Operator precedence: How wrong can I be?

Question

Say I have an expression:

9 * 8 + 1 - 4

This expression can be interpreted in six different ways, depending on operator precedence:

(((9 * 8) + 1) - 4) = 69 (* + -)
((9 * 8) + (1 - 4)) = 69 (* - +)
((9 * (8 + 1)) - 4) = 77 (+ * -)
(9 * ((8 + 1) - 4)) = 45 (+ - *)
((9 * 8) + (1 - 4)) = 69 (- * +)
(9 * (8 + (1 - 4))) = 45 (- + *)

Say I'm a developer, and I don't feel like memorizing precedence tables, etc., so I'm just going to guess.

In this case, the largest margin of error would be 45-77, which is a difference of 32. This means that my guess will only be off by a maximum of 32.

The challenge

Given an expression consisting of numbers and +, -, *, / (integer division) and %, output the absolute difference of the largest and smallest possible value for that expression, based on the precedence of operators.

Specifications

• The input expression will not contain parenthesis and every operator is left-associative.
• The input expression will only contain nonnegative integers. However, subexpressions may evaluate to negatives (e.g. 1 - 4).
• You can take the expression in any reasonable format. For example:
  • "9 * 8 + 1 - 4"
  • "9*8+1-4"
  • [9, "*", 8, "+", 1, "-", 4]
  • [9, 8, 1, 4], ["*", "+", "-"]
• The input will contain at least 1 and at most 10 operators.
• Any expression that contains a division or modulo by 0 should be ignored.
• You can assume that modulo will not be given negative operands.

Test Cases

9 * 8 + 1 - 4             32
1 + 3 * 4                  3
1 + 1                      0
8 - 6 + 1 * 0              8
60 / 8 % 8 * 6 % 4 * 5    63

Answer 1

Python 2, 171 156 bytes

lambda a:max(e(a))-min(e(a))
u=')%s('
def e(a,t=u):
 try:b=[eval(a)]
 except:b=[]
 return sum([e('(%s)'%a.replace(o,t%o),u%t)for o in"+-*/%"if' '+o in a],b)

Try it online!

How it works

We surround each operator with a different number of outward-facing pairs of parentheses to simulate different precedences (in all possible ways), and wrap enough inward-facing pairs of parentheses around the entire string, to get an expression we can eval. For example, with

+ ↦ )+(
* ↦ ))*((
- ↦ )))-(((

we get

9 * 8 + 1 - 4 ↦ (((9 ))*(( 8 )+( 1 )))-((( 4))) = 77.

Answer 2

Jelly, 126 bytes

"Operator Precedence? Parentheses? Pah, who needs that?" - challenges of using Jelly for an operator precedence challenge.

⁾[]i$€Ḥæ%3+\¬œp¹Ḋ€
ǵḟØDO%9µÐṀṪɓœṣ⁹,ṚÑj@¥/
ǵVṾµ1ĿFḟØDḟ”-Lµ?ÐL
5Ḷx@€“]“[”ż⁸j/€,@y³Fɓ³i@€Ṁ’x@“[“]”jÇ
“+_×:%”Œ!Ç€µṾL_L’ỊµÐfV€ṢIS

Try it online!

Input is taken as a string, e.g. "1+2_3×4:5%6". Note multiplication uses "×" instead of "*", division uses ":" instead of "/", and subtraction uses "_" instead of "-".

How it Works The program is divided into three parts: generating all expressions of different operator precedence, evaluating them, and returning the difference between the maximum and minimum.

All expressions are generated with the code:

5Ḷx@€“]“[”ż⁸j/€,@y³Fɓ³i@€Ṁ’x@“[“]”jÇ (4) helper link: returns all outputs given a permutation. Input e.g. "_+:×%"
5Ḷx@€“]“[”           - repeat outer brackets to get ["",""],["]","["],["]]","[["],["]]]","[[["],["]]]]","[[[["]
          ż⁸j/€      - insert the operations in to get "_","]+[","]]:[[","]]]×[[[","]]]]%[[[["
               ,@    - turn this into a mapping equivalent to "_"↦"_","+"↦"]+[",":"↦"]]:[[","×"↦"]]]×[[[","%"↦"]]]]%[[[["
                 y³F - use this mapping to get the right number of outward brackets on each operation. e.g. "1]+[3]]]×[[[4"
ɓ³i@€Ṁ’x@“[“]”j      - add the right number of brackets to the end to get e.g."[[[1]+[3]]]×[[[4]]]"
               Ç     - this calls the link which evaluates the expression
“+_×:%”Œ!Ç€                          (5a) main link. Input e.g. "1+3×4"
“+_×:%”                                 - the string "+_×:%"
       Œ!                               - all permutations
         ǀ                             - apply link (4) to each permutation

