symbolic variable multiplication with numerical matrix

Hi,

I need to multiply a smybolic variable with a numerical matrix. My code is:

syms b1 b2 b3 b4

A = b1.*B + b2.*C + b3.*D + b4.*E

B, C, D and E are matrices that contain numbers. b1 to b4 are 1x1 unspecified variables. When I use the code above I get absurd numbers like (5038398039803537*b1)/36028797018963968. The numbers in numerical matrices are between 0 and 1. I am expecting elements of A that look like, for example, 0.4*b1 + 0.7*b2 + 0.1*b3 + 0.2*b4. I don't even understand where that division is coming from. Thanks in advance.

Answer 1

[Steven Lord]

When you perform that multiplication, Symbolic Math Toolbox converts your numbers into rational numbers rather than leaving them as floating-point numbers. Use the vpa function to approximate those rational numbers to a certain number of decimal points, or use subs to substitute numeric values for your symbolic variables then convert to double if necessary.