String to Integer atoi in DSA Python - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to convert the first character of the string to an integer digit.

DSA Python

digit = ord(s[0]) - [1]

Drag options to blanks, or click blank then click option'

A97

B65

C48

D32

Attempts:

3 left

💡 Hint

Common Mistakes

Using ASCII code of 'A' or 'a' instead of '0'.

Forgetting to subtract the ASCII code.

The ASCII value of '0' is 48, so subtracting 48 converts the character digit to its integer value.

2fill in blank

medium

Complete the code to check if the character is a digit.

DSA Python

if '0' <= s[i] [1] '9':

Drag options to blanks, or click blank then click option'

A<

B==

C>=

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using == instead of <=.

Using > instead of <=.

To check if s[i] is between '0' and '9', use the <= operator.

3fill in blank

hard

Fix the error in the code to correctly build the integer from the string digits.

DSA Python

num = num [1] 10 + (ord(s[i]) - ord('0'))

Drag options to blanks, or click blank then click option'

A*

B-

C+

D//

Attempts:

3 left

💡 Hint

Common Mistakes

Using + instead of * for shifting digits.

Using // or - which are incorrect here.

To build the number, multiply the current number by 10 and add the new digit.

4fill in blank

hard

Fill both blanks to correctly handle the sign and start index for parsing.

DSA Python

sign = 1
if s[0] == [1]:
    sign = -1
    i = [2]
else:
    i = 0

Drag options to blanks, or click blank then click option'

A'-'

B'+'

C1

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' instead of '-' for sign check.

Not updating the start index after sign.

If the first character is '-', sign is -1 and start parsing from index 1.

5fill in blank

hard

Fill all three blanks to complete the atoi function that converts string to integer.

DSA Python

def my_atoi(s):
    s = s.strip()
    if not s:
        return 0
    sign = 1
    i = 0
    if s[0] == [1]:
        sign = -1
        i = 1
    elif s[0] == [2]:
        i = 1
    num = 0
    while i < len(s) and '0' <= s[i] <= '9':
        num = num [3] 10 + (ord(s[i]) - ord('0'))
        i += 1
    return sign * num

Drag options to blanks, or click blank then click option'

A'-'

B'+'

C*

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Not handling '+' sign.

Using '+' instead of '*' when building the number.

The function handles optional '+' or '-' signs and builds the number by multiplying by 10 and adding digits.

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