Mathematica Stack Exchange is a question and answer site for users of Mathematica. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I am trying to make a function that works as follows: Integrate 1 for all t in the interval $ [0,1] $ such that the first element of function vstar is positive. vstar is a function that outputs a pair of real numbers. Unfortunately the test vstar[t, a1, a2, f1, f2, σ]][[1]] > 0 does not work as expected (this is waht I've tried):

D1[f1_, a1_, f2_, a2_,σ_] := NIntegrate [If[vstar[t, a1, a2, f1, f2,σ][[1]] > 0, 1, 0] f[t], {t, 0, tup}, AccuracyGoal -> 8];

The way it is, Mathematica tests if the first slot of vstar is greater than zero (that is $ t>0 $) rather than testing if the first part of the output of vstar is greater than zero. vstar is defined so that it evaluates only for numerical values. I think I need to evaluate the function vstar before the test is performed, I just don't know how to do it - could someone explain how?

p.s. For further reference, here's the code of vstar:

vstar[tt_?NumericQ, aa1_?NumericQ, aa2_?NumericQ, 
  ff1_?NumericQ, ff2_?NumericQ, σσ_?NumericQ] := 
  list = {objm, objs1, objs2}  /. {t -> tt, 
     a1 -> aa1, a2 -> aa2, f1 -> ff1, 
     f2 -> ff2, σ -> σσ}; 
  If[list[[1]] < 0 && list[[2]] < 0 && list[[3]] < 0, {0, 0},
   Which[Ordering[list , -1] == {1}, {v1, v2} /. vm, 
     Ordering[list , -1] == {2}, {v1 /. v1s, 0}, 
     Ordering[list, -1] == {3}, {0, 
      v2 /. v2s}]  /. {t ->tt, a1 -> aa1, 
     a2 -> aa2, f1 -> ff1, f2 -> ff2, σ -> σσ}]]
share|improve this question
I edited your question but your code still has issues so I can't really help much more. At first sight, there are wrong square brackets inserted in your If clause so I suggest you edit the code to amend this (I have not touched your code other than formatting) and also provide the function vstar. May I also suggest you read the relevant section on formatting for future posts. – gpap Sep 17 '13 at 11:26
@gpap Thanks, I spotted the typo and fixed it. – Emilio Calvano Sep 17 '13 at 11:37
Ok, so, this isn't much help now - it depends on other bits of code. The idea is to have a self-containted, minimally working example for people to test, without having to see part sof your code that are irrelevant to the problem. So, if the output of vstar always gives out a pair {a,b} where, say, 0<a,b<1, it will be easier to write vstar[args__]:=RandomReal[{0,1},{2,1}]. Also, the page about formatting that I linked tells you that a code block is formatted easily by selecting the code a pressing ctrl + K. I have done that now, but you should be able to do it yourself in the future – gpap Sep 17 '13 at 12:04

What happens now since vstar only evaluates on numerical arguments Part sees the unevaluated form and extracts the first argument since it doesn't know any better:

vstar[t, a1, a2, f1, f2, s][[1]]
(* t *) 

To get around this you can make a function that extracts the first part only on numerical input:

numericFirst[l_?(VectorQ[#, NumericQ] &)] := l[[1]]

numericFirst[vstar[t, a1, a2, f1, f2, s]]
(* vstar[t, a1, a2, f1, f2, s] *)

(* 0.1 *) 
share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.