Roman to Integer python
Question:
I am trying to solve this python problem with python3, my code looks like this.
class Solution:
def romanToInt(self, s: str) -> int:
# Define integer value to each roman
rom_val = {'I': 1, 'V': 5, 'X': 10, 'L': 50,
'C': 100, 'D': 500, 'M': 1000}
# A list of integer values
value = list(map(rom_val.get, s))
# The subtracted new number
new = 0
# The converted integer number
integer = 0
# List to keep the checked roman
checked = []
for i, j in enumerate(value):
if j > value[i+1] or j == value[i+1]:
checked.append(j)
if j < value[i+1]:
new = value[i+1] - j
checked.append(new)
return sum(checked)
However, I am getting IndexError: list index out of range on the first if statement.
Even though I know this is rather an easy type of question but there is something I don’t understand about.
So I have two questions:
1. of course, why I am getting this index error? how do I fix it?
2. is my way of solving this problem correct?
Thank you very much.
Answers:
Here is a different approach, 5 lines:
d = {'M':1000, 'D':500, 'C':100, 'L':50, 'X':10, 'V':5, 'I':1}
def romanToInt(self, s):
res, p = 0, 'I'
for c in s[::-1]:
res, p = res - d[c] if d[c] < d[p] else res + d[c], c
return res
Basically, going backward, adding each letter to result unless something smaller is in front of something larger in which case a subtraction instead of addition.
Note: The following is not a complete solution, just a fix for your error mentioned in the question. There is a major bug in your algorithm.
Like MCMXCIV should be 1994 but it returns 3099. This is because you consider C as 100 and M as 1000 but should be considering CM as 900. Since there is a solution above, I’ll leave this as an exercise for you.
Problem with your code is that even if you reach the last index, you check for i + 1. You can fix that like this:
def romanToInt(s: str) -> int:
# Define integer value to each roman
rom_val = {'I': 1, 'V': 5, 'X': 10, 'L': 50,
'C': 100, 'D': 500, 'M': 1000}
# A list of integer values
value = list(map(rom_val.get, s))
# List to keep the checked roman
checked = []
for i, j in enumerate(value):
if i == len(value) - 1:
checked.append(j)
elif j >= value[i+1]:
checked.append(j)
elif j < value[i+1]:
checked.append(value[i+1] - j)
print(checked)
return sum(checked)
print(romanToInt("LVIII"))
I have also made your code a bit more concise, removed unnecessary variables. The major change is only the check if it is the last index on in value.
Here, for example, I pass ‘II’ to your function than values becomes [1,1]. Now using enumerate you will have (1,1) as i,j in your second iteration which is causing the index error. Then you can solve this easily.
I know this is an answered post, but I would like to add 3 solutions with their respective runtimes to convert roman numerals to numbers.
Solution 1: (Approx Runtime = 52ms)
def romanToInt(self, s: str) -> int:
roman = {'I':1, 'V':5, 'X':10, 'L':50, 'C':100, 'D':500, 'M':1000 }
num = 0
for i in range(len(s)):
if i!= len(s)-1 and roman[s[i]] < roman[s[i+1]]:
num += roman[s[i]]*-1
else:
num += roman[s[i]]
return num
Solution 2: (Approx Runtime = 60ms)
def romanToInt(self, s: str) -> int:
roman = {'I':1, 'V':5, 'X':10, 'L':50, 'C':100, 'D':500, 'M':1000 }
num = 0
s = s.replace("IV", "IIII").replace("IX", "VIIII")
s = s.replace("XL", "XXXX").replace("XC", "LXXXX")
s = s.replace("CD", "CCCC").replace("CM", "DCCCC")
for x in s:
num += roman[x]
return num
Solution 3: (Approx Runtime = 48ms)
def romanToInt(self, s: str) -> int:
roman = {'I':1, 'V':5, 'X':10, 'L':50, 'C':100, 'D':500, 'M':1000 }
num = 0
for i in range(len(s)-1):
if roman[s[i]] < roman[s[i+1]]:
num += roman[s[i]]*-1
continue
num += roman[s[i]]
num +=roman[s[-1]]
return num
The simplest solution appears to be the best at times 🙂
This question can be solved with the idea that every roman numeral in the roman number are arranged in descending order if that’s not the case then it is a special case like ‘IV’, ‘IX’, ‘XL’, …..
In the below the above peculiar case is handled by the if statement.
def romanToInt(s):
mapping = {
'I': 1,
'V': 5,
'X': 10,
'L': 50,
'C': 100,
'D': 500,
'M': 1000,
}
min_ = None
total = 0
for c in s:
val = mapping[c]
print(val)
if min_ and val > min_:
total -= min_*2
else:
min_ = val
total += val
return total
Take a look at this code. It’s easy to understand the logic.
