python - Roman to Numeral calculator with "for i in X" -

i tried build calculator convert roman numeral can tell me how can choose 2 characters words in high priority?

** roman string

#roman numeral calculator  def romantodecimal(roman): decimalvalue=0 in roman:     if == "cm":         decimalvalue += 900     if == "iv":         decimalvalue += 4     if == "ix":         decimalvalue += 9     if == "xl":         decimalvalue += 40     if == "xc":         decimalvalue += 90     if == "cd":         decimalvalue += 400     elif in roman:         if == "i":             decimalvalue += 1         if == "v":             decimalvalue += 5         if == "x":             decimalvalue += 10         if == "l":             decimalvalue += 50         if == "c":             decimalvalue += 100         if == "d":             decimalvalue += 500         if == "m":             decimalvalue += 1000  return decimalvalue 

and i'm new here please try answer me patience

while there more efficient ways this, follows style of own code easier follow.

the main change replace for loop while loop , manual counter. makes possible skip ahead 2 steps when encounter 1 of our double-chars. next extract characters , inspect them — first looking 2 chars ahead, next looking individually. if find match @ 2 chars long step forward 2 (past them) , restart loop:

def romantodecimal(roman):     decimalvalue=0      = 0 # position in string      while < len(roman):          # test 2 char first             if < len(roman)-1: # make sure we're not on last letter             # current + next letters             s = roman[i:i+2]              add = false              if s == "cm":                 add = 900             elif s == "iv":                 add = 4             elif s == "ix":                 add = 9             elif s == "xl":                 add = 40             elif s == "xc":                 add = 90             elif s == "cd":                 add = 400              if add: # we've found match                 += 2 # step 2                 decimalvalue += add # add value                 continue # next loop          # if here 2 character match failed         # single char match         s = roman[i]           if s == "i":             decimalvalue += 1         elif s == "v":             decimalvalue += 5         elif s == "x":             decimalvalue += 10         elif s == "l":             decimalvalue += 50         elif s == "c":             decimalvalue += 100         elif s == "d":             decimalvalue += 500         elif s == "m":             decimalvalue += 1000          += 1      return decimalvalue  romantodecimal('xixix') 28 

it should noted above not really valid roman numeral string (28 correctly represented xxviii), that's not problem!

let me know if code unclear anywhere , add more comments.