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

Recall & Review

beginner

What does the atoi function do?

It converts a string that represents a number into its integer form, ignoring leading spaces and handling optional '+' or '-' signs.

beginner

How does atoi handle leading spaces in the input string?

It skips all leading whitespace characters before processing the number.

intermediate

What should atoi do if the string contains non-digit characters after the number?

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

intermediate

How does atoi handle integer overflow or underflow?

It clamps the result to the maximum or minimum integer value if the parsed number goes beyond the integer limits.

beginner

Why is it important to check for '+' or '-' signs in atoi?

Because these signs determine whether the resulting integer is positive or negative.

What does atoi return when given the string " -42abc"?

A 0

B 42

C -42

D Error

Leading spaces are ignored, '-' sign is detected, parsing stops at 'a', so result is -42.

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

A 1234

B Error

C 0

D -1234

The '+' sign means positive number, so it returns 1234.

How does atoi treat an empty string ""?

A Returns 0

B Returns 1

C Returns error

D Returns max int

No digits found, so it returns 0.

What happens if the number in the string exceeds the max int value?

A Returns min int

B Throws exception

C Returns 0

D Returns max int

The result is clamped to the maximum integer value.

Which of these strings will atoi parse as 0?

A +0

B All of the above

C -0abc

D abc123

All either start with non-digit or represent zero, so result is 0.

Explain step-by-step how atoi converts the string " -00123abc" to an integer.

Think about ignoring spaces, sign, digits, and stopping at non-digit.

Describe how atoi handles integer overflow when parsing a very large number string.

Consider what happens when number is too big for int.

Practice

1. What does the atoi function do in the context of strings and integers?

easy

A. Converts an integer into a string representation

B. Converts a string representing a number into an integer value

C. Checks if a string contains only digits

D. Splits a string into individual characters

Solution

1. Step 1: Understand the purpose of atoi

   The function atoi stands for ASCII to integer, meaning it converts a string of digits into an integer number.

2. Step 2: Compare options with definition

   Only Converts a string representing a number into an integer value correctly describes converting a numeric string to an integer value.

3. Final Answer:

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

4. Quick Check:

   atoi converts string to int [OK]

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

Common Mistakes:

• Confusing atoi with integer to string conversion
• Thinking atoi splits strings
• Assuming atoi validates string digits only

2. Which of the following Go code snippets correctly converts the string "1234" to an integer using strconv.Atoi?

easy

A. num, err := strconv.Atoi("1234")

B. num := strconv.Atoi("1234")

C. num := strconv.Atoi(1234)

D. num, err := strconv.Atoi(1234)

Solution

1. Step 1: Check function signature of strconv.Atoi

   It returns two values: an integer and an error, so both must be captured.

2. Step 2: Verify argument type

   The argument must be a string, so "1234" is correct, not 1234 (int).

3. Final Answer:

   num, err := strconv.Atoi("1234") -> Option A

4. Quick Check:

   Correct syntax captures int and error [OK]

Hint: Always capture both int and error from strconv.Atoi [OK]

Common Mistakes:

• Not capturing the error return value
• Passing an integer instead of a string
• Using single return variable for two returns

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

package main
import (
  "fmt"
  "strconv"
)
func main() {
  s := "-42"
  num, err := strconv.Atoi(s)
  if err != nil {
    fmt.Println("Error")
  } else {
    fmt.Println(num)
  }
}

medium

A. -42

B. 42

C. Error

D. 0

Solution

1. Step 1: Understand input string and conversion

   The string "-42" is a valid integer representation including a negative sign.

2. Step 2: Check error handling and output

   Since "-42" is valid, err will be nil and num will be -42, so it prints -42.

3. Final Answer:

   -42 -> Option A

4. Quick Check:

   Valid negative string converts to negative int [OK]

Hint: Negative strings convert correctly with atoi [OK]

Common Mistakes:

• Assuming negative sign causes error
• Expecting absolute value output
• Ignoring error check

4. Identify the error in the following Go code snippet that uses strconv.Atoi:

package main
import (
  "fmt"
  "strconv"
)
func main() {
  s := "12a4"
  num, _ := strconv.Atoi(s)
  fmt.Println(num)
}

medium

A. Missing import of strconv package

B. Using fmt.Println instead of fmt.Print

C. Passing a string with letters to strconv.Atoi is allowed

D. Ignoring the error returned by strconv.Atoi

Solution

1. Step 1: Analyze input string validity

   The string "12a4" contains a letter 'a', so it is not a valid integer string.

2. Step 2: Check error handling in code

   The code ignores the error by using underscore _, so it prints zero value 0 without warning.

3. Final Answer:

   Ignoring the error returned by strconv.Atoi -> Option D

4. Quick Check:

   Always check error when converting strings [OK]

Hint: Never ignore error from atoi, always check it [OK]

Common Mistakes:

• Assuming invalid strings convert to partial numbers
• Ignoring error causes silent wrong output
• Thinking fmt.Println usage is wrong

5. You want to convert a list of strings ["10", "-5", "abc", "20"] to integers using strconv.Atoi. Which approach correctly handles invalid strings and collects only valid integers?

hard

A. Use strconv.Atoi only on first string, ignore rest

B. Convert all strings without error check and append results directly

C. Loop over strings, convert each with strconv.Atoi, append to result only if error is nil

D. Convert strings to integers using strconv.Itoa

Solution

1. Step 1: Understand the problem requirements

   We must convert multiple strings to integers and skip invalid ones safely.

2. Step 2: Evaluate each option's correctness

   Loop over strings, convert each with strconv.Atoi, append to result only if error is nil correctly loops, converts, checks error, and appends valid integers only.

3. Final Answer:

   Loop over strings, convert each with strconv.Atoi, append to result only if error is nil -> Option C

4. Quick Check:

   Check error for each conversion before appending [OK]

Hint: Check error for each string before adding integer [OK]

Common Mistakes:

• Appending invalid conversions without error check
• Using strconv.Itoa which converts int to string
• Ignoring invalid strings completely