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.

Practice

1. What does the atoi function do when given a string input?

easy

A. Converts the string to an integer number

B. Converts the string to a floating-point number

C. Reverses the string characters

D. Counts the number of characters in the string

Solution

1. Step 1: Understand the purpose of atoi

   The atoi function is designed to convert a string that represents a number into an integer type.

2. Step 2: Compare options with function behavior

   Only Converts the string to an integer number matches the behavior of converting string to integer. Others describe different operations.

3. Final Answer:

   Converts the string to an integer number -> Option A

4. Quick Check:

   atoi = string to integer [OK]

Hint: Remember atoi means ASCII to integer conversion [OK]

Common Mistakes:

• Thinking atoi converts to float
• Confusing atoi with string reversal
• Assuming atoi counts characters

2. Which of the following Python code snippets correctly converts a string s to an integer similar to atoi?

easy

A. len(s)

B. int(s)

C. float(s)

D. str(s)

Solution

1. Step 1: Identify Python function for string to integer

   Python's built-in int() function converts a string representing a number into an integer.

2. Step 2: Check other options

   str() converts to string, float() converts to float, and len() returns length, so they are incorrect.

3. Final Answer:

   int(s) -> Option B

4. Quick Check:

   int() converts string to integer [OK]

Hint: Use int() to convert string to integer in Python [OK]

Common Mistakes:

• Using str() instead of int()
• Using float() when integer is needed
• Using len() expecting number conversion

3. What will be the output of the following Python code?

def my_atoi(s):
    s = s.strip()
    sign = 1
    if s and s[0] == '-':
        sign = -1
        s = s[1:]
    num = 0
    for char in s:
        if not char.isdigit():
            break
        num = num * 10 + (ord(char) - ord('0'))
    return sign * num

print(my_atoi('  -123abc'))

medium

A. 0

B. 123

C. -123

D. Error

Solution

1. Step 1: Analyze string trimming and sign detection

   The string ' -123abc' is stripped to '-123abc'. The sign is detected as negative and removed from string, leaving '123abc'.

2. Step 2: Convert digits until non-digit character

   Loop reads '1', '2', '3' converting to number 123. Stops at 'a' which is not digit.

3. Final Answer:

   -123 -> Option C

4. Quick Check:

   Parsed number with sign = -123 [OK]

Hint: Stop parsing at first non-digit character [OK]

Common Mistakes:

• Ignoring sign and returning positive number
• Parsing entire string including letters
• Raising error on non-digit characters

4. Find the bug in this atoi implementation and choose the correct fix:

def atoi_buggy(s):
    s = s.strip()
    sign = 1
    if s[0] == '-':
        sign = -1
        s = s[1:]
    num = 0
    for char in s:
        if not char.isdigit():
            return 0
        num = num * 10 + int(char)
    return sign * num

print(atoi_buggy('  -42abc'))

medium

A. Add check if string is empty before accessing s[0]

B. Change return 0 to break inside loop

C. Remove sign variable and always return positive

D. Replace int(char) with ord(char)

Solution

1. Step 1: Identify potential error with empty string

   If input is empty or only spaces, s[0] access causes IndexError. Need to check if s is not empty before accessing.

2. Step 2: Evaluate other fixes

   Changing return 0 to break is good but does not fix crash on empty string. Removing sign loses negative support. Using ord(char) is incorrect for digit conversion.

3. Final Answer:

   Add check if string is empty before accessing s[0] -> Option A

4. Quick Check:

   Check empty string before indexing [OK]

Hint: Always check string length before indexing [OK]

Common Mistakes:

• Not checking empty string before s[0]
• Returning 0 instead of stopping parse
• Ignoring negative sign handling

5. You want to implement atoi that handles leading spaces, optional sign, digits, and clamps output to 32-bit signed integer range [-2^31, 2^31-1]. Which approach correctly applies this?

hard

A. Convert entire string to int directly and then clamp

B. Return 0 if string contains any non-digit characters

C. Ignore sign and clamp only positive values

D. Parse string, convert digits, apply sign, then clamp result between -2147483648 and 2147483647

Solution

1. Step 1: Understand requirements for atoi with clamping

   We must parse string carefully: skip spaces, detect sign, convert digits until non-digit, then clamp to 32-bit integer range.

2. Step 2: Evaluate options for correctness

   Parse string, convert digits, apply sign, then clamp result between -2147483648 and 2147483647 correctly describes parsing and clamping. Convert entire string to int directly and then clamp risks error if string has invalid chars. Ignore sign and clamp only positive values ignores sign which is wrong. Return 0 if string contains any non-digit characters returns 0 too aggressively.

3. Final Answer:

   Parse string, convert digits, apply sign, then clamp result between -2147483648 and 2147483647 -> Option D

4. Quick Check:

   Parse, sign, clamp = correct atoi [OK]

Hint: Clamp after parsing signed integer to avoid overflow [OK]

Common Mistakes:

• Clamping before applying sign
• Not handling non-digit characters properly
• Ignoring integer overflow limits