String to Integer atoi in DSA Go - Time & Space Complexity

Time Complexity: String to Integer atoi

O(n)

Understanding Time Complexity

We want to understand how the time to convert a string to an integer grows as the string gets longer.

How does the number of steps change when the input string size increases?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

func myAtoi(s string) int {
    i, n := 0, len(s)
    sign := 1
    result := 0

    // Skip spaces
    for i < n && s[i] == ' ' {
        i++
    }

    // Check sign
    if i < n {
        if s[i] == '-' {
            sign = -1
            i++
        } else if s[i] == '+' {
            i++
        }
    }

    // Convert digits
    for i < n && s[i] >= '0' && s[i] <= '9' {
        result = result*10 + int(s[i]-'0')
        i++
    }

    return sign * result
}

This code converts a string to an integer by skipping spaces, checking the sign, and then reading digits one by one.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The loops that scan characters in the string.
• How many times: Each loop runs at most once over the string characters, moving forward without going back.

How Execution Grows With Input

As the input string gets longer, the code checks each character once until it finishes or stops at a non-digit.

Input Size (n)	Approx. Operations
10	About 10 character checks
100	About 100 character checks
1000	About 1000 character checks

Pattern observation: The number of steps grows roughly in direct proportion to the string length.

Final Time Complexity

Time Complexity: O(n)

This means the time to convert grows linearly with the length of the input string.

Common Mistake

[X] Wrong: "The conversion takes constant time because it just reads the number."

[OK] Correct: The code must check each character to decide if it is a digit and to build the number, so time depends on input length.

Interview Connect

Understanding this linear time behavior helps you explain how string parsing scales and shows you can analyze simple loops clearly.

Self-Check

"What if the input string was guaranteed to have only digits and no spaces or signs? How would the time complexity change?"

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