How to multiply symbolic with numeric?
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Ali Najem
am 21 Dez. 2020
Kommentiert: Ali Najem
am 23 Dez. 2020
Hi all'
assume that
w=randn(2,2)
a=sym('a',[2 2])
I need to get the answer of multiblication of (a .* w) with smae size [ 2 2] since, I multiplied them like (a .* w) i got this answer
ans =
[ -(4545034027795199*a1_1)/18014398509481984, -(8311781718565711*a1_2)/18014398509481984]
[ -(1390961227654289*a2_1)/1125899906842624, (5388115603050723*a2_2)/18014398509481984]
However, the random values of w is
w =
-0.2523 -0.4614
-1.2354 0.2991
a =
[ a1_1, a1_2]
[ a2_1, a2_2]
I don't belive this correct results however, I need the results like this [ -0.2523 * a1_1, -0.4614* a1_2
-1.2354 * a2_1, 0.2991 * a2_2]
thanks indeed
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Walter Roberson
am 21 Dez. 2020
w = round(sym(randn(2,2),'d'),4)
You are, by the way, not correct about what the value of w is. The actual values for w extend to about 15 significant decimal places, and you are getting confused because you have the default format in effect. I recommend that you use
format long g
and change your preferences to use that.
However, you defined your required output in terms of four decimal places, so I included a call to force four decimal places.
Note: symbolic floating point numbers give the strong impression that they are base 10 internally, but closer to the truth is that they use a base 2^30 representation, which is close enough to base 10^10 representation that it takes real effort to prove the difference.
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Walter Roberson
am 23 Dez. 2020
Taking one derivative of one entry of dL4 took my system about 18 minutes. You have over 65000 derivatives to take at the bottom of your code, each on a value that is more complicated than dL4 is. You would expect a minimum of 20 minutes per derivative, times 65000 derivivates, gives about 130000 minutes -> over 900 days.
Each derivative looks like it is going to be over 2 gigabytes to write out -> 130 petabytes .
Suppose that I whipped up something to run on distributed AWS and (somehow!!) managed to create that 130+ petabyte file tomorrow: what would you do with it?
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