I'm puzzled by the following observation. On the one hand, this works:
for i in 5..10:
echo i
But as soon as I store the slice in a variable, I can no longer iterate over it, i.e., this fails:
var slice = 5..10
for i in slice:
echo i
The error is type mismatch: got (Slice[system.int]), and apparently there is no overloaded signature of the system.items iterator for Slice[T]. This raises the questions:
Why does it work at all in the first case?
Is there a workaround to iterate over a slice in the second case?
With for i in 5..10: you invoke the iterator .. (doc), which is just an alias for countup. Since this is an inline iterator it transforms the for-loop into a while-loop over the values 5 to 10. But inline iterators cannot be assigned to a variable, other than closure iterators.
With var slice = 5..10 you invoke the proc .. (doc), which generates a Slice(a: 5, b: 10). But Slices don't have a default items iterator defined.
You could iterate from slice.a to slice.b, like this:
var slice = 5..10
for i in slice.a .. slice.b:
echo i
Since this is not very nice, the proper solution is to define your own items iterator, like this:
iterator items*[T](s: Slice[T]): T =
for i in s.a .. s.b:
yield i
var slice = 5..10
for i in slice:
echo i
Since this seems pretty reasonable to me, I made a pull request to have this included in the system module: https://github.com/nim-lang/Nim/pull/2449
Related
I am coming from javascript and know how to check if a variable exists. We can use !!var
I have come across an array in Go where I want to know if an index exists:
myArr := []int{1, 2, 3}
if myArr[3] {
fmt.Println("YES")
}
When I run this it gives me an error: Index Out Of Range: 3
Since Go is a compiled language the concept of a variable not existing does not make sense. The closest thing is that some types can take a nil value.
As far as arrays go they just have a length (without gaps). So if the length is N then only indices 0 to N-1 are valid. The built-in len() function works with any array or slice.
I wanted to learn more about for loops, as far as I know there are different types?
For instance,
for i = 1, 5 do
print("hello")
end
^ I know about this one, it's going to print hello 5 times, but there are others like the one below which I do not understand, specifically the index bit (does that mean it is number 1?) and what is the ipairs for
for index, 5 in ipairs(x) do
print("hello")
end
If there are any other types please let me know, I want to learn all of them and if you can provide any further reading I'd be more than greatful to check them out
As you can read in the Lua reference manual
3.3.5 For Statement
The for statement has two forms: one numerical and one generic.
The numerical for loop repeats a block of code while a control
variable runs through an arithmetic progression. It has the following
syntax:
stat ::= for Name ‘=’ exp ‘,’ exp [‘,’ exp] do block end
Example:
for i = 1, 3 do
print(i)
end
Will output
1
2
3
You seem familiar with that one. Read the reference manual section for more details.
The generic for statement works over functions, called iterators. On
each iteration, the iterator function is called to produce a new
value, stopping when this new value is nil. The generic for loop has
the following syntax:
stat ::= for namelist in explist do block end namelist ::= Name {‘,’
Name}
Example:
local myTable = {"a", "b", "c"}
for i, v in ipairs(myTable) do
print(i, v)
end
Will ouput
1 a
2 b
3 c
ipairs is one of those iterator functions mentioned:
Returns three values (an iterator function, the table t, and 0) so
that the construction
for i,v in ipairs(t) do body end will iterate over the key–value pairs (1,t[1]), (2,t[2]), ..., up to the first nil value.
Read more about ipairs and pairs here:
https://www.lua.org/manual/5.3/manual.html#pdf-pairs
https://www.lua.org/manual/5.3/manual.html#pdf-ipairs
Of course you can implement your own iterator functions!
Make sure you also read:
Programming in Lua: 7 Iterators and the Generic for
Yes, It will print hello 5 times
According to this answer on Difference between pairs, ipairs, and next?
ipairs does the exact same thing as pairs, but with a slight twist to it.
ipairs runs through the table, until it finds a nil value, or a value that is non-existent, if that makes sense. So, if you ran the script I showed you for pairs, but just replaced pairs with ipairs, it would do the exact same thing
I'm trying to calculate the necessary length for an array in a merge sort implementation I'm writing in go. It looks like this:
func merge(array []int, start, middle, end int) {
leftLength := middle - start + 1
rightLength := end - middle
var left [leftLength]int
var right [rightLength]int
//...
