Trouble with Nested Do Loops

Question

I have:

myres = {};
Do[
 Do[
  Append[myres, {j, i}], {i, 1, 3}
  ],
 {j, 1, 5}
 ]

However, when I run the code and then enter:

myres

I get:

(* { } *)

Why is it empty? Can someone explain my misunderstanding? Also, could someone suggest some lines in the code that will show some intermediate results as to what is happening?

Rather use Table. Avoid imperative programming in Mathematica. — Pankaj Sejwal, Dec 06 '15 at 06:41
@Rorschach Aha! I should have had myres = Append[myres, {j, i}]. Of course, AppendTo is more efficient. And yes, I like Table better as well, but I am trying to work on some procedural programming style. — David, Dec 06 '15 at 06:44
AppendTo is not efficient, try to run your program with AppendTo for say i=300,j=500. And then try Table with same values. Mathematica has some inherent problems, like any other system. — Pankaj Sejwal, Dec 06 '15 at 06:46
Sow and Reap can be more useful than Table in more complicated cases (e.g., if a list item in not always generated in each iteration). — Michael E2, Dec 06 '15 at 14:45

score 1 · Answer 1 · answered Dec 06 '15 at 06:43

1

Append is not a mutator. You either need myres = Append[...] or AppendTo[...].

As far as monitoring, you could simply add Print statements, or try Monitor.

answered Dec 06 '15 at 06:43

ZachB

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Trouble with Nested Do Loops

1 Answers1