help to fix this numeric symbolic array

I need some help with the code below:
scaleFactors = [ 3.4, 2.3;
1.7, 1.9 ]
xMatSym = sym('X',[ 2, 2 ]);
xScaled = xMatSym./scaleFactors;
I am hoping to get the results shown below:
X1_1/3.4 X1_2/2.3
X2_1/1.7 X2_2/1.9
But, instead I am getting the results below instead:
(5*X1_1)/17 (10*X1_2)/23
(10*X2_1)/17 (10*X2_2)/19
I know the reasons it doesn't work. But I don't know how to fix this. Can someone help me?
Thank you so much!

3 Comments

Jan
Jan on 12 Aug 2017
Edited: Jan on 12 Aug 2017
I've used the "{} code" button to format the code and the numbers. Do you see that this improves the readability? Please use this by your own in the future.
What does "fixing" mean, if the current results are correct? 5*x/17 is the same as x/3.4 mathematically. Working with integer numbers is more accurate from the view point of numerics.
I've looked at doc sym: conversion techniques, but have too few experiences with the symbolic toolbox to adjust the display as wanted.
Hi, Jan, I am not sure what you mean about "{} code". I think that's for cell array. Mind to show what you mean to me with an example?

Answers (1)

Karan Gill
Karan Gill on 12 Aug 2017
Edited: Karan Gill on 17 Oct 2017
You can't have a "numeric symbolic" array because "numeric" and "symbolic" are different data types. For your options, see https://www.mathworks.com/help/symbolic/choose-symbolic-or-numeric-arithmetic.html.
Instead, try the vpa function on your result.

2 Comments

Note that vpa(xScaled) will get you output such as
[ 0.29411764705882352941176470588235*X1_1, 0.43478260869565217391304347826087*X1_2]
[ 0.58823529411764705882352941176471*X2_1, 0.52631578947368421052631578947368*X2_2]
You can also
>> vpa(xScaled,2)
ans =
[ 0.29*X1_1, 0.43*X1_2]
[ 0.59*X2_1, 0.53*X2_2]
Notice these are multiplications, not divisions, and it does matter for precision purposes how many digits you truncate to.
>> 1000/1.7
ans =
588.235294117647
>> 1000*.59
ans =
590
Yes, I do not recommend using vpa(xScaled,2) to anyone who hasn't carefully read the vpa doc :) Too much potential for confusion.

This question is closed.

Asked:

on 9 Aug 2017

Closed:

on 20 Aug 2021

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