I am new to programming and try to calculate the values of A[t] for $t$ from 0 to 0.5 in steps of 0.025 as follows but it does not work;

  Mass = m = 500000;
  StiffnessCoefficient = k =300000000;
  {NaturalFrequency = Sqrt[k/m], Period = 2*Pi/NaturalFrequency, FundamentalFrequency = 2*Pi}//N;
  P0 = 5000000;
  P[t_] := P0*Sin[FundamentalFrequency*t];
  Δτ = 0.025;
  A[0] = 0;
  A[t_] := A[t - Δτ] + P[t - Δτ]*Cos[FundamentalFrequency*(t - Δτ)] + P[t]*Cos[FundamentalFrequency*t];
  Table[A[t], {t, 0.025, 0.5, 0.025}]

I also tried different looping structures using Do, For or While sequences but I don't understand how to achieve it.

Could you help me solve my problem please ?


The issue with your code is that you are indexing into A with real (floating point) numbers. Since these are never exact, there are values that cannot be computed. To fix, remove the Block[ ] and change:

Δτ = 25/1000;
Table[A[t], {t, 25/1000, 1/2, 25/1000}] // N

This indexes with exact numbers and so gives an answer.

  • $\begingroup$ Thank you very much @bill s that is exactly what I wanted! $\endgroup$ – Mav Nov 24 '19 at 15:36
  • $\begingroup$ Probably a more economical change (and possibly faster) is to change the definition of A[0] to A[0.] $\endgroup$ – Carlo Nov 25 '19 at 15:46
  • $\begingroup$ @Carlo -- I don't think that works. The problem is that real/floating point numbers experience round off errors and once an error (no matter how tiny) occurs, you'll end up with undefined terms. $\endgroup$ – bill s Nov 26 '19 at 2:48
  • $\begingroup$ You might be right, I only checked without the Block and the first two iterations were working. $\endgroup$ – Carlo Nov 27 '19 at 7:26

Maybe this produces what you seek

P[t_] := 5000000 Sin[2. Pi t];
Δτ = 0.025;
  P[t - Δτ] Cos[2. Pi (t - Δτ)] + P[t] Cos[2. Pi t], 
  {t, Δτ, 0.5, Δτ}]
  • $\begingroup$ Thank you for your help, but why isn't P evaluated in your piece of code @Henrick Schumacher ? $\endgroup$ – Mav Nov 24 '19 at 15:38
  • $\begingroup$ I do not understand your question; P is evaluated. Maybe you are missing A? That's what Accumulate is good for. $\endgroup$ – Henrik Schumacher Nov 24 '19 at 17:38
  • $\begingroup$ Sorry for that, I said that because I obtain {1. p[0.]+0.987688 p[0.025],1. p[0.]+1.97538 etc... instead of a list of the values I seek. $\endgroup$ – Mav Nov 24 '19 at 18:22
  • $\begingroup$ With my code? Impossible. There is no lower case p. $\endgroup$ – Henrik Schumacher Nov 24 '19 at 18:34
  • 1
    $\begingroup$ Please, restart the kernel (with Exit and Shift+Enter) and copy the code here into your notebook. $\endgroup$ – Henrik Schumacher Nov 24 '19 at 19:13

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