Roman to Integer Conversion in DSA Javascript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Roman to Integer Conversion Function

What is the output of the following JavaScript function when input is 'MCMXCIV'?

DSA Javascript

function romanToInt(s) {
  const map = {I:1, V:5, X:10, L:50, C:100, D:500, M:1000};
  let total = 0;
  for (let i = 0; i < s.length; i++) {
    const current = map[s[i]];
    const next = map[s[i+1]];
    if (next && current < next) {
      total -= current;
    } else {
      total += current;
    }
  }
  return total;
}
console.log(romanToInt('MCMXCIV'));

A 1994

B 1904

C 2094

D None of the above

💡 Hint

Remember that when a smaller numeral appears before a larger one, it is subtracted.

The Roman numeral 'MCMXCIV' breaks down as M (1000) + CM (900) + XC (90) + IV (4) = 1994.

❓ Predict Output

intermediate

Output of Roman to Integer Conversion with Repeated Numerals

What is the output of the function romanToInt when input is 'III'?

DSA Javascript

function romanToInt(s) {
  const map = {I:1, V:5, X:10, L:50, C:100, D:500, M:1000};
  let total = 0;
  for (let i = 0; i < s.length; i++) {
    const current = map[s[i]];
    const next = map[s[i+1]];
    if (next && current < next) {
      total -= current;
    } else {
      total += current;
    }
  }
  return total;
}
console.log(romanToInt('III'));

A 3

B 4

C 2

D 5

💡 Hint

Repeated numerals add up their values.

'III' is three times 'I' which is 1, so total is 3.

❓ Predict Output

advanced

Output of Roman to Integer Conversion with Subtraction Cases

What is the output of the function romanToInt when input is 'XLII'?

DSA Javascript

function romanToInt(s) {
  const map = {I:1, V:5, X:10, L:50, C:100, D:500, M:1000};
  let total = 0;
  for (let i = 0; i < s.length; i++) {
    const current = map[s[i]];
    const next = map[s[i+1]];
    if (next && current < next) {
      total -= current;
    } else {
      total += current;
    }
  }
  return total;
}
console.log(romanToInt('XLII'));

A 52

B 44

C 40

D 42

💡 Hint

XL means 40 because X (10) is before L (50), so subtract 10 from 50.

'XLII' = XL (40) + I (1) + I (1) = 42.

🧠 Conceptual

advanced

Why does the Roman to Integer Conversion use subtraction?

Why does the algorithm subtract the value of a Roman numeral when it appears before a larger numeral?

A Because the algorithm treats all numerals as negative values initially.

B Because the algorithm always subtracts the first numeral regardless of position.

C Because in Roman numerals, a smaller numeral before a larger one means subtraction.

D Because the algorithm adds all numerals and then subtracts the total at the end.

💡 Hint

Think about how Roman numerals like IV and IX are formed.

Roman numerals use subtraction when a smaller numeral precedes a larger one to represent numbers like 4 (IV) or 9 (IX).

🔧 Debug

expert

Identify the error in this Roman to Integer Conversion code

What error does the following code produce when input is 'IX'?

DSA Javascript

function romanToInt(s) {
  const map = {I:1, V:5, X:10, L:50, C:100, D:500, M:1000};
  let total = 0;
  for (let i = 0; i < s.length; i++) {
    const current = map[s[i]];
    const next = map[s[i+1]];
    if (next && current < next) {
      total -= current;
    } else {
      total += current;
    }
  }
  return total;
}
console.log(romanToInt('IX'));

A SyntaxError due to loop condition

B Returns 9 correctly

C TypeError: Cannot read property of undefined

D Returns 11 incorrectly

💡 Hint

Check the loop boundary and array indexing carefully.

The original loop condition was i <= s.length, causing s[i] to be undefined and map[s[i]] to throw a TypeError. Changing it to i < s.length fixes the error.

Practice

1. What is the main rule to convert Roman numerals to integers?

easy

A. Subtract all values from the largest symbol

B. Add values of symbols, subtract if a smaller symbol is before a larger one

C. Add values only if symbols are the same

D. Multiply values of symbols in order

Solution

1. Step 1: Understand Roman numeral rules

   Roman numerals are added normally, but if a smaller numeral appears before a larger one, it is subtracted.