The links are evaluated with this (I could probably improve with a different structure):

⁾[]i$€Ḥæ%3+\¬œp¹Ḋ€      (1) Helper link: Outputs a list of expressions within brackets, e.g. "[[[1]+[3]]]×[[[4]]]"↦"[[1]+[3]]","[[4]]"
⁾[]i$€Ḥæ%3                 - map "[" to 2, "]" to -2, and any other character to 0.
          +\¬              - cumulative sum negated: 1s at characters not in brackets (includes opening brackets), 0s otherwise (includes closing brackets)
             œp¹           - partition the input, not including borders, based on the sum to get "[[[1]+[3]]","[[[4]]"
                Ḋ€         - remove opening brackets
ǵḟØDO%9µÐṀṪɓœṣ⁹,ṚÑj@¥/ (2) Return the input to this link with one of the expressions from (1) evaluated
ǵVṾµ1ĿFḟØDḟ”-Lµ?ÐL     (3) link called from part 1: Evaluates expressions
 µ  µ          µ?          - if:
     1ĿFḟØDḟ”-L            - the input contains no operators within brackets:         
  VṾ                         - evaluate this one expression with normal Jelly calculation and return to string
                           - otherwise:
Ç                            - evaluate one subexpression using link (2)
                  ÐL       - repeat this until a single output is determined

The difference between the maximum and minimum is computed with the code in link (5):

µṾL_L’ỊµÐfV€ṢIS (5b) determine difference between minimum and maximum
µ      µÐf        - filter out outputs involving division or modulo by 0. Determined with:
 ṾL_L’Ị           - actual numbers have their unevaled form Ṿ no more than one byte longer than the non-unevaled form.
          V€      - evaluate each of these valid numbers to get integers from strings
            Ṣ     - sort
             IS   - return the sum of all difference between consecutive elements.

Answer 3

Python 2, 235 234 233 226 bytes

^{-1 byte (and a fix) thanks to Anders Kaseorg!}

^{-7 bytes thanks to Step Hen!}

from itertools import*
def f(e,a=()):
 for o in permutations("+-*/%"):
	l=e[:]
	for c in o:
	 for i in range(len(l),0,-1):
		if l[i-1]==c:l[i-2:i+1]=["("+l[i-2]+l[i-1]+l[i]+")"]
	try:a+=eval(*l),
	except:0
 print max(a)-min(a)

Try it online!

Answer 4

Haskell 582 bytes

This didn't go nearly as well as I hoped it would...

import Data.List
f x=case x of '+'->(+);'-'->(-);'*'->(*);'/'->div;_->rem
e _ s[]=s
e 1 s(')':x:a)|0<-(read$e 0""a),(x=='%'||x=='/')=""|""<-(e 0""s)=""|""<-(e 0""a)=""|0<3=show$(f x)(read$e 0""s)$read$e 0""a
e 1 s")"=e 0""s
e n s(')':a)=e(n-1)(s++")")a
e 0 s('(':a)=e 1 s a
e n s('(':a)=e(n+1)(s++"(")a
e n s(x:a)=e n(s++[x])a
n?s=take n$cycle s
a!b=e 0""(9?"("++(concat$zipWith(++)a(b++[[]]))++9?")")
c#b|l<-[read x|x<-map(c!)(a b),x/=""]=maximum l-minimum l
a c=transpose$map(\x->map((\(Just q)->q).lookup x)$map(\a->zipWith(\x y->(y,x?")"++y:x?"("))[1..5]a)$permutations"+-*%/")c

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Trying to golf a long program just makes me write bad code :(

I tried to use Anders' algorithm in Haskell, but it got out of my control

The function e is like a specific case of eval. (#) takes a list of strings representing integers and a string of operators and returns the difference between the maximum and minimum possible values. e.g

(#) ["9","8","1","4"] "*+-" => 32

Answer 5

Haskell, 254 bytes

import Data.List.Split
import Data.List
f s=(-)<$>maximum<*>minimum$permutations(zip"+-*/%"[p(+),p(-),p(*),c$div,c$mod])>>=(s!)
p=((pure.).)
c o a b=[o a b|b/=0]
s![]=[read s]
s!((x,o):y)=case splitOn[x]s>>=(!y)of[]->[];l->l?o
[a]?_=[a]
(a:b)?o=b?o>>=o a

Try it online!

Input is a whole string, such as 4+5 * 2. It generates all permutations of operations, and for each permutation splits the string recursively. It filters divisions by 0 with the list monad.

