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.