2. Step 2: Apply addition and subtraction rules

   For example, IV is 5 - 1 = 4, while VI is 5 + 1 = 6.

3. Final Answer:

   Add values of symbols, subtract if a smaller symbol is before a larger one -> Option B

4. Quick Check:

   Roman numeral rules = Add and subtract [OK]

Hint: Subtract when smaller numeral precedes larger one [OK]

Common Mistakes:

• Always adding without subtraction
• Multiplying values incorrectly
• Ignoring order of symbols

2. Which of the following is the correct JavaScript syntax to get the integer value of a Roman numeral character from a map object named romanMap?

easy

A. romanMap['V']

B. romanMap.get('V')

C. romanMap.V

D. romanMap('V')

Solution

1. Step 1: Identify the data structure type

   If romanMap is a plain JavaScript object, access values using square brackets with the key as a string.

2. Step 2: Choose correct syntax for object property access

   Using romanMap['V'] correctly accesses the value for key 'V'. romanMap.get('V') is for Map objects, not plain objects.

3. Final Answer:

   romanMap['V'] -> Option A

4. Quick Check:

   Object property access = romanMap['V'] [OK]

Hint: Use square brackets for object keys as strings [OK]

Common Mistakes:

• Using .get() on plain objects
• Using dot notation with string keys
• Calling object like a function

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

function romanToInt(s) {
  const map = {I:1, V:5, X:10, L:50, C:100, D:500, M:1000};
  let total = 0;
  for(let i = 0; i < s.length; i++) {
    const current = map[s[i]];
    const next = map[s[i+1]] || 0;
    if(current < next) {
      total -= current;
    } else {
      total += current;
    }
  }
  return total;
}
console.log(romanToInt('IX'));

medium

A. 9

B. 11

C. 1

D. Error

Solution

1. Step 1: Trace the loop for input 'IX'

   For 'I' (1), next is 'X' (10), since 1 < 10, subtract 1 (total = -1). For 'X' (10), next is undefined (0), so add 10 (total = 9).

2. Step 2: Calculate final total

   Total after loop is 9, which is the integer value of 'IX'.

3. Final Answer:

   9 -> Option A

4. Quick Check:

   'IX' = 9 [OK]

Hint: Subtract if current < next, else add [OK]

Common Mistakes:

• Adding all values without subtraction
• Ignoring next character check
• Off-by-one errors in loop

4. Find the error in this JavaScript function for Roman to Integer conversion:

function romanToInt(s) {
  const map = {I:1, V:5, X:10, L:50, C:100, D:500, M:1000};
  let total = 0;
  for(let i = 0; i <= s.length; i++) {
    const current = map[s[i]];
    const next = map[s[i+1]] || 0;
    if(current < next) {
      total -= current;
    } else {
      total += current;
    }
  }
  return total;
}

medium

A. Use '==' instead of '<' in if condition

B. map object keys should be lowercase

C. total should be initialized to 1

D. Loop condition should be i < s.length, not i <= s.length

Solution

1. Step 1: Analyze loop boundary

   The loop runs from i = 0 to i <= s.length, which means it accesses s[s.length], an undefined index causing current to be undefined.

2. Step 2: Correct loop condition

   Changing loop condition to i < s.length prevents accessing out-of-bound index and avoids errors.

3. Final Answer:

   Loop condition should be i < s.length, not i <= s.length -> Option D

4. Quick Check:

   Loop boundary off-by-one error fixed [OK]

Hint: Use i < length to avoid out-of-bound errors [OK]

Common Mistakes:

• Using <= instead of < in loops
• Wrong case for map keys
• Incorrect comparison operators

5. Given the Roman numeral string 'MCMXCIV', what integer should the conversion function return?

hard

A. 2004

B. 1904

C. 1994

D. 1094

Solution

1. Step 1: Break down the Roman numeral 'MCMXCIV'

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

2. Step 2: Add all parts

   1000 + 900 + 90 + 4 = 1994 total.

3. Final Answer:

   1994 -> Option C

4. Quick Check:

   'MCMXCIV' = 1994 [OK]

Hint: Identify subtractive pairs and sum all values [OK]

Common Mistakes:

• Adding all numerals without subtraction
• Misreading subtractive pairs
• Confusing similar numerals