String to Integer atoi in DSA Python - Execution Trace

Concept Flow - String to Integer atoi

Start

↓

Skip spaces

↓

Check sign '+' or '-'

↓

Read digits one by one

↓

Convert digits to number

↓

Check overflow

↓

Return final integer

The process starts by ignoring spaces, then checks for a sign, reads digits to form a number, handles overflow, and returns the integer.

Execution Sample

DSA Python

def myAtoi(s):
    i, n, sign = 0, len(s), 1
    result = 0
    while i < n and s[i] == ' ':
        i += 1
    if i < n and s[i] in '+-':
        sign = -1 if s[i] == '-' else 1
        i += 1
    while i < n and s[i].isdigit():
        result = result * 10 + int(s[i])
        i += 1
    return max(-2**31, min(sign * result, 2**31 - 1))

This code converts a string to an integer by skipping spaces, handling sign, reading digits, and clamping to 32-bit integer range.

Execution Table

Step	Operation	Index i	Current Char	Sign	Result	Visual State
1	Start parsing	0	' '	1	0	s=' -42'
2	Skip space	1	' '	1	0	s=' -42'
3	Skip space	2	' '	1	0	s=' -42'
4	Check sign	3	'-'	-1	0	s=' -42'
5	Read digit	4	'4'	-1	4	result=4
6	Read digit	5	'2'	-1	42	result=42
7	End digits	6	null	-1	42	final result = -42
8	Clamp and return	6	null	-1	42	return -42

💡 Reached end of digits or string, conversion complete.

Variable Tracker

Variable	Start	After Step 2	After Step 4	After Step 6	Final
i	0	3	4	6	6
sign	1	1	-1	-1	-1
result	0	0	0	42	42

Key Moments - 3 Insights

Why do we skip spaces before processing the number?

Because the function ignores leading spaces to find the actual start of the number, as shown in execution_table steps 1-3 where i moves past spaces.

How does the sign affect the final number?

The sign variable is set at step 4 based on '+' or '-', then multiplied with the result at the end (step 8), changing the number's positivity or negativity.

Why do we stop reading digits at step 7?

We stop when the current character is not a digit or string ends, ensuring only valid digits form the number, as seen when index i reaches 6 with no digit.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'sign' at step 4?

A 0

B 1

C -1

D Undefined

💡 Hint

Check the 'Sign' column at step 4 in the execution_table.

At which step does the variable 'result' become 42?

A Step 6

B Step 7

C Step 5

D Step 8

💡 Hint

Look at the 'Result' column in execution_table rows for steps 5 and 6.

If the input string started with '+' instead of '-', what would be the 'sign' after step 4?

A -1

B 1

C 0

D Undefined

💡 Hint

Refer to how 'sign' is set based on '+' or '-' in execution_table step 4.

Concept Snapshot

String to Integer (atoi) conversion:
- Skip leading spaces
- Check for optional '+' or '-' sign
- Read digits and build number
- Stop at first non-digit
- Clamp result to 32-bit signed int range
- Return final integer

Full Transcript

The String to Integer atoi process starts by skipping any spaces at the beginning of the string. Then it checks if the next character is a '+' or '-' sign to determine if the number is positive or negative. After that, it reads digits one by one, converting them into an integer by multiplying the current result by 10 and adding the new digit. The reading stops when a non-digit character is found or the string ends. Finally, the number is clamped to the 32-bit signed integer range and returned with the correct sign.