Operating on syms class matrices.

My algorithm returns matrices in the class "sym", of a similar form to the following matrix.

matrix =

[ a^2 + b*c, a*b + b*d]

[ a*c + c*d, d^2 + b*c]

I wish to switch the operators in this matrix. Such that, the '*' become '+' and vice versa. I then wish to use subs,

subs(mtx,[a b c d],[1 2 3 4])

in order to solve the matrix. Is there anyway to do this.

I can switch '*' for '+' and vice versa if I convert the matrices elements to char strings and simply search and replace, however this leaves solving the matrix for specific values of a,b,c and d problematic.

Any help would be greatly appreciated.

Regards

Ross

Answer 1

[Andrei Bobrov]

variant

p1 = regexprep(char(A),'+','_')
p2 = regexprep(p1,'*','+')
out = sym(regexprep(p2,'_','*'))

[ross montgomery]

Thanks Andrei.

This method worked well.

Regards

Ross

Answer 2

[Walter Roberson]

What do you intend that a^2 + b*c would transform to?

• (a+a) * (b+c)
• a^2 * (b+c)
• (a^2 * b) + c
• a + (a*b) + c

What should a*b + b*d transform to?

• (a+b) * (b+d)
• a + (b*b) + d
• Or since a*b = b*a, the original expression could also be written b*a + d*b and does that perhaps transform to b + (a*d) + b ?

If you do textual substitutions you need to define the grouping of expressions.

For some of the grouping choices, you can handle the manipulation readily using subs(), such as

newmtx = subs(mtx, {sym('_plus'), sym('_mult')}, {sym('_mult'), sym('_plus')});

The _plus and _mult are the internal MuPAD names of + and * when used as operators: see http://www.mathworks.com/help/toolbox/mupad/stdlib/_mult.html

[ross montgomery]

Hi Walter,

I should have defined this better. I want to keep the original order of the operations, i.e as would be performed in the original term.

I have managed to achieve this by using the "regexpre" function to substitute the operators and by introducing brackets in order to perform the operations in the order wanted. To do this is I simply wrap all '*' in brackets as such, ')*('. I then strcat brackets on to the start and the end of each matrix element in order to balance.

Thanks for your response Walter

Regards

Ross