[Community Puzzle] Place the Parenthesis

Puzzles
#1

https://www.codingame.com/training/easy/place-the-parenthesis

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Created by @dolmen1234,validated by @ToshiTuringMachine,@Klemek and @alze.
If you have any issues, feel free to ping them.

#2

a couple of parenthesis

I read that as if I can place multiple pairs of parentheses. Doesn’t seem to be the case (or at least isn’t required to solve the problem).

Validator 3 has 2 valid solutions, namely:

(-4+1)*2=-6
-(4+1*2)=-6

Testing all possibilities and thus finding both of them gave me 75% on submit.

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#3

in this case 4 is the facto negative

#4

Although this is easy in many dynamic languages, this is definitely not an “easy” level puzzle in languages where you have to write fractional math and the expression evaluator yourself. Although integer-only math passes all the test cases, an input like “2*3/4*2=3” would have complicated things significantly and there was no way knowing if a validator had such a case without submitting a solution.

Even in dynamic languages that allow expression evaluation at runtime, such functionality should never be used for arbitrary input due to it being a massive security hole. Thus, the puzzle teaches bad programming habits.

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#5

Agreed it’s easy when you have an eval function at your disposal, but if you don’t then the puzzle jumps up in difficulty. Also looking at the author’s solution, some stuff is hardcoded and would fail in many situations, like the following +2 offset to see where to place parenthesis :

for (let i = 0 ; i < left.length-2 ; i++)
        for (let j = i+2 ; j <= left.length ; j++)

And there are only 4 tests cases which is probably half of what it should have.

#6

Let’s modify that code:
for (let i = left.length-3; i>=0; i--) {
now another solution gets found for -4+1*2=-6.

#7

I changed validator 3, now all validators have only one solution.

I guess people who tested the problem used some sort of early exit (a return in a function for example) if you encounter a solution, and the expected solution of validator 3 was the first one you get so they didn’t notice the problem.

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