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Finding similar symbolic expressions in a vector

Asked by Jan Kappen on 12 Oct 2012
Hey guys, i have following problem:
Let there be a long vector containing symbolic expressions. To reduce the vector length i try to find elements which are correlated (i don't know if that is the right word for what i mean, maybe an example will help).
For example:
v = [a, b^2+c, d^4/e, -a+b^2+c, -a].'
Here is v(5)=-v(1) or the other way round. And v(4)=-v(1)+v(2)
There might exist other combinations, but these i can see directly.
The big question is: Is there a function or some functions i can use, or should i iterate the whole vector and compare each row to the others?
The first problem (v(x) = t*v(y) where t is a constant, x and y are some rows in the vector) is easier than the second one i think.
I tried some combinations of unique and sort, but unique only shows me identical lines, not similar ones.
Any ideas? Thank you very much :)
ps : i use matlab 2011a

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1 Answer

Answer by Star Strider
on 12 Oct 2012

To start, I suggest children:
syms a b c d e
v = [a, b^2+c, d^4/e, -a+b^2+c, -a].'
cv = children(v)
cv{1:5}
I have 2012b but I believe this is also available in 2011a.

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Nice idea, but unfortunately children does not exist in 2011a.
Sorry.
In that situation, I suggest subexpr, the only other alternative I can find. It should exist in 2011a.
You can use subexpr iteratively:
syms a b c d e
v = [a, b^2+c, d^4/e, -a+b^2+c, -a].'
[r1,s1] = subexpr(v, 'S1')
[r2,s2] = subexpr(r1, 'S2')

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