Conversion to logical from sym is not possible

Question

Nguyen Tien Dung 2020-7-3

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/559088-conversion-to-logical-from-sym-is-not-possible

评论： Nguyen Tien Dung 2020-7-3

Hi everyone, can you help me fix this problem
1. This is my matlab code
clc 
clear all
syms v u w 
w= exp(v)/(v^2 + u)
diffw = diff(w,v)
simplify(diffw)
eq = v^2 - 2*v + u == 0
D1 = 4-4*u < 0
D2 = 4-4*u == 0
D3 = 4-4*u > 0
if D1 < 0
    fprintf('No solution')
    solve(D1,u)
elseif D == 0
   fprintf('Dual solution')
   solveD(D2,u)
else D > 0
    fprintf('Two solution')
    solve(D3,u)
end
prompt1='u=';
u= input(prompt1);
xSol=solve(eq,v)
subs(xSol,u)
Error : Conversion to logical from sym is not possible
2.I don't know matlab can solve > 0 or < 0 with no u(number) so i put another code and it have the same error
clc 
clear all
syms v u w 
w= exp(v)/(v^2 + u)
diffw = diff(w,v)
simplify(diffw)
eq = v^2 - 2*v + u == 0
prompt1='u=';
u= input(prompt1);
D1 = 4-4*u < 0
D2 = 4-4*u == 0
D3 = 4-4*u > 0
if D1 < 0
    fprintf('No solution')
    solve(D1,u)
elseif D == 0
   fprintf('Dual solution')
   solveD(D2,u)
else D > 0
    fprintf('Two solution')
    solve(D3,u)
end
xSol=solve(eq,v)
subs(xSol,u)
Error :When I put u=1
D1 =
  logical
   0
D2 =
  logical
   1
D3 =
  logical
   0
Unrecognized function or variable 'D'.
elseif D == 0
Thank you very much.

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Answer 1

madhan ravi 2020-7-3

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/559088-conversion-to-logical-from-sym-is-not-possible#answer_460682

编辑：madhan ravi 2020-7-3

在 MATLAB Online 中打开

4 * u
% ^ 

Give assumptions for u:

assume(u > 0)
D1 = isAlways(4-4*u < 0)
D2 = isAlways(4-4*u == 0)
D3 = isAlways(4-4*u > 0)

And in if block remove the comparator.

if D1 
    fprintf('No solution')
    solve(D1,u)
elseif D2
   fprintf('Dual solution')
   solveD(D2,u)
else D3 
    fprintf('Two solution')
    solve(D3,u)
end