Create a Roman Numeral calculator

JavaScript (ES6), 238

c=s=>{X={M:1e3,CM:900,D:500,CD:400,C:100,XC:90,L:50,XL:40,X:10,IX:9,V:5,IV:4,I:1}
n=eval('W='+s.replace(/[\w]+/g,n=>(o=0,n.replace(/[MDLV]|C[MD]?|X[CL]?|I[XV]?/g,d=>o+=X[d]),
o+';W=W')));o='';for(i in X)while(n>=X[i])o+=i,n-=X[i];return o}

Usage:

c("XIX + LXXX")
> "XCIX"
c('XCIX + I / L * D + IV')
> "MIV"

Annotated version:

/**
 * Process basic calculation for roman numerals.
 * 
 * @param {String} s The calculation to perform
 * @return {String} The result in roman numerals
 */
c = s => {
  // Create a lookup table.
  X = {
    M: 1e3, CM: 900, D: 500, CD: 400, C: 100, XC: 90, 
    L: 50,  XL: 40,  X: 10,  IX: 9,   V: 5,   IV: 4, I: 1
  };
  // Do the calculation.
  // 
  // The evaluated string is instrumented to as below:
  //   99+1/50*500+4 -> W=99;W=W+1;W=W/50;W=W*500;W=W+4;W=W
  //                 -> 1004
  n = eval('W=' + s.replace(
    // Match all roman numerals.
    /[\w]+/g,
    // Convert the roman number into an integer.
    n => (
      o = 0,
      n.replace(
        /[MDLV]|C[MD]?|X[CL]?|I[XV]?/g,
        d => o += X[d]
      ),
      // Instrument number to operate left-side operations.
      o + ';W=W'
    )
  ));

  // Convert the result into roman numerals.
  o = '';
  for (i in X)
    while (n >= X[i])
      o += i,
      n -= X[i];

  // Return calculation result.
  return o
}

T-SQL, 1974 - 50 = 1924 bytes

I know that golfing in SQL is equivalent to playing 18 holes with nothing but a sand wedge, but I relished the challenge of this one, and I think I managed to do a few interesting things methodologically.

This does support the vinculum for both input and output. I adopted the convention of using a trailing tilde to represent it , so V~ is 5000, X~ is 10000, etc. It should also handle outputs up to 399,999 according to standard modern Roman numeral usage. After that, it will do partially non-standard Roman encoding of anything in INT's supported range.

Because it's all integer math, any non-integer results are implicitly rounded.

DECLARE @i VARCHAR(MAX)
SET @i='I+V*IV+IX*MXLVII+X~C~DCCVI'
SELECT @i

DECLARE @t TABLE(i INT IDENTITY,n VARCHAR(4),v INT)
DECLARE @u TABLE(n VARCHAR(50),v INT)
DECLARE @o TABLE(n INT IDENTITY,v CHAR(1))
DECLARE @r TABLE(n INT IDENTITY,v INT,r VARCHAR(MAX))
DECLARE @s TABLE(v INT,s VARCHAR(MAX))
DECLARE @p INT,@x VARCHAR(4000)='SELECT ',@j INT=1,@m INT,@y INT,@z VARCHAR(2),@q VARCHAR(50)='+-/*~]%'
INSERT @t(n,v) VALUES('i',1),('iv',4),('v',5),('ix',9),('x',10),('xl',50),('l',50),('xc',90),('c',100),('cd',400),('d',500),('cm',900),('m',1000),('mv~',4000),('v~',5000),('mx~',9000),('x~',10000),('x~l~',40000),('l~',50000),('x~c~',90000),('c~',100000)
INSERT @u VALUES('%i[^i'+@q,-2),('%v[^vi'+@q,-10),('%x[^xvi'+@q,-20),('%l[^lxvi'+@q,-100),('%c[^clxvi'+@q,-200),('%d[^dclxvi'+@q,-1000),('%mx~%',-2010),('%x~l~%',-20060),('%x~c~%',-20110)
WHILE PATINDEX('%[+-/*]%', @i)!=0
BEGIN
    SET @p=PATINDEX('%[+-/*]%', @i)
    INSERT @o(v) SELECT SUBSTRING(@i,@p,1)
    INSERT @r(r) SELECT SUBSTRING(@i,1,@p-1)
    SET @i=STUFF(@i,1,@p,'')
END 
INSERT @r(r) SELECT @i
UPDATE r SET v=COALESCE(q.v,0) FROM @r r LEFT JOIN (SELECT r.r,SUM(u.v)v FROM @u u JOIN @r r ON r.r LIKE u.n GROUP BY r.r)q ON q.r=r.r
UPDATE r SET v=r.v+q.v FROM @r r JOIN (SELECT r.n,r.r,SUM((LEN(r.r)-LEN(REPLACE(r.r,t.n,REPLICATE(' ',LEN(t.n)-1))))*t.v) v FROM @r r JOIN @t t ON CHARINDEX(t.n,r.r) != 0 AND (LEN(t.n)=1 OR (LEN(t.n)=2 AND RIGHT(t.n,1)='~')) GROUP BY r.n,r.r) q ON q.r=r.r AND q.n = r.n
SELECT @m=MAX(n) FROM @o
SELECT @x=@x+REPLICATE('(',@m)+CAST(v AS VARCHAR) FROM @r WHERE n=1
WHILE @j<=@m
BEGIN
    SELECT @x=@x+o.v+CAST(r.v AS VARCHAR)+')'
    FROM @o o JOIN @r r ON r.n=o.n+1 WHERE o.n=@j
    SET @j=@j+1
END 
INSERT @s(v,s) EXEC(@x+',''''')
UPDATE @s SET s=s+CAST(v AS VARCHAR(MAX))+' = '
SET @j=21
WHILE @j>0
BEGIN
    SELECT @y=v,@z=n FROM @t WHERE i = @j
    WHILE @y<=(SELECT v FROM @s)
    BEGIN
        UPDATE @s SET v=v-@y,s=s+@z
    END  
    SET @j=@j-1
END
SELECT @x+' = '+UPPER(s) FROM @s

