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Consider you have some expression like
i = j = 0
supposing this is well-defined in your language of choice. Would it generally be better to split this up into two expressions like
i = 0
j = 0
I see this sometimes in library code. It doesn't seem buy you much in terms of brevity and shouldn't perform any better than the two statements (though that may be compiler dependant). So, is there a reason to use one over the other? Or is it just personal preference? I know this sounds like a silly question but it's bugging me for a long time now :-).
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In highly-object-oriented languages, double assignment results in the same object being assigned to multiple variables, so changes in one variable are reflected in the other.
The key to managing variables at different levels is the double arrow assignment operator <<- . Unlike the usual single arrow assignment ( <- ) that always works on the current level, the double arrow operator can modify variables in parent levels.
As you all know, R comes from S. But you might not know a lot about S (I don't). This language used <- as an assignment operator. It's partly because it was inspired by a language called APL, which also had this sign for assignment.
expn. this statements tells the python to evaluate all the expression on the right and assign them to the corresponding variables on the left. Simultaneous Assignments is helpful to swap values of two variables. For e.g. >>> x = 1 >>> y = 2 >>> y, x = x, y # assign y value to x and x value to y.
Once upon a time there was a performance difference, which is one of the reason that this kind of assignment was used. The compilers would turn i = 0; j = 0; into:
load 0
store [i]
load 0
store [j]
So you could save an instruction by using i = j = 0 as the compiler would turn this into:
load 0
store [j]
store [i]
Nowadays compilers can do this type of optimisations by themselves. Also, as the current CPUs run several instructions at once, performance can no longer simply be measured in number of instructions. Instructions where one action doesn't rely on the result of another can run in parallel, so the version that uses a separate value for each variable might actually be faster.
Regarding programming style, you should use the way that best expresses the intention of the code.
You can for example chain the assignments when you simply want to clear some variables, and make it separate assignments when the value has a specific meaning. Especially if the meaning of setting one variable to the value is different from setting the other variable to the same value.
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