# How to drop small terms in symbolic expression?

38 views (last 30 days)
JT on 18 Nov 2020
Commented: Walter Roberson on 16 Mar 2021
Hello,
I have a large complicated matrix with symbolic expressions within such as:
(3.7e-33*A*E)/R + (12.0*E*I)/R^3
Attached is a larger subset of the large matrice to give you an idea.
I want to loop through each cell to remove the small terms such as (3.7e-33*A*E)/R or (6.1e-17*A*E)/R - (7.3e-16*E*I)/R^3. If only I could target any numbers less than a certain threshold (like 1e-10) and make them 0. That would effectively accomplish that task. Any ideas how I would do this?
Thanks!

Prudhvi Peddagoni on 25 Nov 2020
Hi,
You can use conditional indexing this
a={1; 2; 3; 4; 5};
a(cell2mat(a)>3)={0}
Hope this helps.
JT on 27 Nov 2020
Hello,
Unfortunately, the terms I want to drop are within each cell. For example, in a cell that says:
(3.7e-33*A*E)/R + (12.0*E*I)/R^3
I want it to be edited to:
(12.0*E*I)/R^3
And the variables are symbolic which adds complication.
Thanks

Walter Roberson on 27 Nov 2020
I was sure that I had posted code for this at one point, but I cannot find it.
threshold = 1e-10;
syms A E I R
M = (sym('3.7e-33')*A*E)/R + (sym('12.0')*E*I)/R^3
M =
mapSymType(M, 'vpareal', @(x) piecewise(abs(x)<=threshold, 0, x))
ans =
##### 2 CommentsShowHide 1 older comment
Walter Roberson on 16 Mar 2021
You cannot set this as default.
There are cases, such as working with the bessel functions, where values smaller than 1e-10 (or 1e-16) are important to the calculation. Bessel function often occur in ratios, so although the individual components might be small, the ratio might be subtantial enough to affect the calculation. So in the general case having this as a default facility would lead to results that would be difficult to explain.