How do I perform a zip iterator in reverse? - Chapel

Question

How can I perform a zip iterator in reverse order? I need to shift the elements of a sub array.

My code is as follows:

for (x,y) in zip({c..d by stride},{a..b by stride},){
    A1[x]=A1[y];
}

I need to perform this in reverse order(i.e b-->a & d-->c ) so as to avoid overwriting in case of an overlapping region . (a..b is always before c..d).

Answer 1

A few things to point out.

First, in your code sample, it uses

{c..d by stride}

for example. The { } create a domain variable, but you just want to iterate over it. You can iterate over a range directly and that is both syntactically simpler and faster. I.e. don't write

for i in {1..10} { ... } // unncessary domain

instead, write this

for i in 1..10 { ... } // good

Now, to your question. Iterating over a range in reverse is done with a negative stride. Like this:

for i in 1..5 by -1 {
  writeln(i);
}

outputs

5
4
3
2
1

Such reverse iteration can be zippered, like this:

for (i,j) in zip( 1..5 by -1, 10..50 by -10 ) {
  writeln( (i,j) );
}

outputs

(5, 50)
(4, 40)
(3, 30)
(2, 20)
(1, 10)