Show Timing but assign the result directly

Question

The code a=2+2//Timing assigns the variable a to {time,4} . How can I show the evaluation time, but still assign a with the result of 2+2 directly (in one line)?

I have tried defining a custom Timing function (working now)

showTiming[in_] :=  Block[{res}, res = Timing[in]; Print[res[[1]], " s"]; res[[2]] ]
SetAttributes[showTiming, HoldAll] (* <- IMPORTANT!*)
a=2+2//showTiming

Obviously 2+2 is going to show zero... 2+2 is extremely fast to calculate. If you want to test it more, try timing something difficult like factoring big numbers e.g a = FactorInteger[Times @@ RandomPrime[2^70, 2]] — flinty, May 21 '21 at 12:41

score 5 · Accepted Answer · answered May 21 '21 at 14:16

5

The only thing missing from your definition of showTiming was a HoldAll or HoldFirst attribute. Without that attribute the argument in is evaluated before being handed off to showTiming.

Since V12.2 there is a function, EchoTiming, that effectively does the same thing as your function, but using Echo instead of Print to display the time.

answered May 21 '21 at 14:16

Brett Champion

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Thank you very much for looking at my attempt and explaining the HoldAll function. – Albercoc May 21 '21 at 14:25

flinty · Answer 2 · 2021-05-21T12:37:49.980

1

In one line this looks a bit ugly:

(a = Last@#; Print@*First@#) &@Timing[2 + 2]
(* 0. and a is set to 4 *)

With a harder workload:

(a = Last@#; Print@*First@#) &@Timing[
  FactorInteger[Times @@ RandomPrime[2^70, 2]]
]
(* 0.34375, and a is set to the factorization *)

Maybe you could have this as a function like:

TimedExpr[expr_] := (Last@#; Print@*First@#;) &@Timing[expr]
SetAttributes[TimedExpr, HoldAll]
(* example *)
TimedExpr[
 b = FactorInteger[Times @@ RandomPrime[2^70, 2]]
]

edited May 21 '21 at 12:37

answered May 21 '21 at 12:30

flinty

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(for other users): this does not show the result of the evaluation. But removing the semicolon in the parenthesis achieves that, (Print@*First@#; Last@#) &. – Albercoc May 21 '21 at 14:31
Thank you @flinty for taking the time to do this function. I would have liked to accept both answers. – Albercoc May 21 '21 at 14:33

score 1 · Answer 3 · answered May 21 '21 at 14:34

1

a = 2+2; // Timing // First

You can leave out the // First for simplicity.

answered May 21 '21 at 14:34

Roman

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Show Timing but assign the result directly

3 Answers3