Multiple assignment in one line

Question

I just come across the statement in embedded c (dsPIC33)

sample1 = sample2 = 0; 

Would this mean

sample1 = 0;  sample2 = 0; 

Why do they type it this way? Is this good or bad coding?

Answer 1

Remember that assignment is done right to left, and that they are normal expressions. So from the compilers perspective the line

sample1 = sample2 = 0; 

is the same as

sample1 = (sample2 = 0); 

which is the same as

sample2 = 0; sample1 = sample2; 

That is, sample2 is assigned zero, then sample1 is assigned the value of sample2. In practice the same as assigning both to zero as you guessed.

Answer 2

Formally, for two variables t and u of type T and U respectively

T t; U u; 

the assignment

t = u = X; 

(where X is some value) is interpreted as

t = (u = X); 

and is equivalent to a pair of independent assignments

u = X; t = (U) X; 

Note that the value of X is supposed to reach variable t "as if" it has passed through variable u first, but there's no requirement for it to literally happen that way. X simply has to get converted to type of u before being assigned to t. The value does not have to be assigned to u first and then copied from u to t. The above two assignments are actually not sequenced and can happen in any order, meaning that

t = (U) X; u = X; 

is also a valid execution schedule for this expression. (Note that this sequencing freedom is specific to C language, in which the result of an assignment in an rvalue. In C++ assignment evaluates to an lvalue, which requires "chained" assignments to be sequenced.)

There's no way to say whether it is a good or bad programming practice without seeing more context. In cases when the two variables are tightly related (like x and y coordinate of a point), setting them to some common value using "chained" assignment is actually perfectly good practice (I'd even say "recommended practice"). But when the variables are completely unrelated, then mixing them in a single "chained" assignment is definitely not a good idea. Especially if these variables have different types, which can lead to unintended consequences.