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.