; program ends after printing characters.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
s	undefined	"hello"	"hello"	"hello"

Key Moments - 3 Insights

Why does s[1] give 'e' and not 'h'?

Because string indexing starts at 0, s[0] is 'h', so s[1] is the next character 'e' as shown in execution_table row 2.

What does a negative index like s[-1] mean?

Negative indexes count from the end of the string. s[-1] is the last character 'o', as shown in execution_table row 3.

What happens if the index is out of range?

Python will give an error. This example does not show that, but if you try s[10] on 'hello', it will cause an IndexError.

Visual Quiz - 3 Questions

Look at the execution table, what character does s[1] retrieve?

A "h"

B "e"

C "l"

D "o"

💡 Hint

Check execution_table row 2 under 'Character Retrieved'

At which step does the program get the last character of the string?

A Step 3

B Step 2

C Step 1

D No step gets the last character

💡 Hint

Look at execution_table row 3 where index -1 is used

If we change s[-1] to s[-2], what character would be retrieved?

A "o"

B "e"

C "l"

D "h"

💡 Hint

Negative index -2 means second last character; check variable s value in variable_tracker

Concept Snapshot

String indexing lets you get characters by position.
Positive index: 0 is first character.
Negative index: -1 is last character.
Example: s = "hello"; s[1] = 'e', s[-1] = 'o'.
Out of range index causes error.

Full Transcript

We start with a string variable s holding 'hello'. Using s[1] gets the second character 'e' because counting starts at zero. Using s[-1] gets the last character 'o' because negative indexes count from the end. If you try an index outside the string length, Python will give an error. This example shows how positive and negative indexing works step by step.

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