What do the semicolons of the following bit of code do?

Question

I am trying to understand code used to solve the 100 doors problem of rosettacode. The code in question is this

n=100;
tmp=ConstantArray[-1,n];
Do[tmp[[i;;;;i]]*=-1;,{i,n}];
Do[Print["door ",i," is ",If[tmp[[i]]==-1,"closed","open"]],
{i,1,Length[tmp]}]

I understand every line except the third. I have never seen the construct

Do[tmp[[i;;;;i]]*=-1;,{i,n}];

what do these semicolons inside a list do. What is the * for?

Look at the FullForm, e.g., Hold[tmp[[i;;;;i]]] //FullForm or Hold[tmp[[i ;; ;; i]] *= -1] // FullForm — Carl Woll, Oct 18 '17 at 15:36
Possible duplicate of What are the most common pitfalls awaiting new users? — Szabolcs, Oct 19 '17 at 19:03

Anxon Pués · Answer 1 · 2017-10-18T16:15:17.633

1

I look in the help that aaa[[b;;c;;d]] is understood as Part from b to c in steps of d. Then if c is missed that means to the end. Only the positions that are squares are opened. So it cames from this [[b;; ;;d]]

edited Oct 18 '17 at 16:15

answered Oct 18 '17 at 16:06

Anxon Pués

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What do the semicolons of the following bit of code do?

1 Answers1