Roman numeral to decimal converter

Question

Program written in TI-Basic that works on TI-83/84/+/SE calculators. Automatically detects whether input is a number of a Roman numeral, and converts to the opposite. Please try to optimize for size, if possible.

DelVar IDelVar VDelVar XDelVar LDelVar CDelVar DDelVar MClrHome
Input Str1
expr(Str1->A
If A
Then
A->B
"?->Str2
For(I,1,iPart(A/|E3
Str2+"M->Str2
B-|E3->B
End
If B>899
Then
Str2+"CM->Str2
B-900->B
End
If B>499
Then
Str2+"D->Str2
B-500->B
End
If B>399
Then
Str2+"CD->Str2
B-400->B
End
B->A
For(I,1,iPart(A.01
Str2+"C->Str2
B-|E2->B
End
If B>89
Then
Str2+"XC->Str2
B-90->B
End
If B>49
Then
Str2+"L->Str2
B-50->B
End
If B>39
Then
Str2+"XL->Str2
B-40->B
End
B->A
For(I,1,iPart(A.1
Str2+"X->Str2
B-10->B
End
If B=9
Then
DelVar BStr2+"IX->Str2
End
If B>4
Then
Str2+"V->Str2
B-5->B
End
If B=4
Then
DelVar BStr2+"IV->Str2
End
For(I,1,B
Str2+"I->Str2
End
sub(Str2,2,length(Str2)-1
Else
"?"+Str1+"?->Str1
For(I,0,5
inString(Str1,sub("IVIXXLXCCDCM",2I+1,2
If Ans
Then
sub(Str1,1,Ans-1)+sub(Str1,Ans+2,length(Str1)-Ans-1->Str1
C+(4+10fPart(I/2))10^(iPart(I/2->C
End
End
For(I,1,length(Str1
inString("IVXLCDM",sub(Str1,I,1
If Ans
C+(5-8fPart(Ans/2))10^(iPart(.5(Ans-1->C
End
C
End
Ans

Answer 1

You have optimized this very well. However, using a different algorithm can reduce code size from 665 bytes to 411 bytes:

DelVar IDelVar VDelVar XDelVar LDelVar CDelVar DDelVar M"?->Str2
"IVXLCDM->Str3
ClrHome
Input Str1
expr(Str1->N
If N
Then
For(I,0,log(N
10fPart(.1N->A
While A
sum({4.1,4.1,9.2}(A={4,5,9->B
sub(Str3,2I+10fPart(B)+1,1)+Str2->Str2
abs(A-1-int(B->A
End
int(.1N->N
End
sub(Str2,1,length(Str2)-1
Else
Str2+Str1+"?->Str1
For(I,0,5
inString(Str1,sub("IVIXXLXCCDCM",2I+1,2
If Ans
Then
sub(Str1,1,Ans-1)+sub(Str1,Ans+2,length(Str1)-Ans-1->Str1
C+(4+10fPart(I/2))10^(iPart(I/2->C
End
End
For(I,1,length(Str1
inString(Str3,sub(Str1,I,1
If Ans
C+(5-8fPart(.5Ans))10^(iPart(.5Ans-.5->C
End
C
End
Ans

Answer 2

Improving FlyAwayBirdie's answer from 411 bytes to 188 bytes:

This code has a much smaller Roman→decimal conversion. Rather than throwing an error when given an invalid Roman numeral like "VL", it will give an unpredictable result. It simply subtracts letters if the next letter has greater value, and adds the rest. It seems FlyAwayBirdie got the decimal→Roman conversion from here; I could only save seven bytes plus the substring command at the end. Close-quotes have been left on for formatting.

"IVXLCDM"→Str3
ClrHome 
Input Str1 
expr("0"+Str1→N
If N
Then
" "→Str1
For(I,0,log(N
.1int(N→N
10fPart(N→A
While A
.1sum({22,42,74}(A={4,5,9→B
A-1-int(B→A
sub(Str3,2I+5fPart(B)+1,1)+Str1→Str1
End
End
Else
.5seq(inString(Str3,sub(Str1,X,1)),X,1,length(Str1
sum(10^(int(Ans))(.5+fPart(Ans)-2(Ans<ΔList(cumSum(augment(Ans,{0
End
Ans

Edit: Improved with ideas from M. I. Wright. I had thought of using the seq( method, which I knew would be faster, but I didn't realize it would be smaller as well.

Edit: Optimized the decimal->Roman part some more; I saved seven bytes plus the sub(Ans,1,length(Ans-1at the end, because removing the trailing space is unnecessary.