Why does swapping the LHS and RHS of an equation give an error?

Question

def add_10(x):
    tenplus=x+10
    return tenplus
add_30=add_10(30)
add_90=add_10(90)

If I swap the last two lines such that I get:

def add_10(x):
    tenplus=x+10
    return tenplus
add_10(30)=add_30
add_10(30)=add_90

There is an error. How does a computer read the second version?

I don't think that would work in any language. How can you make an assignment to a function call? It's not clear to me why you expect that to work — roganjosh, Jun 08 '19 at 13:20
you assign by `x=value` not `value=x` and for comparing use `x==value` — anky, Jun 08 '19 at 13:21
I think you're thinking too much in terms of commutative examples from mathematics. In programming (in Python at least), `=` is an assignment and separate. Other languages vary in the syntax, but I don't think they vary in the ordering of the assignment. — roganjosh, Jun 08 '19 at 13:23
Many programming languages use `=` for assignment even though this is mathematically confusing and/or annoying. To avoid this, e.g. Pascal uses `:=` instead; this is [being made available as a synonym in a forthcoming version of Python](/a/50071110/874188), too, although the canonical `=` assignment is not going away any time soon. (I believe `:=` was chosen as an ASCII approximation of ⇐ back in the dark ages.) — tripleee, Jun 08 '19 at 13:39
(Looking at PEP572 in more detail, you won't be able to use `:=` as a direct replacement for `=` even in 3.8.) — tripleee, Jun 08 '19 at 13:47

John Kugelman · Accepted Answer · 2019-06-08T13:34:57.003

They're not equations; they're assignments. Don't confuse = for a mathematical equality symbol. It's an assignment operator. It assigns the RHS to the LHS.

Imagine that the assignment operator were ← instead of =. Then this would assign the result of add_10(30) to a variable named add_30:

add_30 ← add_10(30)

And this would... assign add_30 to the function call add_10(30)? What the heck does that mean? It doesn't exactly make sense, does it?

add_10(30) ← add_30     # huh?

Indeed, I too was confused about = when I first started programming. We programmers are all quite used to = nowadays, but it wasn't necessarily the best choice back in the 70's when C was invented. In fact, Pascal—which was designed as a teaching language—deliberately invented a new, non-symmetric assignment operator so as not to confuse math students. = was the equality operator rather than assignment.

add_30 := add_10(30)
if add_30 = 40 then ...

Too bad it didn't catch on. C won the influence wars and most modern languages base their syntax on C, so = for assignment it is.

score 1 · Answer 2 · answered Jun 08 '19 at 13:21

1

Here, "=" is an assignment operator. In the first case, you assign a known value to an unknown variable - this makes sense. In the second case, I am not sure what you are trying to achieve.

answered Jun 08 '19 at 13:21

Yulia V

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Why does swapping the LHS and RHS of an equation give an error?

2 Answers2