CODEWITHSUNDEEP
luacomparison
Plakhoy
Plakhoy

Reputation: 1872

Why does Lua do comparison the way it does?

Pardon me if the title doesn't make much sense. I had to choose between Is Lua's way of comparison usefull? and Comparison in Lua.

I wanted to do something like this today: 

 if currChar == nextChar == "-" then
    ... 
 end

but it kept returning false everytime: 

> currChar="-"
> nextChar="-"
> =currChar == nextChar == "-"
false
>
-- All true in Python
print(5 == 5)                    -- true
print(5 == 5 == 5)               -- false
print((5 == 5) == (5 == 5))      -- true
print(5 == (4 + 1) == (6 - 1))   -- false

I fiddled with the values for some time and found out that for some reason, Lua compares values pairwise from left to right: 

> = 52 > 3 > 2
stdin:1: attempt to compare number with boolean
stack traceback:
        stdin:1: in main chunk
        [C]: in ?
>

I's there a case when such a form of comparison is usefull?
Why do comparisons that way?

Upvotes: 2

Views: 598

Answers (1)

Dave
Dave

Reputation: 4291

Lua's comparison operators are true binary operators. They work on two operands and that's it. In Lua, 5 == 5 == 5 is evaluated as (5 == 5) == 5, which simplifies to True == 5 and is false. In Python, on the other hand, 5 == 5 == 5 is evaluated as 5 == 5 and 5 == 5, which is true.

Python is atypical in supporting chaining of comparison operators, where x < y < z is converted to x < y and y < z. There aren't many languages of which I am aware that support that syntax.

As to whether or not it is useful, that's completely arbitrary. The chaining syntax is simply shorthand.

Upvotes: 5

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