String to Integer atoi in DSA Go - Build from Scratch

String to Integer atoi

📖 Scenario: Imagine you are building a simple calculator app. Users enter numbers as text, but your program needs to convert these text numbers into actual numbers to do math.

🎯 Goal: Build a small program that converts a string containing digits into an integer number, similar to the atoi function.

📋 What You'll Learn

Create a string variable with a number as text

Create a variable to hold the result number

Write a loop to convert each character to its digit value and build the integer

Print the final integer number

💡 Why This Matters

🌍 Real World

Converting user input from text to numbers is common in calculators, forms, and data processing apps.

💼 Career

Understanding how to parse strings into numbers is a fundamental skill for software developers working with input validation and data conversion.

1

Create the input string

Create a string variable called input and set it to the exact value "12345".

DSA Go

# Create a string variable called input with value "12345"
# Your code here

Hint

Use input := "12345" to create the string variable.

2

Create the result variable

Create an integer variable called result and set it to 0.

DSA Go

package main

func main() {
    input := "12345"
    // Create an integer variable called result and set it to 0
    // Your code here
}

Hint

Use result := 0 to create the integer variable.

3

Convert string to integer

Use a for loop with variable i to iterate over the string input. Inside the loop, convert each character to its digit value by subtracting '0', then update result by multiplying it by 10 and adding the digit.

DSA Go

package main

func main() {
    input := "12345"
    result := 0
    // Use a for loop with i to iterate over input
    // Convert each character to digit and update result
    // Your code here
}

Hint

Use for i := 0; i < len(input); i++ to loop. Convert with int(input[i] - '0').

4

Print the integer result

Use fmt.Println(result) to print the integer value stored in result. Remember to import the fmt package.

DSA Go

package main

import "fmt"

func main() {
    input := "12345"
    result := 0
    for i := 0; i < len(input); i++ {
        digit := int(input[i] - '0')
        result = result*10 + digit
    }
    // Print the result
    // Your code here
}

Hint

Use fmt.Println(result) to show the number.

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