String to Integer atoi in DSA C++ - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this atoi implementation?

Given the following C++ code that converts a string to an integer, what will be printed?

DSA C++

int myAtoi(string s) {
    int i = 0, sign = 1, result = 0;
    while (i < s.size() && s[i] == ' ') i++;
    if (i < s.size() && (s[i] == '+' || s[i] == '-')) {
        sign = (s[i] == '-') ? -1 : 1;
        i++;
    }
    while (i < s.size() && isdigit(s[i])) {
        int digit = s[i] - '0';
        if (result > (INT_MAX - digit) / 10) {
            return sign == 1 ? INT_MAX : INT_MIN;
        }
        result = result * 10 + digit;
        i++;
    }
    return sign * result;
}

int main() {
    cout << myAtoi("   -42abc") << endl;
    return 0;
}

A -2147483648

B 42

C -42

D 0

💡 Hint

Focus on how the function skips spaces and handles the sign before digits.

The function skips leading spaces, detects the '-' sign, then reads digits until a non-digit character. It converts "-42" correctly and stops at 'a'.

❓ Predict Output

intermediate

What does this atoi variant output?

Consider this C++ code snippet. What will it print?

DSA C++

int myAtoi(string s) {
    int i = 0, sign = 1, result = 0;
    while (i < s.size() && s[i] == ' ') i++;
    if (i < s.size() && (s[i] == '+' || s[i] == '-')) {
        sign = (s[i] == '-') ? -1 : 1;
        i++;
    }
    while (i < s.size() && isdigit(s[i])) {
        int digit = s[i] - '0';
        if (result > (INT_MAX - digit) / 10) {
            return sign == 1 ? INT_MAX : INT_MIN;
        }
        result = result * 10 + digit;
        i++;
    }
    return sign * result;
}

int main() {
    cout << myAtoi("4193 with words") << endl;
    return 0;
}

A 4193

B 2147483647

C 0

D -4193

💡 Hint

Check how digits are read until a non-digit character.

The function reads digits '4193' and stops at the space before 'with'. It returns 4193.

❓ Predict Output

advanced

What is the output when input causes overflow?

What will this code print when converting a string representing a number larger than INT_MAX?

DSA C++

int myAtoi(string s) {
    int i = 0, sign = 1, result = 0;
    while (i < s.size() && s[i] == ' ') i++;
    if (i < s.size() && (s[i] == '+' || s[i] == '-')) {
        sign = (s[i] == '-') ? -1 : 1;
        i++;
    }
    while (i < s.size() && isdigit(s[i])) {
        int digit = s[i] - '0';
        if (result > (INT_MAX - digit) / 10) {
            return sign == 1 ? INT_MAX : INT_MIN;
        }
        result = result * 10 + digit;
        i++;
    }
    return sign * result;
}

int main() {
    cout << myAtoi("9223372036854775808") << endl;
    return 0;
}

A 2147483647

B 9223372036854775808

C -2147483648

D 0

💡 Hint

Check how the function handles overflow by comparing with INT_MAX.

The input exceeds INT_MAX, so the function returns INT_MAX (2147483647) to avoid overflow.

❓ Predict Output

advanced

What is the output when input has no digits?

What will this code print when the input string has no digits after optional spaces and sign?

DSA C++

int myAtoi(string s) {
    int i = 0, sign = 1, result = 0;
    while (i < s.size() && s[i] == ' ') i++;
    if (i < s.size() && (s[i] == '+' || s[i] == '-')) {
        sign = (s[i] == '-') ? -1 : 1;
        i++;
    }
    while (i < s.size() && isdigit(s[i])) {
        int digit = s[i] - '0';
        if (result > (INT_MAX - digit) / 10) {
            return sign == 1 ? INT_MAX : INT_MIN;
        }
        result = result * 10 + digit;
        i++;
    }
    return sign * result;
}

int main() {
    cout << myAtoi("   +abc") << endl;
    return 0;
}

A -1

B 0

C 2147483647

D +abc

💡 Hint

If no digits are found after optional sign, what does the function return?

No digits found, so result remains 0 and function returns 0.

🧠 Conceptual

expert

Which step is crucial to prevent integer overflow in atoi?

In the atoi implementation, which step ensures the integer does not overflow when converting large strings?

A Stopping parsing at the first non-digit character

B Skipping leading spaces before parsing digits

C Handling the sign '+' or '-' correctly

D Checking if result > (INT_MAX - digit) / 10 before adding the digit

💡 Hint

Think about how to detect if adding a new digit will exceed the integer limit.

