How do I evaluate expressions from text?

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#1

I am trying to make a calculator, and I am getting an expression from the textbox, but I do not know how to evaluate the expression. I tried printing it, but it would come out as a normal string. For example, if I input 3+2 it would print 3+2:
hacjk

equal.MouseButton1Click:Connect(function()	
	local str = text.Text
	print(str)
end)

Please help!

#2

This is actually a harder problem than it seems. You could just loadstring the equation, but that’s generally a bad idea.

The fully correct way to do it is to tokenize the expression, run that token sequence through the shunting-yard algorithm to transform it into a reverse-polish-notation sequence, then process that sequence to get the answer.

#3

Use string.split(str, “+”) to store the table where the character will be separated by the + character.
Store the result in a variable, which will actually be a table, because that’s what string.split() returns.

For example

str = "3+2"
tablee = string.split(str, "+") --tablee[1] stores "3", and tablee[2] stores "2"

You then can create a variable, and store the sum in it.

sum = tonumber(tablee[1]) + tonumber(tablee[2]) -- tonumber() needed because string.split returns strings as elements.

You will need to loop through the string before you do all of these to find what operator is used there, (+, -, /, *), check it with if, each iteration. However, if you have more than 2 operators, or more than 2 numbers, then you probably need to make the code more complex. You can loop through the table using ipairs, to add more than 2 numbers, but I am not sure how you would make the code work with more than 2 operators.

#4

This helped a lot, but I do not know how to check if it would give out nil. I tried this:

equal.MouseButton1Click:Connect(function()	
	local str = text.Text
	local tabplus = string.split(str, "+")
	local tabminus = string.split(str, "-")
	local sum = tonumber(tabplus[1]) + tonumber(tabplus[2])
	if sum == nil then
		local diff = tonumber(tabminus[1]) - tonumber(tabminus[2])
		text.Text = diff
	else
		text.Text = sum
	end
end)

but when I try to subtract, I get this:
rodlocks

#5

You can add a simple condition to check if the values of these elements are not nil.

if tabplus[1] and tabplus [2] then
--your code for addition (including the sum variable initialisation)
elseif tabminus[1] and tabminus[2] then
--your code for subtraction (including the diff variable initialisation)
end
#6

Sorry I already solved it but forgot to check it as the solution.

#7

You could use an existing parser. Work’s already been done for you.
https://www.roblox.com/library/5005473860/Luaxp-Expression-Parser

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