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502

answers:

8

What is associativity (for an operator) and why is it important?

Updated: operator associativity

+1  A: 

I assume you mean operator associativity...

It's the order of binding of operands to an operator. Basically:

a - b + c

might be evaluated as (assuming - and + have the same precedence):

((a - b) + c) or,
(a - (b + c))

If operators are left associative (bind immediately to the left operand), it'll be evaluated as the first. If they are right associative, it'll be evaluated as the second.

Mehrdad Afshari
+4  A: 

There are three kinds of associativity:

The Associative property in mathematics

Order of Operations in programming languages

Associativity in CPU caches.

The Associative property in mathematics is a property of operators such as addition (+). This property allows you to rearrange parentheses without changing the value of a statement, i.e.:

(a + b) + c = a + (b + c)

In programming languages, the associativity (or fixity) of an operator is a property that determines how operators of the same precedence are grouped in the absence of parentheses; i.e. in what order each operator is evaluated. This can differ between programming languages.

In CPU caches, associativity is a method of optimizing performance.

Robert Harvey
A: 

If you mean operator associativity:

It defines the way expressions are parsed. It gives a standard, so every expression is parsed the same way.

It's mostly important for operations which have the same precedense, when there could be side effects.

Ikke
+4  A: 

For operators, associativity means that when the same operator appears in a row, then to which direction the evaluation binds to. In the following, let Q be the operator

a Q b Q c

If Q is left associative, then it evaluates as

(a Q b) Q c

And if it is right associative, then it evaluates as

a Q (b Q c)

It's important, since it changes the meaning of an expression. Consider the division operator with integer arithmetic, which is left associative

4 / 2 / 3    <=>    (4 / 2) / 3    <=> 2 / 3     = 0

If it were right associative, it would evaluate to an undefined expression, since you would divide by zero

4 / 2 / 3    <=>    4 / (2 / 3)    <=> 4 / 0     = undefined
Johannes Schaub - litb
A: 

it is the order of evaluate for operators of the same precedence. The LEFT TO RIGHT or RIGHT TO LEFT order matters. For

3 - 2 - 1

if it is LEFT to RIGHT, then it is

(3 - 2) - 1

and is 0. If it is RIGHT to LEFT, then it is

3 - (2 - 1)

and it is 2. In most languages, we say that the minus operator has a LEFT TO RIGHT associativity.

動靜能量
If you already knew the answer, then why did you ask the question?
Robert Harvey
it was to help out new people. i remember learning C long time ago and didn't know what associativity really was until later.
動靜能量
I suspect that most people learning C can do without your "help".
anon
hm, for example, is associativity limited to the same operator, or is it for operators on the same precedence level? Can many people answer that for sure without checking books or references?
動靜能量
@Neil Butterworth, why so hostile? I think it's acceptable to post an answer to your own question. This is in the FAQ, and it's been mentioned in the podcast several times.
Jay Conrod
@jay Why not address this question to Robert Harvey? He questioned whether this answer was needed, I didn't.
anon
A: 

If you are referring to "operator associativity" - it is how a language determines how operators of the same precedence are grouped in the absence of parentheses.

For example, the + and - operators in C-based languages have the same precedence. When you write an expression that uses both of them (without parentheses) the compiler must determine what order to evaluate them in.

If you write 12 - 5 + 3, the possible evaluations include:

  1. (12 - 5) + 3 = 10
  2. 12 - (5 + 3) = 4

Depending on the order you evaluate the expression in, you can get different results. In C-based languages, + and - have left associativity, which means that the expression above would evaluate as the first case.

All language have strongly-defined rules for both precedence and associativity. You can learn more about the rules for C# here. The general concepts of operator associativity and precedence are well covered on wikipedia.

LBushkin
Your examples would be clearer if they all used the same operands.
Michael Carman
A: 

Most of the previous examples have used constants. If the arguments happen to be function calls, the order that the calls are made in may be determined by the association rules, depending of course on your compiler. And if those functions have side effects ..

JustJeff
A: 

We all know that precedence is important but so is associativity in interpreting the meaning of an expression. For a really simple intro try Power of Operators.

mikej