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Behavior of subs() function in Symbolic Math Toolbox

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Kosuke
Kosuke 2017-9-18
编辑： Walter Roberson 2017-9-20
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

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采纳的回答

Nicolas Schmit
Nicolas Schmit 2017-9-19
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)
  1 个评论
Kosuke
Kosuke 2017-9-19
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

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更多回答（1 个）

Walter Roberson
Walter Roberson 2017-9-19
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)
  1 个评论
Kosuke
Kosuke 2017-9-19
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

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