Note:
The idea is to traverse the roman number in a reverse manner character by character and add if the next character is greater than or equal to the previous one.
def romanToDecimal(self, S):
d = {
'I':1, 'V':5, 'X':10, 'L':50, 'C':100, 'D':500, 'M':1000
}
res = 0
old = 'I'
for i in S[::-1]:
if d[i] >= d[old]:
res = res + d[i]
else:
res = res - d[i]
old = i
return res
I am trying to solve this python problem with python3, my code looks like this.
class Solution:
def romanToInt(self, s: str) -> int:
# Define integer value to each roman
rom_val = {'I': 1, 'V': 5, 'X': 10, 'L': 50,
'C': 100, 'D': 500, 'M': 1000}
# A list of integer values
value = list(map(rom_val.get, s))
# The subtracted new number
new = 0
# The converted integer number
integer = 0
# List to keep the checked roman
checked = []
for i, j in enumerate(value):
if j > value[i+1] or j == value[i+1]:
checked.append(j)
if j < value[i+1]:
new = value[i+1] - j
checked.append(new)
return sum(checked)
However, I am getting IndexError: list index out of range on the first if statement.
Even though I know this is rather an easy type of question but there is something I don’t understand about.
So I have two questions:
1. of course, why I am getting this index error? how do I fix it?
2. is my way of solving this problem correct?
Thank you very much.
Here is a different approach, 5 lines:
d = {'M':1000, 'D':500, 'C':100, 'L':50, 'X':10, 'V':5, 'I':1}
def romanToInt(self, s):
res, p = 0, 'I'
for c in s[::-1]:
res, p = res - d[c] if d[c] < d[p] else res + d[c], c
return res
Basically, going backward, adding each letter to result unless something smaller is in front of something larger in which case a subtraction instead of addition.
Note: The following is not a complete solution, just a fix for your error mentioned in the question. There is a major bug in your algorithm.
Like MCMXCIV should be 1994 but it returns 3099. This is because you consider C as 100 and M as 1000 but should be considering CM as 900. Since there is a solution above, I’ll leave this as an exercise for you.
Problem with your code is that even if you reach the last index, you check for i + 1. You can fix that like this:
def romanToInt(s: str) -> int:
# Define integer value to each roman
rom_val = {'I': 1, 'V': 5, 'X': 10, 'L': 50,
'C': 100, 'D': 500, 'M': 1000}
# A list of integer values
value = list(map(rom_val.get, s))
# List to keep the checked roman
checked = []
for i, j in enumerate(value):
if i == len(value) - 1:
checked.append(j)
elif j >= value[i+1]:
checked.append(j)
elif j < value[i+1]:
checked.append(value[i+1] - j)
print(checked)
return sum(checked)
print(romanToInt("LVIII"))
I have also made your code a bit more concise, removed unnecessary variables. The major change is only the check if it is the last index on in value.
Here, for example, I pass ‘II’ to your function than values becomes [1,1]. Now using enumerate you will have (1,1) as i,j in your second iteration which is causing the index error. Then you can solve this easily.
I know this is an answered post, but I would like to add 3 solutions with their respective runtimes to convert roman numerals to numbers.
Solution 1: (Approx Runtime = 52ms)
def romanToInt(self, s: str) -> int:
roman = {'I':1, 'V':5, 'X':10, 'L':50, 'C':100, 'D':500, 'M':1000 }
num = 0
for i in range(len(s)):
if i!= len(s)-1 and roman[s[i]] < roman[s[i+1]]:
num += roman[s[i]]*-1
else:
num += roman[s[i]]
return num
Solution 2: (Approx Runtime = 60ms)
def romanToInt(self, s: str) -> int:
roman = {'I':1, 'V':5, 'X':10, 'L':50, 'C':100, 'D':500, 'M':1000 }
num = 0
s = s.replace("IV", "IIII").replace("IX", "VIIII")
s = s.replace("XL", "XXXX").replace("XC", "LXXXX")
s = s.replace("CD", "CCCC").replace("CM", "DCCCC")
for x in s:
num += roman[x]
return num
Solution 3: (Approx Runtime = 48ms)
def romanToInt(self, s: str) -> int:
roman = {'I':1, 'V':5, 'X':10, 'L':50, 'C':100, 'D':500, 'M':1000 }
num = 0
for i in range(len(s)-1):
if roman[s[i]] < roman[s[i+1]]:
num += roman[s[i]]*-1
continue
num += roman[s[i]]
num +=roman[s[-1]]
return num
The simplest solution appears to be the best at times 🙂
This question can be solved with the idea that every roman numeral in the roman number are arranged in descending order if that’s not the case then it is a special case like ‘IV’, ‘IX’, ‘XL’, …..
In the below the above peculiar case is handled by the if statement.
def romanToInt(s):
mapping = {
'I': 1,
'V': 5,
'X': 10,
'L': 50,
'C': 100,
'D': 500,
'M': 1000,
}
min_ = None
total = 0
for c in s:
val = mapping[c]
print(val)
if min_ and val > min_:
total -= min_*2
else:
min_ = val
total += val
return total
Take a look at this code. It’s easy to understand the logic.
Note:
The idea is to traverse the roman number in a reverse manner character by character and add if the next character is greater than or equal to the previous one.
def romanToDecimal(self, S):
d = {
'I':1, 'V':5, 'X':10, 'L':50, 'C':100, 'D':500, 'M':1000
}
res = 0
old = 'I'
for i in S[::-1]:
if d[i] >= d[old]:
res = res + d[i]
else:
res = res - d[i]
old = i
return res