Problem while trying to solve 3 inequalities with solve function

Asked by Pietro Rinaudo

Pietro Rinaudo (view profile)

on 14 Sep 2019
Latest activity Edited by Pietro Rinaudo

Pietro Rinaudo (view profile)

on 14 Sep 2019
Hi everyone, thanks in advance for any help you will provide me.
I'm struggling with a - in my opinion - basic problem involving the symbolic function solve and 3 easy inequalities: even though a solution does exist, matlab doesn't return it as output. Here is the code:
clear
clc
char_filler = '0';
for i = 1:size(s,1)
ss(i) = rmmissing(s(i));
end
s = ss;
for i =1:size(s,2)-11
%for i =1:size(s,2)
s(i)=strcat(s(i),char_filler);
strin = s(i);
position = regexp(strin,'');
dim = length(position);
for j = 1:dim
pos = position(j)*(j==1)+(position(j)+(j-1))*(j>1);
new_s(i)=insertAfter(s(i),pos,'*');
end
end
if exist('new_s','var')
sym_table = str2sym(new_s);
else
sym_table = str2sym(s);
end
clearvars -except sym_table new_s s
%syms a b e
soluzione = solve(sym_table,'ReturnConditions',true);
%% print
sym_table
soluzione.conditions
While the .txt files is simply (14 lines)
-e+4a>=
-e+4b<=
-e-e+3a<=
-e-e+e+2a>=
-e-e+2b+a<=
-e-e+e+b+c<=
-e+3b+a>=
-e+c+b+2a>=
-e+c+2b+a<=
-e+2c+2a>=
-e+2b+2a>=
-e+d+c+b+a<=
-e+2a+c+d>=
-c+3d+e>=
I already proved that the extraction of line is fine, and so is the final system of expressions (sym_table, 3 expressions inside as expected). The problem arises when MATLAB tries to solve it, in fact it returns the warning
Warning: Unable to find explicit solution. For options, see help.
However, with another .txt file
a+b>
a-b<
a<=
A solution if found.
I tried explicing the 3 variable (as commented) inside solve, i tried several assumptions for them, but nothing works.
Am i missing something, or simply matlab sometimes struggles with this? Because some online solvers manage to find expressions (even for bigger sets of expressions), so i'm quite in doubt.