问题
Does anyone know why this works in MATLAB?
>> 1 ++ 2
ans =
3
Coming from coding in C, python, Java etc, I find it most counterintuitive that this should work at all. Presumably there's something important about the parser that I don't understand?
回答1:
There's a difference between plus and uplus. I suspect MATLAB takes the first + as plus, and all the others as uplus. Since uplus is by default just "return what's behind", you add 1 and 2, and use a lot of "return what's behind" in between.
a=2;
c=+a % unitary plus
c =
2
1+2 % addition
ans =
3
1+++2 % addition and two uplusses
ans =
3
The reason uplus exists is to allow operator overloading in classes. The same works in other languages, e.g. in C#, to allow for operator overloading in confined classes.
The other reason mentioned in that C# thread is that is changes unsigned shorts to integers, which is not the case for MATLAB:
d=uint8(1)
d =
uint8
1
+d
ans =
uint8
1
a=+d
a =
uint8
1
It does, however, convert a boolean to a double, thanks to Cris Lunego for pointing that out:
+true
ans =
1
+false
ans =
0
The following however remains a mystery to me, inspired by Sanjay Manohar's comment:
>> [1 ++ 2]
ans =
1 2 % Two unary plusses
>> [1 + + 2]
ans =
3 % A normal plus and a unary one
>> [1++2]
ans =
3 % A normal plus and a unary one
The same works with multiple plusses, [1 +++..+++ 2], so with all plusses consecutively in the middle generates [1 2], all other combinations (as far as I tested) result in 3. I asked a separate question about this: Why do the plus and unary plus behave strange in array syntax?
来源：https://stackoverflow.com/questions/52555709/why-are-multiple-consecutive-plusses-allowable-syntax-in-matlab