Roman to Integer Conversion in DSA Go - Build from Scratch

Roman to Integer Conversion

📖 Scenario: You are building a simple tool to convert Roman numerals into regular numbers. Roman numerals are used in clocks, book chapters, and movie titles. Your program will help translate these into numbers we use every day.

🎯 Goal: Create a Go program that converts a Roman numeral string into its integer value using a map and a loop.

📋 What You'll Learn

Create a map with Roman numeral characters as keys and their integer values as values.

Create a variable to hold the total integer value.

Use a for loop to iterate over the Roman numeral string and apply the conversion logic.

Print the final integer value.

💡 Why This Matters

🌍 Real World

Roman numerals appear in clocks, book chapters, movie titles, and historical dates. Converting them to integers helps computers understand and process these values.

💼 Career

Understanding how to map characters to values and process strings with loops and conditions is fundamental in programming jobs, especially in data parsing and conversion tasks.

1

Create the Roman numeral map

Create a map called romanMap with these exact entries: 'I': 1, 'V': 5, 'X': 10, 'L': 50, 'C': 100, 'D': 500, 'M': 1000.

DSA Go

// Create the romanMap with Roman numeral keys and integer values
// Your code here

Hint

Use map[rune]int to create the map with rune keys and int values.

2

Create the input Roman numeral and total variable

Create a string variable called roman and set it to "MCMXCIV". Create an integer variable called total and set it to 0.

DSA Go

package main

import "fmt"

func main() {
    romanMap := map[rune]int{'I': 1, 'V': 5, 'X': 10, 'L': 50, 'C': 100, 'D': 500, 'M': 1000}
    // Create roman string and total integer
    // Your code here
}

Hint

Use roman := "MCMXCIV" and total := 0.

3

Convert Roman numeral to integer

Use a for loop with index i from 0 to len(roman)-1. Inside the loop, get the value of the current Roman character from romanMap. If i + 1 is less than len(roman) and the value of the current character is less than the value of the next character, subtract the current value from total. Otherwise, add the current value to total.

DSA Go

package main

import "fmt"

func main() {
    romanMap := map[rune]int{'I': 1, 'V': 5, 'X': 10, 'L': 50, 'C': 100, 'D': 500, 'M': 1000}
    roman := "MCMXCIV"
    total := 0
    // Add for loop to convert roman to integer
    // Your code here
}

Hint

Use for i := 0; i < len(roman); i++ and check if current value is less than next value to decide addition or subtraction.

4

Print the integer result

Print the value of total using fmt.Println(total).

DSA Go

package main

import "fmt"

func main() {
    romanMap := map[rune]int{'I': 1, 'V': 5, 'X': 10, 'L': 50, 'C': 100, 'D': 500, 'M': 1000}
    roman := "MCMXCIV"
    total := 0
    for i := 0; i < len(roman); i++ {
        value := romanMap[rune(roman[i])]
        if i+1 < len(roman) && value < romanMap[rune(roman[i+1])] {
            total -= value
        } else {
            total += value
        }
    }
    // Print the total
    // Your code here
}

Hint

Use fmt.Println(total) to display the final integer.

Practice

1. What is the value of the Roman numeral IX when converted to an integer?

easy

A. 6

B. 11

C. 19

D. 9

Solution

1. Step 1: Understand Roman numeral rules for subtraction

   In Roman numerals, when a smaller numeral appears before a larger one, it is subtracted. Here, I (1) is before X (10), so 1 is subtracted from 10.

2. Step 2: Calculate the integer value

   10 - 1 = 9.

3. Final Answer:

   9 -> Option D

4. Quick Check:

   IX = 9 [OK]

Hint: Subtract when smaller numeral is before larger one [OK]

Common Mistakes:

• Adding values instead of subtracting
• Confusing IX with XI
• Ignoring subtraction rule

2. Which of the following Go code snippets correctly initializes a map for Roman numeral values?

easy

A. romanMap := map[string]int{"I" = 1, "V" = 5, "X" = 10}

B. romanMap := map[int]string{1: "I", 5: "V", 10: "X"}

C. romanMap := map[string]int{"I": 1, "V": 5, "X": 10}

D. romanMap := map[string]int{I: 1, V: 5, X: 10}

Solution

1. Step 1: Check map key and value types

   The map should have string keys (Roman numerals) and int values (their integer equivalents). romanMap := map[string]int{"I": 1, "V": 5, "X": 10} uses map[string]int correctly.

