Confused about assignments

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hasen
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Confused about assignments

Post by hasen » Tue Jan 30, 2018 8:37 am

I've been studying various bits of code I found around the place and on this forum and I was slightly confused about the assignments going on in one block I came across:

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player.x, player.y, cols, len = world:move(player, player.x + player.dx, player.y + player.dy)

    player.xAcc, player.yAcc = 0, player.yAcc * 0.45
I'm aware that you can make assignment like this in lua:

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SW, SH = 10, 12
But the above assignments seem to be slightly different. I'm not sure what's getting assigned to what in these lines.

grump
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Re: Confused about assignments

Post by grump » Tue Jan 30, 2018 9:01 am

But the above assignments seem to be slightly different. I'm not sure what's getting assigned to what in these lines.
The right side of an assignment is always assigned to the left side. There can only be variables on the left, while the right side can be arbitrary expressions. Lua evaluates all expressions on the right side (going from left to right) and assigns the results to the variables on the left (also from left to right).

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left1, left2, left3 = expression1, expression2, expression3
hasen wrote:
Tue Jan 30, 2018 8:37 am

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player.x, player.y, cols, len = world:move(player, player.x + player.dx, player.y + player.dy)
Functions in Lua can have more than one return value. The function world:move() probably returns 4 values. This line assigns these values to the variables player.x, player.y, cols and len.

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player.xAcc, player.yAcc = 0, player.yAcc * 0.45
Same here, just without the function call. player.xAcc is set to 0, player.yAcc is set to player.yAcc * 0.45. It's equivalent to

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player.xAcc = 0
player.yAcc = player.yAcc * 0.45

hasen
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Re: Confused about assignments

Post by hasen » Tue Jan 30, 2018 9:16 am

grump wrote:
Tue Jan 30, 2018 9:01 am
But the above assignments seem to be slightly different. I'm not sure what's getting assigned to what in these lines.
The right side of an assignment is always assigned to the left side. There can only be variables on the left, while the right side can be arbitrary expressions. Lua evaluates all expressions on the right side (going from left to right) and assigns the results to the variables on the left (also from left to right).

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left1, left2, left3 = expression1, expression2, expression3
hasen wrote:
Tue Jan 30, 2018 8:37 am

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player.x, player.y, cols, len = world:move(player, player.x + player.dx, player.y + player.dy)
Functions in Lua can have more than one return value. The function world:move() probably returns 4 values. This line assigns these values to the variables player.x, player.y, cols and len.

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player.xAcc, player.yAcc = 0, player.yAcc * 0.45
Same here, just without the function call. player.xAcc is set to 0, player.yAcc is set to player.yAcc * 0.45. It's equivalent to

Code: Select all

player.xAcc = 0
player.yAcc = player.yAcc * 0.45
Ok I see, so it was like I thought, it was just that the world:move was returning four values. And for the second one it seems obvious now, don't know why I didn't get that before. Thanks very much for your help.

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zorg
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Re: Confused about assignments

Post by zorg » Tue Jan 30, 2018 9:45 am

hasen wrote:
Tue Jan 30, 2018 9:16 am
grump wrote:
Tue Jan 30, 2018 9:01 am
...
Ok I see, so it was like I thought, it was just that the world:move was returning four values. And for the second one it seems obvious now, don't know why I didn't get that before. Thanks very much for your help.
Keep in mind two more things;
lua functions can return more than one value (without using tables, i mean)
the comma (as well as extra parentheses around function calls) limit the number of returned values to 1.
So in the example code:

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player.x, player.y, cols, len = world:move(player, player.x + player.dx, player.y + player.dy) -- this probably returns 4 values, although there's no rule that it needs to; if it returns more than how many vars are on the left side of the equals sign, they get discarded, if it returns less, then the rightmost vars will be set to nil.

a,b = fn(), 'whatever', -- fn will return one value because of the comma
d,d = (fn()) -- fn will return one value because of the parentheses around it
Me and my stuff :3True Neutral Aspirant. Why, yes, i do indeed enjoy sarcastically correcting others when they make the most blatant of spelling mistakes. No bullying or trolling the innocent tho.

hasen
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Re: Confused about assignments

Post by hasen » Tue Jan 30, 2018 11:27 am

Ok I see, thanks for elaborating on that, Zorg. That would make sense since in Lua anything that isn't something get's labeled as nil. Which is pretty convenient really. Good tips regarding the functions too.

grump
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Re: Confused about assignments

Post by grump » Tue Jan 30, 2018 9:33 pm

Thanks zorg. Sematics of multiple return values is one of the more difficult, least intuitive parts about Lua, and it keeps messing with me to this day.

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pgimeno
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Re: Confused about assignments

Post by pgimeno » Tue Jan 30, 2018 11:45 pm

By the way, that applies to parameters in function calls too. Only the last element can see multiple values, e.g.

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function f()
  return 1, 2
end

function g(a,b,c,d)
  print(a,b,c,d)
end

g(f(),f(),f()) -- prints 1,1,1,2
w,x,y,z = f(), f(), f() -- w=1 x=1 y=1 z=2
w,x,y,z = f(), f(), (f()) -- w=1 x=1 y=1 z=nil

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zorg
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Re: Confused about assignments

Post by zorg » Wed Jan 31, 2018 12:21 am

I think it's correct to say, that in anything that features commas as variable delimiters; tables, function (even anonymous and "functionalized table via metatable") parameters, and parentheses around a multi-value returning construct will have its return values trimmed to one.

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function myprint(t)
    for i,v in ipairs(t) do io.write(tostring(v)) end
    io.write('\n')
end

myprint({1,2,(function() return 3,4 end)()}) -- table + anon function at the end, not wrapped in parentheses --> 1,2,3,4
myprint({1,2,(function() return 3,4 end)(),5}) -- table + anon function not at the end, not wrapped in parentheses --> 1,2,3,5
myprint({1,2,((function() return 3,4 end)())}) -- table + anon function at the end, wrapped in parentheses --> 1,2,3
myprint({1,2,((function() return 3,4 end)()),5}) -- table + anon function not at the end, wrapped in parentheses --> 1,2,3,5

-- and just for some extra shenanigans, parameters to the anonymous function while it's at the last table slot

myprint({1,2,(function(a,b) return a,b end)(3,4)}) --> 1,2,3,4
myprint({1,2,(function(a,b) return a,b end)(3)}) --> 1,2,3
myprint({1,2,(function(a,b) return a,b end)(nil,4)}) --> 1,2,nil,4
myprint({1,2,(function(a) return a end)(3,4)}) --> 1,2,3
myprint({1,2,(function(a) return a end)(nil,4)}) --> 1,2
Me and my stuff :3True Neutral Aspirant. Why, yes, i do indeed enjoy sarcastically correcting others when they make the most blatant of spelling mistakes. No bullying or trolling the innocent tho.

choopos
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Re: Confused about assignments

Post by choopos » Thu Sep 12, 2019 8:38 am

Here's my solution to the problem https://www.assignmentexpert.com/progra ... ce-project. They haven't break my trust yet.

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Sasha264
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Re: Confused about assignments

Post by Sasha264 » Thu Sep 12, 2019 9:39 am

Is here an explanation why the comma limits the number of returned values to 1?

I can imagine negative side of that solution. For example this call:

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love.graphics.draw(drawable, x, y, r, sx, sy, ox, oy, kx, ky)
can not be made like this:

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love.graphics.draw(drawable, obj:getPosition(), r, obj:getScale(), ox, oy, kx, ky)
instead it requires a bunch of temp variables.

But what is positive side?

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