colon-equals

问题 In some programming languages, I see (ex.): x := y What is this := operator generally called and what does it do? 回答1: In all languages that support an operator := it means assignment. In languages that support an operator := , the = operator usually means an equality comparison. In languages where = means assignment, == is typically used for equality comparison. does := mean = ? I can't recall any languages where := means the same as = . In MySQL := and = are both used for assignment,

问题 For variable assignment in Make, I see := and = operator. What's the difference between them? 回答1: This is described in the GNU Make documentation, in the section titled 6.2 The Two Flavors of Variables . In short, variables defined with := are expanded once, but variables defined with = are expanded whenever they are used. 回答2: Simple assignment := A simple assignment expression is evaluated only once, at the very first occurrence. For example, if CC :=${GCC} ${FLAGS} during the first

问题 For variable assignment in Make, I see := and = operator. What's the difference between them? 回答1: This is described in the GNU Make documentation, in the section titled 6.2 The Two Flavors of Variables . In short, variables defined with := are expanded once, but variables defined with = are expanded whenever they are used. 回答2: Simple assignment := A simple assignment expression is evaluated only once, at the very first occurrence. For example, if CC :=${GCC} ${FLAGS} during the first

问题 in MySQL what is the difference between these two command? They work perfectly and the result is always the same: set @numRecords = (select count(*) from config); set @numRecords := (select count(*) from config); Thanks Davide 回答1: Quoting the MySQL 5.7 Reference Manual, section 10.4 User-Defined Variables: For SET, either = or := can be used as the assignment operator. You can also assign a value to a user variable in statements other than SET. In this case, the assignment operator must be :

问题 What is the difference in between set test_var = 20; and set test_var:=20; as they both seem to assign the value ? 回答1: Both of them are assignment operators but one thing I can find their differences is that = can be used to perform boolean operation while := cannot. valid : SUM(val = 0) Invalid: SUM(val := 0) FROM User-Defined Variables One more thing, You can also assign a value to a user variable in statements other than SET. In this case, the assignment operator must be := and not =

问题 This question already has answers here : Closed 8 years ago . Possible Duplicate: What is the use of the := syntax? I've tried hunting down the MDSN documentation for := in VB.NET as well as scoured Google only to be linked to a dead MSDN page... What would the purpose of := be? 回答1: It strongly names arguments, allowing you to call a method with arguments in an order other than that specified in the method definition. For example: sub foo (byval x As Long, byval y As Long) debug.print

问题 Currently, I am learning some FPGA design techniques using VHDL, my problem is whether we can use := and <= interchangeably in VHDL or not, though I've seen the use of := in constants declarations and <= in assignments? Thanks in advance! 回答1: The rules are a little more complex than this, but basically: you use <= to do signal assignment, which takes effect on the next delta cycle. You use := to do variable assignment, which takes place immediately. So if you have a signal, you always use <=

问题 In the Go web server example here: http://golang.org/doc/effective_go.html#web_server The following line of code works var addr = flag.String("addr", ":1718", "http service address") but changing it to addr := flag.String("addr", ":1718", "http service address") is a compilation error. Why? Does it have anything to do with the face that the return type of the function is *string instead of string ? What difference does that make? UPDATE : Thanks for pointing out that := is not allowed at the

问题 I frequently find myself doing a long series of chained calculations using := on the same data table. For example, something like this test = data.table(1:10, 1:10, 1:10, 1:10) test[, V1 := V1^2] test[, V2 := V1*V2] test[, V3 := V2/V3] test[, V4 := sqrt(V3)] test[, new := letters[V4]] Having to write test[, ...] on every line 1) takes longer to type (not a big issue I can cope with that). But, more importantly, also distracts visually from the flow and content of the calculation. I would much

问题 Using R's data.table package, This works: instruction = "a = data.table(name=1:3, value=1:3, blah=1:3); a[,c('value', 'blah'):=NULL]" eval(parse(text=instruction)) # name #1: 1 #2: 2 #3: 3 This works: myFunc = function(instruction) { eval(parse(text=instruction)) } myFunc(instruction) # name #1: 1 #2: 2 #3: 3 Now, put this function into a package, load it, and try to call it. This doesn't work: myFuncInPackage(instruction) #Error in `:=`(c("value", "blah"), NULL) : # Check that is.data.table