String to Integer atoi in DSA C# - 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 C#

int digit = str[0] - [1];

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

A'0'

B'9'

C'a'

D'1'

Attempts:

3 left

💡 Hint

Common Mistakes

Using '1' instead of '0' causes wrong digit calculation.

Using character 'a' or '9' leads to incorrect results.

Subtracting the character '0' from a digit character converts it to its integer value.

2fill in blank

medium

Complete the code to skip leading whitespace characters in the string.

DSA C#

while (index < str.Length && str[index] == [1]) { index++; }

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

A'\t'

B'\n'

C'0'

D' '

Attempts:

3 left

💡 Hint

Common Mistakes

Skipping only tabs or newlines misses spaces.

Using '0' character skips digits incorrectly.

Leading spaces are represented by the space character ' '.

3fill in blank

hard

Fix the error in the code to correctly check if the current character is a digit.

DSA C#

if (str[index] >= [1] && str[index] <= [2]) { /* process digit */ }

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

A'0'

B'9'

C'a'

D'z'

Attempts:

3 left

💡 Hint

Common Mistakes

Using letters 'a' or 'z' instead of digits causes wrong checks.

Checking only one boundary leads to errors.

Digits in ASCII range from '0' to '9'. Checking this range ensures the character is a digit.

4fill in blank

hard

Fill both blanks to correctly update the result integer while parsing digits.

DSA C#

result = result [1] 10 [2] (str[index] - '0');

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

A*

B+

C-

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction or division instead of addition causes wrong results.

Not multiplying by 10 loses place value.

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

5fill in blank

hard

Fill all three blanks to handle sign and return the final integer result.

DSA C#

int sign = [1];
if (str[index] == '-') { sign = [2]; index++; }
return result [3] sign;

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

A1

B-1

C*

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Adding sign instead of multiplying changes the number incorrectly.

Not updating sign when '-' is found causes wrong sign.

Sign is 1 by default, changes to -1 if '-' found, and final result is multiplied by sign.

Practice

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

easy

A. Reverses the string characters

B. Converts the string to a floating-point number

C. Converts the string to an integer number

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: Differentiate from other string operations

   It does not reverse or count characters, nor convert to floating-point; it specifically converts to integer.

3. Final Answer:

   Converts the string to an integer number -> Option C

4. Quick Check:

   atoi converts string to integer [OK]

Hint: atoi always converts string digits to integer number [OK]

Common Mistakes:

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

2. Which of the following C# code snippets correctly converts a string "1234" to an integer using atoi logic?

easy

A. int num = Convert.ToString(1234);

B. int num = "1234".ToInt();

C. int num = int.ToString("1234");

D. int num = int.Parse("1234");

Solution

1. Step 1: Identify correct C# method for string to int

   int.Parse(string) converts a numeric string to an integer correctly.

2. Step 2: Check other options for syntax errors

   "1234".ToInt() and int.ToString("1234") are invalid methods; Convert.ToString(1234) converts int to string, not string to int.

3. Final Answer:

   int num = int.Parse("1234"); -> Option D

4. Quick Check:

   int.Parse converts string to int [OK]

Hint: Use int.Parse to convert string digits to int in C# [OK]

Common Mistakes:

• Using invalid method names like ToInt()
• Confusing Convert.ToString with conversion to int
• Trying to call ToString on string input

3. What is the output of the following C# code?

string s = "  -42abc";
int result = MyAtoi(s);
Console.WriteLine(result);

int MyAtoi(string str) {
    int i = 0, sign = 1, num = 0;
    while (i < str.Length && str[i] == ' ') i++;
    if (i < str.Length && (str[i] == '+' || str[i] == '-')) {
        sign = (str[i] == '-') ? -1 : 1;
        i++;
    }
    while (i < str.Length && char.IsDigit(str[i])) {
        num = num * 10 + (str[i] - '0');
        i++;
    }
    return sign * num;
}

medium

A. Exception thrown

B. -42

C. 42

D. 0

Solution

1. Step 1: Skip leading spaces and detect sign

   The code skips spaces, finds '-', sets sign to -1, then moves to digits.

2. Step 2: Parse digits until non-digit

   It reads '4' and '2', stops at 'a', so number is 42, multiplied by sign -1 gives -42.

3. Final Answer:

   -42 -> Option B

4. Quick Check:

   Parsed number with sign = -42 [OK]

Hint: Skip spaces, check sign, parse digits until non-digit [OK]

Common Mistakes:

• Not skipping spaces before sign
• Parsing non-digit characters
• Ignoring sign character

4. Identify the bug in this MyAtoi function and select the fix:

int MyAtoi(string str) {
    int i = 0, sign = 1, num = 0;
    while (str[i] == ' ') i++;
    if (str[i] == '+' || str[i] == '-') {
        sign = (str[i] == '-') ? -1 : 1;
        i++;
    }
    while (char.IsDigit(str[i])) {
        num = num * 10 + (str[i] - '0');
        i++;
    }
    return sign * num;
}

medium

A. Add boundary checks for index i before accessing str[i]

B. Remove the sign check to simplify code

C. Change char.IsDigit to char.IsLetter

D. Initialize num to -1 instead of 0

Solution

1. Step 1: Check for index out of range errors

   The code accesses str[i] without checking if i < str.Length, risking runtime error.

2. Step 2: Fix by adding boundary checks in all loops

   Adding conditions like i < str.Length in while loops prevents out-of-range exceptions.

3. Final Answer:

   Add boundary checks for index i before accessing str[i] -> Option A

4. Quick Check:

   Always check index bounds before accessing string [OK]

Hint: Always check string length before accessing characters [OK]

Common Mistakes:

• Ignoring string length checks causing exceptions
• Removing sign check breaks negative numbers
• Using char.IsLetter instead of char.IsDigit

5. Given the input string "+-12", what should a robust atoi implementation return according to standard rules?

hard

A. 0

B. -12

C. 12

D. Exception thrown

Solution

1. Step 1: Analyze sign characters in input

   The string starts with '+' followed immediately by '-', which is invalid for atoi parsing.

2. Step 2: According to atoi rules, invalid sign sequence returns 0

   Parsing stops at invalid sequence, so result is 0.

3. Final Answer:

   0 -> Option A

4. Quick Check:

   Invalid sign sequence returns 0 [OK]

Hint: Invalid sign sequences return 0 in atoi [OK]

Common Mistakes:

• Parsing multiple signs as valid
• Returning partial number ignoring invalid signs
• Throwing exceptions instead of returning 0