Answer 6

Pyth, 45 bytes

KS.nm.x.vj\ u.nm+*H/kHckHGd]s.iFQY.p_{eQ-eKhK

I'm sure that a lot more optimization can be done, but I like it so far.

Takes input like this: [9, 8, 1, 4], ["*", "+", "-"].

Try it online!

Answer 7

Mathematica, 186 164 159 bytes

eMax@#-Min@#&[Fold[#//.{m___,x_,#2[[0]],y_,n___}:>{m,x~Last@#2~y,n}&,e,#]&/@Permutations@{"+"@Plus,"-"[#-#2&],"*"@Times,"/"@Quotient,"%"@Mod}/. 0/0|1/0->{}]

\[Function] takes 3 bytes.

Some alternatives (keeps bytecount the same)

#2-#&@MinMax[...] to replace Max@#-Min@#&[...]

Head@#2 to replace #2[[0]]

Try it online at http://sandbox.open.wolframcloud.com : enter ( .... )[{60, "/", 8, "%", 8, "*", 6, "%", 4, "*", 5}] with .... replaced by code above for test case 60 / 8 % 8 * 6 % 4 * 5. Press Shift + enter to evaluate.

Answer 8

Javascript, 280 bytes

Note: The integer division rounds using the floor function, which means that negative numbers round away from zero.

This solution is based on this answer.

b=>(Math.max(...(f=(a,h="(",i=")",r=[...a[d="replace"](/[^-+*/%]|(.)(?=.*\1)/g,"")])=>(r[0]?(r.map((c,j)=>s=s.concat(f(h+a[d](RegExp("\\"+(n=r.concat()).splice(j,1),"g"),i+c+h)+i,h+"(",i+")",n)),s=[]),s):(a=eval(`(${a})`[d](/\(/g,"Math.floor(")))==a&&1/a?a:r))(b))-Math.min(...f(b)))

Example code snippet:

g=

b=>(Math.max(...(f=(a,h="(",i=")",r=[...a[d="replace"](/[^-+*/%]|(.)(?=.*\1)/g,"")])=>(r[0]?(r.map((c,j)=>s=s.concat(f(h+a[d](RegExp("\\"+(n=r.concat()).splice(j,1),"g"),i+c+h)+i,h+"(",i+")",n)),s=[]),s):(a=eval(`(${a})`[d](/\(/g,"Math.floor(")))==a&&1/a?a:r))(b))-Math.min(...f(b)))

for(k=0;k<5;k++)
  v=["9*8+1-4","1+3*4","1+1","8-6+1*0","60/8%8*6%4*5"][k],
  console.log(`g(${v}) = ${g(v)}`)

Answer 9

Python 2, 262 256 254 bytes

from itertools import*
def r(s,o):
 try:
  while o in s:i=s.index(o)-1;s[i:i+3]=[`eval(''.join(s[i:i+3]))`]
  return s
 except:0
def f(s):
 u=[int(v[0])for v in [reduce(r,O,s.split(' '))for O in permutations('*/%+-')]if v!=None];return abs(max(u)-min(u))

Try it online!

Answer 10

PHP, 316 bytes

<?for(;$t++<54322;)count_chars($t,3)!=12345?:$p[]=$t;foreach($p as$x){for(list($z,$q)=$_GET,$b=1,$i=0;$y=strtr($x,12345,"/%*+-")[$i++];)while(-1<$k=array_flip($q)[$y]){$n=$k+1;if($b&=$z[$n]||ord($y)%10<6)eval("\$z[$k]=$z[$k]$y$z[$n]^0;");($s=array_splice)($z,$n,1);$s($q,$k,1);}$b?$r[]=$z[0]:0;}echo max($r)-min($r);

Try it online!

Expanded
for(;$t++<54322;)
  count_chars($t,3)!=12345?:$p[]=$t;
foreach($p as$x){
  for(list($z,$q)=$_GET,$b=1,$i=0;$y=strtr($x,12345,"/%*+-")[$i++];)
    while(-1<$k=array_flip($q)[$y]){
      $n=$k+1;
      if($b&=$z[$n]||ord($y)%10<6)
        eval("\$z[$k]=$z[$k]$y$z[$n]^0;");
      ($s=array_splice)($z,$n,1);
      $s($q,$k,1);
    }
  $b?$r[]=$z[0]:0;
}
echo max($r)-min($r);

Answer 11

JavaScript (ES6), 210 bytes

Input as an array of numbers and operators

k=>(m=n=-k,r=(o,k,j=0)=>{for(o||(m=m>k?m:k,n=n<k?n:k);q=o[j++];(q>'%'&q<'/'||z)&&r(o.slice(0,j-1)+o.slice(j),h))for(h=[...k],z=1;i=h.indexOf(q)+1;h.splice(i-2,3,eval(a=h[i-2]+q+h[i])|0))z*=h[i]})('+-*/%',k)|m-n

