I am trying to figure out why the case C is not working. As you can see when I use 'yield' and '<<' sugar syntax it raises an error, but if I use the method's name 'acc.push' it works. In the other hand, if I use the 'result' variable to get yield result and then add to acc array using << syntax, it works. I just would like to understand why it does not work in the case C. Thanks.
Case A - Working fine
def my_map(my_arr)
c = 0 # the counter
acc = [] # new array
until c == my_arr.length
acc.push(yield my_arr[c])
c += 1
end
acc
end
p my_map( [1,2,3,4] ) { |each| each * 10 }
Case B - Working fine
def my_map(my_arr)
c = 0 # the counter
acc = [] # new array
until c == my_arr.length
result = yield my_arr[c]
acc << result
c += 1
end
acc
end
p my_map( [1,2,3,4] ) { |each| each * 10 }
Case C - Error: syntax error, unexpected local variable or method, expecting `end' acc << yield my_arr[c]
def my_map(my_arr)
c = 0 # the counter
acc = [] # new array
until c == my_arr.length
acc << yield my_arr[c]
c += 1
end
acc
end
p my_map( [1,2,3,4] ) { |each| each * 10 }
While you expect that Ruby interprets acc << yield my_arr[c] as
acc.<<(yield(my_arr[c]))
Ruby actually understands it like this
acc.<<(yield)(my_arr[c])
Which doesn't make much sense. It can be fixed by using parentheses as others already mentioned:
acc << yield(my_arr[c])
You're hitting a case of operator precedence, which Ruby can't resolve for itself.
You can fix it by using parentheses to provide enough clarity for Ruby to work out how to parse the offending line:
acc << yield(my_arr[c])
Just use parenthesis for the yield method call.
acc << yield(my_arr[c])
My guess is that the << tries to add the yield directly to the array which results in the syntax error.
Related
I was working on lua and looped over two tables and wanted to create a new table out of it, with no nil values in it. So this is basically a cross product. E.g:
{1,2,3} x {3,4,5} -> {1*3,1*4,1*5,2*3,2*4,2*5,3*3,3*4,3*5}
Of course this is not hard to do:
t = {1,2,3}
s = {3,4,5}
xs = {}
q = 1
for i,h in ipairs(t) do
for j,k in ipairs(s) do
xs[q] = h * k
q = q + 1
end
end
We keep a counter q and add 1 every iteration. And this works fine. However is it also possible without a counter? Can I fill up x so with just i and j such that there are no gaps in x?
t = {1,2,3}
s = {3,4,5}
xs = {}
for i,h in ipairs(t) do
for j,k in ipairs(s) do
q = f(i,j) -- <- I want to know if f is possible to write
xs[q] = h * k
end
end
I would say not, at least I was not able to find one myself easily.
EDIT: It is possible though if I am allowed to use the size of s.
s = {1,2,3}
t = {4,5,6}
xs = {}
for i,h in ipairs(s) do
for j,k in ipairs(t) do
q = i + (j - 1) * #t
xs[q] = h * k
end
end
You can use table.insert, there is no reason to specify the index in your case.
s = {1,2,3}
t = {4,5,6}
xs = {}
for i,h in ipairs(s) do
for j,k in ipairs(t) do
table.insert(xs, h * k)
end
end
for _, v in ipairs(xs) do
print(v)
end
Resource on insert:
https://www.lua.org/pil/19.2.html
I wonder if there's a cheap(performance wise) option to search an index of array element which meets certain criteria starting from an index?
Array.tryFindIndex method doesn't have an argument startIndex. I could do Array.skip(n) and then search there but it seems expensive to create an array just for search. How do I do this?
I looked List also doesn't have that argument.
Do I have to use while ... do? Is there a nicer way?
The base libraries try to provide functions for your convenience but they cannot possibly anticipate all use cases. Nothing wrong with writing your own if need be:
module Array =
let tryFindIndexFrom i p (a : _ []) =
let rec loop k =
if k >= a.Length then None
elif p a.[k] then Some k
else loop (k + 1)
if i < 0 then None else loop i
EDIT: p is the predicate testing the array elements. tryFindIndexFrom has the same signature as tryFindIndex but with the starting index added as first parameter.
EDIT 2: Added test for k < 0 for fool-proof usage.
EDIT 3: Moved test for k < 0 out of the loop as it needs to be checked only once.
Here's a way to do it using a lazy sequence of array indexes:
let input = [| 'a' .. 'z' |]
seq { 4 .. input.Length - 1 }
|> Seq.tryFind (fun i -> input |> Array.tryItem i = Some 'x')
I'll leave it to you to generalise this into a helper function if you think that's necessary.
