String to Integer atoi in DSA Python - Build from Scratch

String to Integer atoi

📖 Scenario: Imagine you are building a simple calculator app. Users type numbers as text, but your app needs to convert these text numbers into actual numbers to do math.

🎯 Goal: Build a small program that converts a string containing a number into an integer number, just like the atoi function does.

📋 What You'll Learn

Create a string variable with a number in text form

Create a variable to hold the converted integer

Write code to convert the string to an integer

Print the integer to see the result

💡 Why This Matters

🌍 Real World

Converting text input from users into numbers is common in calculators, forms, and data processing apps.

💼 Career

Understanding how to convert strings to numbers is a basic skill for software developers working with user input and data parsing.

1

Create the string number

Create a string variable called num_str and set it to the exact value "1234".

DSA Python

# Create a string variable called num_str with value "1234"
# Your code here

Hint

Use quotes to create a string. For example: num_str = "1234"

2

Prepare the integer variable

Create a variable called num_int and set it to 0. This will hold the converted number.

DSA Python

num_str = "1234"
# Create a variable num_int and set it to 0
# Your code here

Hint

Just write num_int = 0 to start with zero.

3

Convert the string to integer

Use the built-in int() function to convert num_str to an integer and assign it to num_int.

DSA Python

num_str = "1234"
num_int = 0
# Convert num_str to integer and store in num_int
# Your code here

Hint

Use num_int = int(num_str) to convert the string to a number.

4

Print the integer result

Print the value of num_int using print(num_int) to see the converted number.

DSA Python

num_str = "1234"
num_int = 0
num_int = int(num_str)
# Print num_int
# Your code here

Hint

Use print(num_int) to show the number on screen.

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