The check before adding a digit prevents overflow by ensuring the next multiplication and addition won't exceed INT_MAX.

Practice

1. What does the atoi function do in C++?

easy

A. Converts a string representing a number into an integer

B. Converts an integer into a string

C. Sorts characters in a string alphabetically

D. Calculates the length of a string

Solution

1. Step 1: Understand the purpose of atoi

   The function atoi stands for ASCII to integer and converts a numeric string to an integer value.

2. Step 2: Differentiate from other string functions

   It does not convert integers to strings, sort strings, or calculate string length.

3. Final Answer:

   Converts a string representing a number into an integer -> Option A

4. Quick Check:

   atoi = string to int conversion [OK]

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

Common Mistakes:

• Confusing atoi with itoa (integer to string)
• Thinking atoi sorts or measures strings
• Assuming atoi handles non-numeric strings safely

2. Which of the following is the correct syntax to use atoi in C++?

easy

A. int num = atoi("1234");

B. int num = atoi(1234);

C. int num = atoi('1234');

D. int num = atoi(\"1234\");

Solution

1. Step 1: Check the parameter type for atoi

   The atoi function expects a C-style string (const char*), so the argument must be a string literal like "1234".

2. Step 2: Analyze each option

   int num = atoi("1234"); passes a string literal correctly. int num = atoi(1234); passes an integer, which is invalid. int num = atoi('1234'); uses single quotes, which denote a char, not a string. int num = atoi(\"1234\"); has escaped quotes incorrectly.

3. Final Answer:

   int num = atoi("1234"); -> Option A

4. Quick Check:

   atoi needs a string literal argument [OK]

Hint: atoi requires a string literal, not a number or char [OK]

Common Mistakes:

• Passing integer instead of string
• Using single quotes instead of double quotes
• Incorrect escaping of quotes

3. What will be the output of the following C++ code?

char str[] = "-42abc";
int val = atoi(str);
std::cout << val << std::endl;

medium

A. 0

B. Runtime error

C. 42

D. -42

Solution

1. Step 1: Understand atoi behavior with non-digit characters

   atoi converts the initial part of the string that forms a valid integer. It stops reading when it encounters a non-digit after the number.

2. Step 2: Analyze the string "-42abc"

   It starts with '-', then '4', '2' which form -42. The characters 'a', 'b', 'c' are ignored after the number.

3. Final Answer:

   -42 -> Option D

4. Quick Check:

   atoi stops at first non-digit after number [OK]

Hint: atoi reads number until non-digit character [OK]

Common Mistakes:

• Expecting atoi to parse entire string including letters
• Assuming atoi returns 0 for strings with trailing letters
• Thinking it throws runtime error on letters

4. Identify the error in the following code snippet that uses atoi:

std::string s = "123";
int num = atoi(s);
std::cout << num << std::endl;

medium

A. Missing header file for atoi

B. atoi requires integer argument

C. Cannot pass std::string directly to atoi

D. No error, code runs fine

Solution

1. Step 1: Check parameter type for atoi

   atoi expects a C-style string (const char*), but s is a std::string object.

2. Step 2: Correct usage

   To use atoi with std::string, call s.c_str() to get a C-style string.

3. Final Answer:

   Cannot pass std::string directly to atoi -> Option C

4. Quick Check:

   atoi needs const char*, not std::string [OK]

Hint: Use s.c_str() to convert std::string for atoi [OK]

Common Mistakes:

• Passing std::string directly to atoi
• Ignoring need for c_str() method
• Assuming atoi accepts std::string

5. You want to convert a string to integer but also handle invalid inputs safely without crashing. Which approach is best in C++?

hard

A. Use atoi and check if result is zero to detect errors

B. Use stoi inside try-catch block to catch exceptions

C. Use atoi and ignore invalid inputs

D. Use printf to print the string and guess the number

Solution

1. Step 1: Understand limitations of atoi

   atoi returns 0 for invalid inputs but cannot distinguish between actual zero and error, so it is unsafe for error handling.

2. Step 2: Use stoi with exception handling

   stoi throws exceptions on invalid input, allowing safe detection and handling of errors using try-catch blocks.

3. Final Answer:

   Use stoi inside try-catch block to catch exceptions -> Option B

4. Quick Check:

   stoi with try-catch is safe for invalid input [OK]

Hint: Use stoi with try-catch for safe string to int conversion [OK]

Common Mistakes:

• Relying on atoi to detect invalid input
• Ignoring exceptions from stoi
• Using printf to guess numbers