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'9'

B'1'

C'a'

D'0'

Attempts:

3 left

💡 Hint

Common Mistakes

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

Using character 'a' causes incorrect subtraction.

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

2fill in blank

medium

Complete the code to skip leading spaces in the string.

DSA C++

while (str[i] == '[1]') i++;

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

A'\n'

B'\t'

C' '

D'0'

Attempts:

3 left

💡 Hint

Common Mistakes

Using '\n' skips newlines, not spaces.

Using '0' skips digits, not spaces.

Leading spaces are represented by the space character ' '.

3fill in blank

hard

Fix the error in checking if the current character is a digit.

DSA C++

if (str[i] >= '[1]' && str[i] <= '9') {

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

A'0'

B'a'

C'1'

D' '

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'a' checks for letters, not digits.

Using ' ' checks for space, not digits.

Digits in ASCII range from '0' to '9'.

4fill in blank

hard

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

DSA C++

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

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

A*

B+

C-

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-' instead of '+' causes wrong result.

Using '/' instead of '*' breaks number building.

Multiply result by 10 and add the new digit value.

5fill in blank

hard

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

DSA C++

int sign = [1];
if (str[i] == '-') {
    sign = [2];
    i++;
}
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 causes wrong output.

Not updating sign when '-' found causes incorrect sign.

Sign is 1 by default, -1 if negative sign found, multiply result by sign.

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