}
I then get this complaint when running go test:
./mergesort.go:6: non-constant array bound leftLength
./mergesort.go:7: non-constant array bound rightLength
I assume go does not enjoy users instantiating an Array's length with a calculated value. It only accepts constants. Should I just give up and use a slice instead? I expect a slice is a dynamic array meaning it's either a linked list or copies into a larger array when it gets full.
You can't instantiate an array like that with a value calculated at runtime. Instead use make to initialize a slice with the desired length. It would look like this;
left := make([]int, leftLength)
I'm wondering why it's not possible to do the following in go:
func main() {
stuff := []string{"baz", "bla"}
foo("bar", stuff...)
}
func foo(s ...string) {
fmt.Println(s)
}
In my understanding, slice... "explodes" the slice so it can be used for multi argument function calls. So the above example should actually expand to foo("bar", "baz", "bla").
foo(stuff...) works as expected, no surprises here, but in the example above, the compiler complains about too many arguments.
Is this a desired limitation? I'm coming from a ruby background where a foo("bar", *stuff) is perfectly fine (and is, at least in my book, the same thing), that's why this surprises me.
The value for a variadic argument can be specified either by enumerating the elements, or using an existing slice, specified by its name followed by ....
You want to mix the 2 possible ways which is not permitted by the Go Language Specification (Passing arguments to ... parameters).
If the first form is used (enumerating the elements):
The value passed [as the variadic parameter] is a new slice of type []T with a new underlying array whose successive elements are the actual arguments.
If the latter is used (passing an existing slice followed by ...) no new slice is created, the one you pass is used as is. And the passed slice can only be used to specify the value of one – the final – variadic parameter. Attempting to pass both a single element and a slice will not match the signature (the parameter list in this case) of your function and you'll get an error:
too many arguments in call to foo
There is no actual "exploding" involved in Go, the term is just used in other languages to help visualize that the passed array or slice will not be an element of the variadic parameter but will be the value of variadic parameter itself.
Mixing the 2 would require to allocate a new slice because obviously the existing slice cannot be used.
The ugly way to get this to work is make it into a new variadic.
foo(append([]string{"bar"}, stuff...)...)
And if the order doesn't matter:
foo(append(stuff, "bar")...)
https://play.golang.org/p/mY6y0vScfPB
The specification on this is at the "Passing arguments to ... parameters":
If f is variadic with a final parameter p of type ...T, then within f the type of p is equivalent to type []T.
If f is invoked with no actual arguments for p, the value passed to p is nil.
Otherwise, the value passed is a new slice of type []T with a new underlying array whose successive elements are the actual arguments, which all must be assignable to T.
In your case, where stuff... works:
If the final argument is assignable to a slice type []T, it may be passed unchanged as the value for a ...T parameter if the argument is followed by .... In this case no new slice is created.
But "bar", stuff... doesn't match either case specified above.
T, []T doesn't match f([]T).
I ran into this situation when preparing arguments to feed to external commands. If possible, just build an one argument slice, then you don't have to worry about combining scalars with slices when it's time to call the function:
package main
import "os/exec"
func main() {
stuff := []string{"bar"}
stuff = append(stuff, "baz", "bla")
exec.Command("name", stuff...).Run()
}
I'm executing a template with 2 parallel arrays (same size) and I want to list items from both arrays in parallel, how do I use index inside of range?
this obviously doesn't work:
{{range $i, $e := .First}}$e - {{index .Second $i}}{{end}}
One of the predefined global template functions is index.
index Returns the result of indexing its first argument by the
following arguments. Thus index x 1 2 3 is, in Go syntax,
x[1][2][3]. Each indexed item must be a map, slice, or array.
So you are on the right track. The only issue is that you are not accounting for the fact the dot has been reassigned within the range block.
So you need to get back to the original dot, for that we have the following
When execution begins, $ is set to the data argument passed to Execute, that is, to the starting value of dot.
So (assuming there is nothing else going on in your template) you should be able to do:
{{range $i, $e := .First}}$e - {{index $.Second $i}}{{end}}
Personally though, I would create a template function called zip that accepts multiple slices and returns a slice of each pair of values. It would look cleaner in your template and probably get reused somewhere.