How can I perform Element wise Substitution using Subs without using 'for' loop?

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Harshvardhan
Harshvardhan 2012-11-5
Hello Everyone
I would appreciate if anyone can help me understand how I can use 'subs' for simultaneous substitutions (row/column-wise) in a vector of functions. For example, if I enter
>> syms x y z a b
>> func =
[ x + y + 2*z;
x^2 + y^2 + 3;
3*y + z + 3];
>> subs(func, [x y; y z; z x]; [a b; b a; a a])
I expected to see
[a + b + 2*z;
x^2 + b^2 + 3;
3*y + a + 3]
(This because I want 'a' for 'x' and 'b' for 'y' only in the first function, 'b' for 'y' and 'a' for 'z' only in the second function and so on)
Instead, I see the following result
3*a + b
a^2 + b^2 + 3
a + 3*b + 3
I would like to have a simultaneous substitutions across functions to avoid writing a 'for' loop as I have to perform substitutions in a huge vector of functions and using a for loop is slowing down my program drastically.
Thanks

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回答（1 个）

Walter Roberson
Walter Roberson 2012-11-5
When you subs() specifying a symbol, all occurrences of the symbol are replaced in the expression. When you subs() specifying an expression, only occurrences of the expression are substituted for, and all such occurrences are done -- but "hidden" occurrences of the expression are not touched. For example if you had 2*x + y and substituted on x + y then subs() would not know to break this down into x + x + y and do the substitution; it also might not find x + y in an expression x + z + y . You also cannot, in such a case, use a "pattern".
If you want to substitute into part of an expression, use indexing, or subsop(). See also the MuPAD match() routine.

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