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As everyone knows, JavaScript "Hoists" the variables to the top of the file or scope. But from my understanding, using let is the same as var only let is confined to the scope it is defined in.
Being a compiled language instead of an interpreted, can we assume Swift does NOT do this?
For example:
x = 10
var y = x + 10
var x
can we assume Swift does NOT do this?
You can assume whatever you want, but no matter the programming language, your absolute best bet it to just try it and find out. If you would have pasted your sample code into a Playground or in any Swift-capable IDE, or just tried running it through the command line, you'd quickly find out that this simply does not work.
Your question is somewhat confusing, but I think I can address all or at least most of your questions.
x = 10
var y = x + 10
var x
Assuming there is no other code to go with your original sample, it simply does not compile. All three lines have a problem.
The first two lines complain about the use of an unresolved identified 'x'. In English, this means Swift can't figure out what variable you're talking about. Variables in Swift must be declared before they are used, so the declaration on line three doesn't help the next two lines.
The third line complains that it can't figure out what type x should be. "Type annotation missing in pattern". In some cases, Swift can figure out what type our variable should be. For example, with var x = 10, Swift can figure out that x's type should be Int. If we want something else, we must specify. But if we aren't assigning a value in the declaration, Swift has no idea and must be told: var x: Int?
What about the case where x exists at a different scope?
Well, Swift allows variable shadowing. That is to say, a variable declared at one scope hides a variable declared at another scope.
So, for example:
class Foo {
let x = 10
func foo(value: Int) -> Int {
let a = value * self.x
let x = 10
return a * x
}
}
Now we can use some x before we've declared a locally scoped x, but these are different variables. Also, perhaps most importantly, notice the self. prepended to x here. It's required. Without it, Swift will refuse to compile this code and will complain: "Use of local variable 'x' before its declaration."
However, within functions of classes is not the only place we can shadow variables. We can also do it within if blocks (among other places) which is where things can get a little more confusing. Consider this:
class Foo {
let x = 10
func foo(value: Int) -> Int {
print(x)
if x > 3 {
let x = 2
print(x)
}
return x
}
}
Here, we've used x twice before declaring it. And we didn't have to make use of the self. and it doesn't complain and compiles perfectly fine. But it's important to note that outside the if block (including the x > 3 conditional) the x we're referencing is the instance variable, but inside the if block, we've created a new variable called x which shadows the instance variable. We can also create the same sort of shadowing by using if let and if var constructs (but not guard let).
The result of calling this function will be that the value 10 is printed, we enter the if block, the value of 2 is printed, then we exit the if block and the value of 10 is returned.
Now let's through var into the mix here. First, if you aren't already aware, you should start by reading this which explains the difference between let and var (one is a constant, the other is not).
Let's combine let vs var with scoping and variable shadowing to see how it effects things.
class Foo {
let x = 10
func foo(value: Int) -> Int {
print(x)
if x > 3 {
var x = 2
while x < 10 {
print(x)
x += 3
}
return x
}
return x
}
}
Okay, so this is the same as before but with a slightly more complicated bit within the if block.
Here, our locally-scoped variable is declared as a var while our instance variable remains a constant let. We cannot modify the instance variable, but we can modify the local variable. And we do so, on each iteration of the while loop.
But importantly, this variable is an entirely different variable from the instance variable. It might as well have a completely different name (and in practice, it basically always should have a different name). So modifying our local variable x doesn't change anything about our more broadly scoped instance variable x. They're different variables that reside in different memory locations. And once a variable is declared as a let or a var, that variable cannot be changed to the other.
'let' and 'var' do not have a difference in scope.
Global variables are variables that are defined outside of any
function, method, closure, or type context. Local variables are
variables that are defined within a function, method, or closure
context.
Related
When I try and run my code in Xcode playground I get a warning:
Variable 'n' was never mutated; consider changing to 'let' constant.
First of all, I am changing the variable in the body of the loop so why is it telling me to change it to a let (constant) data type.
func multiples (n : Int) {
var n = 1
for _ in (3 ..< 1000) {
var n = n + 1
let multiple3 = 3 * n
print(multiple3)
}
}
I am changing the variable in the body of the loop
No, you’re not. The one in the body of the loop is a different n.