Less golfed

k=>(
  m = n = NaN,
  r =(o, k, j=0) => {
    // try all operators in o
    for(;q = o[j]; j++)
    {  
      // q : current operator, 
      // look for q inside the expression to evaluate
      for(h = [...k], z = 1; i = h.indexOf(q) + 1;)
      {
        a = h[i - 2]
        b = h[i]
        z *= b // trace if any second operand is zero
        // subst subexpression with its value
        h.splice(i - 2, 3, eval(a + q + b) | 0)
      }
      // now all subexp involving current operator are evaluated
      // the result is ok if current operator is not % or /
      //  OR if no second operand was zero
      (q > '%' & q < '/' || z) && 
        // try again recursively
        // using the remaining operators and the remaining expression
        r(o.slice(0, j) + o.slice(j+1), h) 
    }
    // if no more operators to try, check max and min
    // k is an array with 1 element, can be used like a single number
    o || (
      m = m > k ? m : k, 
      n = n < k ? n : k
    )
  },
  r('+-*/%', k),
  m-n
)

Test

var F=
k=>(m=n=-k,r=(o,k,j=0)=>{for(o||(m=m>k?m:k,n=n<k?n:k);q=o[j++];(q>'%'&q<'/'||z)&&r(o.slice(0,j-1)+o.slice(j),h))for(h=[...k],z=1;i=h.indexOf(q)+1;h.splice(i-2,3,eval(a=h[i-2]+q+h[i])|0))z*=h[i]})('+-*/%',k)|m-n

function update() {
  var input = I.value.match(/\d+|\S/g)
  var result = F(input)
  O.textContent = I.value + ' -> ' + result + ' (max:'+m+' min:'+n+')'
}

update()

<input id=I value="60 / 8 % 8 * 6 % 4 * 5" oninput='update()'>
<pre id=O></pre>

Answer 12

Python 3, 284 bytes

Edit: seems like something's wrong with evaluating the last example. I'll look into it tomorrow.

Another Python answer. Couldn't outgolf everyone else, but I spent too long on this to not put it up.

from itertools import*
def f(n,o):
 z=[]
 for p in permutations("+-*/%"):
  try:
   p,x,a=[*p],n[:],o[:]
   while(p):
    for i,d in enumerate(a):
     if d==p[0]:x[i+1]=str(eval(x[i]+d+x[i+1]));x.pop(i);a.pop(i)
    p.pop(0)
   z+=x
  except:0
 z=[*map(float,z)];return max(z)-min(z)

Try it online!

Answer 13

Clojure (+ combinatorics), 342 377 + 41 = 418 bytes

+35 bytes because of a bug.

(fn[x y](let[l filter s first z #(l(fn[y]y)%)r(sort(z(for[e(q/permutations[+ - * quot mod])](try(loop[t e m y a x](if(=[]t)(s a)(let[j(s t)i(reverse(keep-indexed #(if(= j %2)%)m))](recur(rest t)(l #(not= j %)m)(loop[d 0 h a](if(=(count i)d)h(let[c(nth i d)f(inc c)](recur(inc d)(vec(z(assoc h c(j(nth h c)(nth h f))f nil)))))))))))(catch Exception _ nil)))))](-(last r)(s r))))

Try it online!

For this function to work, you have to use the clojure.math.combinatorics library (41 bytes):

(use '[clojure.math.combinatorics :as q])

Nuances:

This function is an anonymous function, which means you must do this to use it:

((fn[x y]...) numbers operators)

Also, I'm using the word quot instead of / (since Clojure does fraction division by default), and mod instead of %.

Ungolfed program:

(defn precedence [numbers operators]
  (let [results
        (sort
          (for [permute (c/permutations [+ - * / mod])]
            (loop [p-temp permute
                  o-temp operators
                  n-temp numbers]
              (if (empty? o-temp) (first n-temp)
                (let [first-p (first p-temp)
                      indices (reverse (keep-indexed #(when (= first-p %2) %) o-temp))]
                  (recur
                    (rest p-temp)
                    (filter #(not= first-p %) o-temp)
                    (loop [ind 0
                          n-through n-temp]
                      (if (= ind (count indices)) n-through
                        (let [current-ind (nth indices ind)]
                          (recur
                            (inc ind)
                            (vec
                              (filter #(not (nil? %))
                                (assoc n-through
                                  current-ind (first-p (nth n-through current-ind) (nth n-through (inc current-ind)))
                                  (inc current-ind) nil)))))))))))))]
    (- (last results) (first results))))