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

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

Answer 1

[Walter Roberson]

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.