I'm still tinkering with a set-based solution to replace some of the WHILE looping that might whittle down the byte count and be a more elegant example of idiomatic SQL. There are also some bytes to be gained by reducing use of table aliases to a bare minimum. But as it's essentially un-winnable in this language, I'm mostly just here to show off my Don Quixote outfit. :)

SELECT @i at the top repeats the input:

I+V*IV+IX*MXLVII+X~C~DCCVI

And the SELECT at the end returns:

SELECT (((((1+5)*4)+9)*1047)+90706) = 125257 = C~X~X~V~CCLVII

And you can test it yourself at this SQLFiddle

And I will be returning to add some commentary on how it works, because why post an obviously losing answer if you're not going to exploit it for educational value?

Javascript - 482 476 characters

String.prototype.m=String.prototype.replace;eval("function r(a){return a>999?'Mk1e3j899?'CMk900j499?'Dk500j399?'CDk400j99?'Ck100j89?'XCk90j49?'Lk50j39?'XLk40j9?'Xk10j8?'IX':a>4?'Vk5j3?'IV':a>0?'Ik1):''}".m(/k/g,"'+r(a-").m(/j/g,"):a>"));s=prompt();h=s.match(/\w+/gi);for(k in h)s=s.m(h[k],eval(eval("'0'+h[k].m(/IVu4pIXu9pXLu40pXCu90pCDu400pCMu900pMu1000pDu500pCu100pLu50pXu10pVu5pIu1')".m(/u/g,"/g,'+").m(/p/g,"').m(/")))+")");for(k in h)s="("+s;alert(r(Math.floor(eval(s))))

The sample input/output works:

XIX + LXXX -> XCIX
XCIX + I / L * D + IV -> MIV

It badly handles large numbers too:

MMM+MMM -> MMMMMM
M*C -> MMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM

And it accepts, but does not requires, spaces too.

But, since I was golfing it has some problems:

• It does not validates if the input is well-formed. If the input is not well-formed, the behaviour is undefined (and in practice it is very bizarre and strange).
• It truncates fraction numbers on output (but it is able to do intermediate calculations with them).
• It really abuses the eval function.
• It does not attempt to handle negative numbers.
• It is case-sensitive.

This alternative version handles numbers over 5000 upto 99999, but it has 600 598 584 characters:

String.prototype.m=String.prototype.replace;eval("function r(a){return a>8zz?'XqCqk9e4j4zz?'Lqk5e4j3zz?'XqLqk4e4jzz?'Xqk1e4j89z?'IqXqk9e3j49z?'Vqk5e3j9z?'Mk1e3j8z?'CMk900j4z?'Dk500j3z?'CDk400jz?'Ck100j89?'XCk90j49?'Lk50j39?'XLk40j9?'Xk10j8?'IX':a>4?'Vk5j3?'IV':a>0?'Ik1):''}".m(/k/g,"'+r(a-").m(/j/g,"):a>").m(/q/g,"\u0305").m(/z/g,"99"));s=prompt();h=s.match(/\w+/gi);for(k in h)s=s.m(h[k],eval(eval("'0'+h[k].m(/IVu4pIXu9pXLu40pXCu90pCDu400pCMu900pMu1000pDu500pCu100pLu50pXu10pVu5pIu1')".m(/u/g,"/g,'+").m(/p/g,"').m(/")))+")");for(k in h)s="("+s;console.log(r(Math.floor(eval(s))))