RosettaCodeData/Task/Roman-numerals-Decode/BBC-BASIC/roman-numerals-decode.basic

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PRINT "MCMXCIX", FNromandecode("MCMXCIX")
PRINT "MMXII", FNromandecode("MMXII")
PRINT "MDCLXVI", FNromandecode("MDCLXVI")
PRINT "MMMDCCCLXXXVIII", FNromandecode("MMMDCCCLXXXVIII")
END
DEF FNromandecode(roman$)
LOCAL i%, j%, p%, n%, r%()
DIM r%(7) : r%() = 0,1,5,10,50,100,500,1000
FOR i% = LEN(roman$) TO 1 STEP -1
j% = INSTR("IVXLCDM", MID$(roman$,i%,1))
IF j%=0 ERROR 100, "Invalid character"
IF j%>=p% n% += r%(j%) ELSE n% -= r%(j%)
p% = j%
NEXT
= n%