To fix that, change
var n = n + 1
To
n = n + 1
3 little notes:
a) If you read carefully messages from Xcode, you will understand about vars' lifetime and usage. ("Variable 'n' was never mutated; consider changing to 'let' constant" )
b) you have two var with same name in different scope
c) the you enter "for", n on the left will be computed using N in outer scope, so inner n will always be == 2
d) using debugger You will see as in picture.
Those are two different variables named n. One is unchanged and one is created for each new iteration of the for loop.
The reason you can have two variables with the same name is that they exist in different scopes and the one inside the for loop temporarily overrides the one outside the loop for the duration of the loop but only inside it.
I know the question seems a bit heretical. Indeed, having much appreciated lambdas in C++11, I was quite thrilled to learn a language which was built to support them from the beginning rather than as a contrived addition.
However, I cannot figure out how to do with Scala all I can do with C++11 lambdas.
Suppose I want to make a function which adds to a number passed as a parameter some value contained in a variable a. In C++, I can do both
int a = 5;
auto lambdaVal = [ a](int par) { return par + a; };
auto lambdaRef = [&a](int par) { return par + a; };
Now, if I change a, the second version will change its behavior; but the first will keep adding 5.
In Scala, if I do this
var a = 5
val lambdaOnly = (par:Int) => par + a
I essentially get the lambdaRef model: changing a will immediately change what the function does.
(which seems somewhat specious to me given that this time a isn't even mentioned in the declaration of the lambda, only in its code. But let it be)
Am I missing the way to obtain lambdaVal? Or do I have to first copy a to a val to be free to modify it afterwards without side effects?
The a in the function definition refers the variable a. If you want to use the current value of a when the lambda has been created, you have to copy the value like this:
val lambdaConst = {
val aNow = a
(par:Int) => par + aNow
}
Why does for loop require var while for..in does not allow use of var?
for loop
for var index = 0; index < 10; i++ {
}
for..in loop
for index in "test" {
}
instead of:
for var index in "test" {
}
The Swift documentation sums it up pretty nicely:
index is a constant whose value is automatically set at the start of each iteration of the loop. As such, it does not have to be declared before it is used. It is implicitly declared simply by its inclusion in the loop declaration, without the need for a let declaration keyword.
In other words, the variable used in a for/in loop can only be a constant; thus, there's really no need to require that let be used.
The variable(s) used in a "traditional" for loop, however, can be variables or constants, so var or let is required. (Generally, they will be variables, but it is possible to use a constant in a for loop.) They can also be declared outside of the for loop (i.e., before it) and still used in the for loop. Because of this flexibility, you are required to declare it as a constant or variable.
The compiler expands for x in 0..<5 to the following:
var g = (0..<5).generate() {
while let x = g.next() {
// Use x in loop body
}
Every time around the loop, x is a freshly declared variable, the value of unwrapping the next result from calling next on the generator.
Now, that while can be rewritten in this fashion:
while var x = g.next() {
// use x here
}
I guess for this reason, for...in doesn't support the var syntax for declaring the loop counter so that it doesn't give you the wrong impression that x is mutable.
In the subsection Capturing Values of the section Closures, Apple's book The Swift Programming Language says
The closure can then refer to and modify the values of those constants and variables from within its body, even if the original scope that defined the constants and variables no longer exists.
It should be written as "refer to constants and refer to and modify variables" Try the following in a playground:
let x = 1
println(x)
{ x = x + 1 }()
println(x)
If you run this code, the variable f seems to shadow the function f. Is there anyway to reach the function f?
func f (a:Int)->Int{
return a + 43
}
var f = {(a:Int) in a + 42}
var z = f(1)
println(z)
No.
In Swift, function declarations are simply shortcuts for what you did with that closure + variable thing. That is, function names are essentially constants and should always be viewed as such (you can even pass around the function name, without brackets, as a reference).
What you're doing is you're redeclaring the name f to the variable closure. It seems Swift has a compiler issue not complaining about this. However, this problem would never occur in good code, so it's not a real problem.
It can be a bit confusing, though.