What does ":=" do?

Question

I've seen := used in several code samples, but never with an accompanying explanation. It's not exactly possible to google its use without knowing the proper name for it.

What does it do?

Solution 1

http://en.wikipedia.org/wiki/Equals_sign#In_computer_programming

In computer programming languages, the equals sign typically denotes either a boolean operator to test equality of values (e.g. as in Pascal or Eiffel), which is consistent with the symbol's usage in mathematics, or an assignment operator (e.g. as in C-like languages). Languages making the former choice often use a colon-equals (:=) or ≔ to denote their assignment operator. Languages making the latter choice often use a double equals sign (==) to denote their boolean equality operator.

Note: I found this by searching for colon equals operator

Solution 2

It's the assignment operator in Pascal and is often used in proofs and pseudo-code. It's the same thing as = in C-dialect languages.

Historically, computer science papers used = for equality comparisons and ← for assignments. Pascal used := to stand in for the hard-to-type left arrow. C went a different direction and instead decided on the = and == operators.

Solution 3

In the statically typed language Go := is initialization and assignment in one step. It is done to allow for interpreted-like creation of variables in a compiled language.

// Creates and assigns
answer := 42

// Creates and assigns
var answer = 42

Solution 4

Another interpretation from outside the world of programming languages comes from Wolfram Mathworld, et al:

If A and B are equal by definition (i.e., A is defined as B), then this is written symbolically as A=B, A:=B, or sometimes A≜B.

■ http://mathworld.wolfram.com/Defined.html

■ https://math.stackexchange.com/questions/182101/appropriate-notation-equiv-versus

Solution 5

Some language uses := to act as the assignment operator.

View more solutions

Comments

[Beta]

In what language?

[svens]

You normally use := when you define something, to separate it from regular variable changes.. What programming language are we talking about?

[Andrew]

PL/SQL it is for assignment. But given a different language, that answer isn't guarenteed to hold true - so which languages was the example in?

[PRINCE KUMAR]

To google something like this, spell it out and enclose it in quotes, like so: "colon equals"

[Admin]

I think Pascal's got this operator !

[Shamaoke]

You can search for special symbols using this service.

[sevko]

The OP might have been referring to pseudocode, in which I've often seen :=.

[nighthawk454]

Ironically, this answer is now above Wikipedia when searching for colon equals operator.

[TigOldBitties]

@Pacerier see this post stackoverflow.com/questions/7462322/… as to why the answer to your question can be both "Yes" and "No".

[Pacerier]

@TigOldBitties, Good gotcha from Erwin down there.

[ATLUS]

If we keep typing colon equals operator, we work magic on Google's SEO to make this the top result

[Variadicism]

If they wanted it close to the left arrow, they could have used <- like Haskell did. They weren't trying to get close to the left arrow with :=, they were using the mathematical 'is defined as' operator: mathworld.wolfram.com/Defined.html

[Michael restore Monica Cellio]

Pedant alert: <- in Haskell is not assignment. Haskell does not have destructive assignment in the way of Pascal, Ada etc. <- is part of the do-notation syntax for parameter substitution. It is more analogous to the process of substituting values into parameters in a subroutine call.

[Variadicism]

@Michael Fair enough. You're right. My bad. Anyway, the point remains that if they were trying to imitate the left arrow, they would not have used :=, they would have used <-.

[Gabriel Staples]

That link looks old. Here's the updated link (I think), but the quote seems to be massively changed since then and I can't track down the new quote exactly: en.wikipedia.org/wiki/Assignment_(computer_science).