Behavior of subs() function in Symbolic Math Toolbox

Dear MATLAB Central members,

I would like to ask a behavior of subs() in Symbolic Math Toolbox (R2016b, Windows 64 bit, OS: Windows 7).

Here is an example.

syms a b aM bM

aM=sym([2 3;4 5]);

bM=sym([6 7;8 9]);

s_temp=a*b;

>> s_temp=subs(s_temp,a,'aM')

s_temp =

aM*b

>> s_temp=subs(s_temp,b,'bM')

s_temp =

aM*bM

>> subs(s_temp)

ans =

[ 12, 21]
[ 32, 45]

which is same as

>> aM.*bM

ans =

[ 12, 21]
[ 32, 45]

but what I like to get the result from subs(s_temp) is a result of

>> aM*bM

ans =

[ 36, 41]
[ 64, 73]

Can I get this result using subs function, or should I do something else?

Thank you in advance for your help.

Regards,

Kosuke

Answer 1

[Nicolas Schmit]

You have to define a and b as 2x2 symbolic matrices. Also, do not put the variables aM and bM into quotes when using subs()

a = sym('a', [2 2]);
b = sym('b', [2 2]);

aM=sym([2 3;4 5]);
bM=sym([6 7;8 9]);

s_temp=a*b;
s_temp=subs(s_temp,a,aM)
s_temp=subs(s_temp,b,bM)
subs(s_temp)

[Kosuke]

Dear Nicolas,

Thank you for your reply.

I learned a new thing of subs() function.

What I wanted was to handle a and b more likely strings,

and the actual calculation was done by substitution of a to aM and b to bM,

then evaluate aM*bM.

e.g.,

user types s_temp=a*b;

Then if the user wants to calculate a matrix representation of a*b,

use a function (say st2mat()) including subs() function,

M = st2mat(s_temp);

Thank you again,

Kosuke

Answer 2

[Walter Roberson]

No. The symbolic toolbox treats all unassigned variables as scalar and that is not possible to change (it is the behavior of the symbolic engine itself)

[Kosuke]

Dear Walter,

Thank you for your reply.

It seems that the behavior of symbolic toolbox is not simple as I expected.

Although, It is good to know it is not possible so that I can change my coding or put a caution that a*b as an input is not converted to aM*bM correctly.

Thank you again, Kosuke