String to Integer atoi in DSA C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the atoi function do in C?

It converts a string representing a number into its integer value. For example, "123" becomes the integer 123.

beginner

What should atoi do when the string contains leading spaces?

It should ignore all leading spaces before starting to convert digits into an integer.

beginner

How does atoi handle signs like '+' or '-' in the string?

If the string starts with '+' or '-', atoi uses it to determine if the resulting integer is positive or negative.

intermediate

What happens if atoi encounters non-digit characters after reading digits?

It stops converting at the first non-digit character and returns the integer value parsed so far.

advanced

How should atoi handle integer overflow or underflow?

It should clamp the result to the maximum or minimum integer value allowed (e.g., INT_MAX or INT_MIN) to avoid overflow.

What is the output of atoi(" -42abc")?

A 42

B -42

C 0

D -420

Leading spaces are ignored, '-' sign makes the number negative, and parsing stops at first non-digit character 'a'.

If the input string is "+1234", what integer does atoi return?

A 1234

B -1234

C 0

D 1

The '+' sign indicates a positive number, so the integer returned is 1234.

What should atoi return for the input "words and 987"?

A 0

B 987

C words

D -987

Since the string does not start with digits or sign, atoi returns 0.

How does atoi treat the string "2147483648" if INT_MAX is 2147483647?

A -2147483648

B 2147483648

C 0

D 2147483647

The number exceeds INT_MAX, so atoi clamps it to INT_MAX.

Which step is NOT part of a typical atoi implementation?

A Ignoring leading spaces

B Parsing digits until a non-digit character

C Converting letters to their ASCII values

D Handling optional '+' or '-' sign

Letters are not converted to ASCII values; parsing stops when a non-digit character is found.

Explain step-by-step how to convert a string like " -123abc" to an integer using the atoi logic.

Think about reading the string from left to right carefully.

Describe how to handle integer overflow when converting a string to an integer with atoi.

Remember the limits of 32-bit signed integers.

Practice

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

easy

A. Checks if a string contains only digits

B. Converts a string representing a number into an integer

C. Calculates the length of a string

D. Converts an integer into a string

Solution

1. Step 1: Understand the purpose of atoi

   The atoi function stands for ASCII to integer, which means it converts a string of digits into an integer value.

2. Step 2: Compare options with definition

   Only Converts a string representing a number into an integer correctly describes this conversion from string to integer.

3. Final Answer:

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

4. Quick Check:

   atoi = string to integer conversion [OK]

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

Common Mistakes:

• Confusing atoi with itoa (integer to ASCII)
• Thinking atoi calculates string length
• Assuming atoi validates string digits

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

easy

A. int num = atoi(char *str);

B. int num = atoi();

C. int num = atoi(int str);

D. int num = atoi(str);

Solution

1. Step 1: Check function parameter type

   The atoi function takes a single argument of type const char *, which is a string pointer.

2. Step 2: Validate correct syntax

   int num = atoi(str); correctly shows calling atoi with a string variable str without specifying type in call.

3. Final Answer:

   int num = atoi(str); -> Option D

4. Quick Check:

   atoi takes string variable as argument [OK]

Hint: Call atoi with just the string variable name [OK]

Common Mistakes:

• Including type in function call parentheses
• Calling atoi without arguments
• Passing integer instead of string

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

char str[] = "-123abc";
int val = atoi(str);
printf("%d", val);

medium

A. -123

B. 0

C. 123

D. Error

Solution

1. Step 1: Analyze atoi behavior with mixed string

   atoi converts initial numeric part including sign until non-digit character. Here, "-123" is converted, stops at 'a'.

2. Step 2: Determine output value

   So, val becomes -123 and printf prints -123.

3. Final Answer:

   -123 -> Option A

4. Quick Check:

   atoi stops at first non-digit, returns -123 [OK]

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

Common Mistakes:

• Expecting atoi to ignore leading sign
• Thinking atoi returns 0 on trailing letters
• Assuming runtime error due to letters

4. Identify the error in this code snippet using atoi:

char *str = NULL;
int num = atoi(str);
printf("%d", num);

medium

A. Passing NULL pointer to atoi causes undefined behavior

B. Missing header file

C. atoi cannot convert strings with digits

D. No error, code runs fine

Solution

1. Step 1: Check argument passed to atoi

   The code passes a NULL pointer to atoi, which expects a valid string pointer.

2. Step 2: Understand consequences of NULL argument

   Passing NULL leads to undefined behavior, often causing a crash or runtime error.

3. Final Answer:

   Passing NULL pointer to atoi causes undefined behavior -> Option A

4. Quick Check:

   atoi needs valid string pointer, NULL causes error [OK]

Hint: Never pass NULL to atoi; always check string pointer [OK]

Common Mistakes:

• Ignoring NULL pointer checks
• Assuming atoi handles NULL safely
• Confusing missing header with runtime error

5. You want to convert a string to integer but also detect if the string contains invalid characters after the number. Which approach is best in C?

hard

A. Use atoi and then check the rest of the string manually

B. Use atoi and assume input is always valid

C. Use strtol which provides end pointer to check leftover characters

D. Use printf to convert string to integer

Solution

1. Step 1: Understand limitation of atoi

   atoi converts initial number but does not provide info about leftover invalid characters.

2. Step 2: Use strtol for better control

   strtol converts string to long and returns pointer to first invalid character, allowing validation.

3. Step 3: Choose best approach

   Use strtol which provides end pointer to check leftover characters uses strtol with end pointer to detect invalid trailing characters safely.

4. Final Answer:

   Use strtol which provides end pointer to check leftover characters -> Option C

5. Quick Check:

   strtol + end pointer = safe string to int with validation [OK]

Hint: Use strtol with end pointer to detect invalid chars [OK]

Common Mistakes:

• Relying only on atoi for input validation
• Ignoring leftover characters after number
• Using printf incorrectly for conversion