CODEWITHSUNDEEP
if-statementluaarguments
user1415451
user1415451

Reputation: 11

In Lua what does an if statement with only one argument mean?

I've been taught to program in Java. Lua is new to me and I've tried to do my homework but am not sure what an if statement of the following nature means.

The code is as follows:

local function getMinHeight(self)
    local minHeight = 0
    for i=1, minimizedLines do
        local line = select(9+i, self:GetRegions())
        **if(line) then
            minHeight = minHeight + line:GetHeight() + 2.5
        end**
    end
    if(minHeight == 0) then
        minHeight = select(2, self:GetFont()) + 2.5
    end
    return minHeight
end

The if statement with the ** before and after is the part I'm not sure about. I don't know what the if statement is checking. If the line is not nil? If the line exists? If what?

Upvotes: 1

Views: 455

Answers (2)

Mud
Mud

Reputation: 29000

In Lua, anything that's not nil or false evaluates to true in a conditional.

If the line is not nil? If the line exists?

Yes to both, because they kinda mean the same thing.

The select function returns a specific argument from it's list of arguments. It's used primarily with ..., but in this case it's being used to select the (i+9)th value returned by self:GetRegions. If there is no such value (for instance, if GetRegions only returns 5 values), then select returns nil.

if(line) is checking to see that it got a value back from select.

if(line) is being used as a shortcut for if(line ~= nil), since nil evaluates to false in a conditional.

It's worth pointing out that this shortcut is not always appropriate. For instance, we can iterate all the values in a table like this:

key, val = next(lookup)
while key do
    print(key, val)
    key, val = next(lookup, key)
end

However, this will fail if one of the table's keys happens be false:

lookup = { 
    ["fred"] = "Fred Flinstone",
    [true] = "True",
    [false] = "False", 
}

So we have to explicitly check for nil:

key, val = next(lookup)
while key ~= nil do
    print(key, val)
    key, val = next(lookup, key)
end

Upvotes: 8

kikito
kikito

Reputation: 52668

As Mud says, in lua anything other than nil and false is considered truthy. So the if above will pass as long as line is not nil or false.

That said, it worries me a bit the way you have phrased the question - "an if with only one argument".

First, it's not called "argument" - it's called expression. And in most languages is always one. In java, for example, you could do something like this:

bool found = false

...

if(found) {
  ...
}

ifs only care about the final value of the expression; they don't care whether it's a single variable or a more complex construction.

Upvotes: 2

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