The nice thing about the current form is that it's quite flexible. You can change the maximum index easily, or search backwards, e.g. seq { input.Length - 1 .. -1 .. 4 }.
Follow your instinct. Considering Array.skip but noting the obvious waste of allocating a second array, you can take it one step further and generalize to the lazily evaluated Seq.skip, compose it with the standard Seq.tryFindIndex function and add the offset, if applicable.
let tryFindIndexMin n p =
Seq.skip n
>> Seq.tryFindIndex p
>> Option.map ((+) n)
// val tryFindIndexMin : n:int -> p:('a -> bool) -> (seq<'a> -> int option)
[ for i in 0..3 ->
[|"a"; "b"; "a"; "b"|]
|> tryFindIndexMin i ((=) "a") ]
// val it : int option list = [Some 0; Some 2; Some 2; null]
local sometable = {a = "ag", b = "fa"}
for k, v in ipairs(sometable) do
print(k, v)
end
The code above is my effort, so how do i print a, b in that table?!
You are using the wrong iterator. ipairs is for sequences. For hash-like tables, use pairs instead:
for k, v in pairs(sometable) do
ipairs only traversers the array part of a table. What you can do is simply writing
print(sometable.a, sometable.b)
or you can cycle through both the dictionary and array parts of the table by using
for key, value in pairs(sometable)
You could also define your own iterator to only cycle through the dictionary part of the table. In my mind it would look like
function cycle(dict)
local contentarray = {}
for k, v in pairs(dict) do
contentarray[#contentarray + 1] = {k, v}
end
local n = 0
return function()
n = n + 1
if not contentarray[n] then
return
else
while type(contentarray[n][1]) ~= "string" do
n = n + 1
end
return contentarray[n][1], contentarray[n][2]
end
end
end
But that would be higly inefficient.
The issue happens when the variable, that the array was built from, was a nil initially.
y = (1..2).map do
v = nil
v = 1
v
end
p y # => [1, 1]
p y.class # => Array(Int32)
p y.sum # => 2
When v stops being nil on a condition, that is potentially computational and not solvable while compiling:
z = (1..2).map do
v = nil
v = 1 if true
v
end
p z # [1, 1]
p z.class # => Array(Nil | Int32)
The array gets more complex type, that isn't compatible with current sum implementation, so p z.sum causes compile time error:
undefined method 'zero' for Nil:Class (compile-time type is (Nil | Int32):Class)
def sum(initial = T.zero)
^~~~
How am I supposed to fight this properly?
Or maybe it waits for some better implementation of stdlib sum method or anything else?
UPD: inject gives the same:
p z.inject{ |i, j| i + j }
undefined method '+' for Nil (compile-time type is (Nil | Int32))
You can use Iterator#compact_map to select non-nil values. The compiler will be able to infer a Array(Int32) in that case.
http://play.crystal-lang.org/#/r/e85
z = (1..2).map do
v = nil
v = 1 if true
v
end
pp typeof(z) # => Array(Nil | Int32)
pp z # => z = [1, 1]
y = z.compact_map(&.itself)
pp typeof(y) # => Array(Int32)
pp y # => y = [1, 1]
Also, notice that typeof(Expr) and Expr.class might lead to different results. The first is the compile time type and the later is the runtime type.
An alternative solution to what Brian says is to use sum with a block:
http://play.crystal-lang.org/#/r/ein
z = (1..2).map do
v = nil
v = 1 if true
v
end
puts z.sum { |x| x || 0 } #=> 2
I have two hashes:
p = {"name"=>"TRICHI", "subdistrict"=>{"WANDIWASH"=>"1234"}}
q = {"name"=>"VELLORE", "subdistrict"=>{"WANDIWASH"=>"4183"}}
I need to make this as
r = [{"name"=>"VELLORE", "subdistrict"=>{"WANDIWASH"=>"4183"}},
{"name"=>"TRICHI", "subdistrict"=>{"WANDIWASH"=>"1234"}}]
I guess you want this:
r = [] << p << q
# or r = [p, q]
# either way you'll get:
# [ {"name"=>"VELLORE", "subdistrict"=>{"WANDIWASH"=>"4183"}},
# {"name"=>"TRICHI", "subdistrict"=>{"WANDIWASH"=>"1234"}} ]
This way you will have an array with 2 hashes.
As Tim pointed, r doesn't seem to be a Hash, maybe you meant an Array, in which case you can do
r = [p,q]
or
r = []
r << p
r << q
.. keep going for any other entry you want to push into r