2. Step 2: Verify syntax correctness

   romanMap := map[string]int{"I": 1, "V": 5, "X": 10} uses correct Go syntax with colon ':' to assign values. Options A and C have syntax errors, and B reverses key-value types.

3. Final Answer:

   romanMap := map[string]int{"I": 1, "V": 5, "X": 10} -> Option C

4. Quick Check:

   Correct map syntax uses colons and proper types [OK]

Hint: Use colon ':' for map key-value pairs in Go [OK]

Common Mistakes:

• Using '=' instead of ':' in map initialization
• Swapping key and value types
• Omitting quotes around string keys

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

func romanToInt(s string) int {
    romanMap := map[byte]int{'I': 1, 'V': 5, 'X': 10}
    total := 0
    for i := 0; i < len(s); i++ {
        if i+1 < len(s) && romanMap[s[i]] < romanMap[s[i+1]] {
            total -= romanMap[s[i]]
        } else {
            total += romanMap[s[i]]
        }
    }
    return total
}

fmt.Println(romanToInt("IV"))

medium

A. 6

B. 4

C. 9

D. Error

Solution

1. Step 1: Analyze the loop and subtraction logic

   The code subtracts the value if the current numeral is less than the next one, else adds it. For "IV", 'I' (1) < 'V' (5), so subtract 1, then add 5.

2. Step 2: Calculate total value

   Total = -1 + 5 = 4.

3. Final Answer:

   4 -> Option B

4. Quick Check:

   IV = 4 using subtraction rule [OK]

Hint: Subtract when current < next numeral [OK]

Common Mistakes:

• Adding all values without subtraction
• Index out of range errors
• Confusing byte and string keys in map

4. The following Go function is intended to convert Roman numerals to integers. What is the bug?

func romanToInt(s string) int {
    romanMap := map[byte]int{'I': 1, 'V': 5, 'X': 10}
    total := 0
    for i := 0; i < len(s); i++ {
        if romanMap[s[i]] < romanMap[s[i+1]] {
            total -= romanMap[s[i]]
        } else {
            total += romanMap[s[i]]
        }
    }
    return total
}

medium

A. Missing check for i+1 < len(s) causing index out of range

B. Using map with byte keys instead of string keys

C. Incorrect subtraction logic for Roman numerals

D. Returning total before loop ends

Solution

1. Step 1: Identify loop boundary issue

   The code accesses s[i+1] without checking if i+1 is within string length, causing possible index out of range error.

2. Step 2: Explain fix

   Adding condition 'i+1 < len(s)' before accessing s[i+1] prevents runtime error.

3. Final Answer:

   Missing check for i+1 < len(s) causing index out of range -> Option A

4. Quick Check:

   Always check bounds before accessing next element [OK]

Hint: Check i+1 < len(s) before accessing s[i+1] [OK]

Common Mistakes:

• Ignoring string length check for next index
• Confusing byte and string keys (not a bug here)
• Incorrect subtraction logic (logic is correct)

5. Given the Roman numeral MCMXCIV, what integer does it convert to using the standard Roman to Integer rules?

hard

A. 1994

B. 1984

C. 2004

D. 1949

Solution

1. Step 1: Break down the numeral into parts

   M = 1000, CM = 900 (100 before 1000 means subtract 100), XC = 90 (10 before 100), IV = 4 (1 before 5).

2. Step 2: Sum all parts

   1000 + 900 + 90 + 4 = 1994.

3. Final Answer:

   1994 -> Option A

4. Quick Check:

   MCMXCIV = 1994 by adding and subtracting parts [OK]

Hint: Add values, subtract when smaller before larger [OK]

Common Mistakes:

• Adding all numerals without subtraction
• Misreading CM as 1100 instead of 900
• Ignoring subtraction pairs