String to Integer atoi in DSA Python

Mental Model

Convert a string of characters into a number by reading each digit one by one and building the number step by step.

Analogy: Like reading a number written on a paper from left to right, ignoring spaces and signs, and stopping when you see something that is not a digit.

Input string: "  -42abc"
Parsing steps:
[ ] -> ' ' -> '-' -> '4' -> '2' -> 'a' -> 'b' -> 'c'
Result number: 0 -> -4 -> -42 -> stop at 'a'

Dry Run Walkthrough

Input: string: " -42abc"

Goal: Convert the string to the integer -42 by ignoring spaces, reading sign, digits, and stopping at first non-digit

Step 1: Skip leading spaces

"-42abc" (start parsing at '-')

Why: Spaces do not affect the number, so we ignore them

Step 2: Read sign '-' and set sign to negative

sign = -1, next chars: '4' -> '2' -> 'a' -> 'b' -> 'c'

Why: Sign tells if the number is positive or negative

Step 3: Read digit '4', number = 4

number = 4, next chars: '2' -> 'a' -> 'b' -> 'c'

Why: Start building the number from digits

Step 4: Read digit '2', number = 4*10 + 2 = 42

number = 42, next chars: 'a' -> 'b' -> 'c'

Why: Add next digit to the number by shifting previous digits left

Step 5: Encounter non-digit 'a', stop parsing

final number = 42, apply sign -> -42

Why: Stop when non-digit found, number is complete

Result:

final number: -42

Annotated Code

DSA Python

class Solution:
    def myAtoi(self, s: str) -> int:
        i = 0
        n = len(s)
        # Skip leading spaces
        while i < n and s[i] == ' ':
            i += 1
        # Check if string is empty after spaces
        if i == n:
            return 0
        # Check sign
        sign = 1
        if s[i] == '-':
            sign = -1
            i += 1
        elif s[i] == '+':
            i += 1
        # Convert digits to number
        num = 0
        while i < n and s[i].isdigit():
            num = num * 10 + (ord(s[i]) - ord('0'))
            i += 1
        return sign * num

# Driver code
sol = Solution()
print(sol.myAtoi("  -42abc"))

while i < n and s[i] == ' ':

skip all leading spaces to find start of number

if i == n: return 0

handle empty string or only spaces by returning zero

if s[i] == '-': sign = -1 i += 1 elif s[i] == '+': i += 1

detect and record sign, move index past sign

while i < n and s[i].isdigit(): num = num * 10 + (ord(s[i]) - ord('0')) i += 1

build number by reading each digit and shifting previous digits

return sign * num

apply sign to the number and return

Output

-42

Complexity Analysis

Time: O(n) because we scan the string once from left to right

Space: O(1) because we use only a few variables regardless of input size

vs Alternative: This is optimal compared to parsing with regex or converting whole string at once which may be slower or use more memory

Edge Cases

empty string or string with only spaces

returns 0 because no digits found

DSA Python

if i == n:
    return 0

string with only sign and no digits, e.g. '+' or '-'

returns 0 because no digits after sign

DSA Python

while i < n and s[i].isdigit():

string starting with non-digit non-sign character

returns 0 because no valid number starts

DSA Python

while i < n and s[i].isdigit():

When to Use This Pattern

When you see a problem asking to convert a string to a number with rules about spaces, signs, and stopping at non-digits, use the String to Integer atoi pattern to parse carefully step by step.

Common Mistakes

Mistake: Not skipping leading spaces before parsing sign and digits
Fix: Add a loop to skip spaces before checking sign or digits

Mistake: Not handling sign correctly or ignoring it
Fix: Check for '+' or '-' sign and record it before parsing digits

Mistake: Continuing to parse after encountering a non-digit character
Fix: Stop parsing digits as soon as a non-digit character is found

Summary

Converts a string into an integer by reading characters one by one, handling spaces, sign, and digits.

Use when you need to safely parse a number from a string that may have extra characters or spaces.

The key insight is to carefully skip spaces, detect sign, then build the number digit by digit until a non-digit stops parsing.

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