Home/Questions/How to fix "solve() throws this result empty sym: 0-by-1 and fails to get the correct results"?
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Charles Walsh
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Asked: 2022-05-18T19:00:16+00:00 In:

How to fix “solve() throws this result empty sym: 0-by-1 and fails to get the correct results”?

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How to fix “solve() throws this result Empty sym: 0-by-1 and fails to get the correct results”? Here is the code that I run:

syms v1 v2 v3
 eqns = [5*v1 -2*v2 -3*v3 == 24, ...
  4*v1 +5*v2 == 0, ...
  -6*v1 +12*v2 +8*v3 == 0];
 vars = [v1 v2 v3];
 answer = solve(eqns,vars);
 vpa(answer.v1)
 vpa(answer.v2)
 vpa(answer.v3)
ans =
 Empty sym: 0-by-1
 ans =
 Empty sym: 0-by-1
 ans =
 Empty sym: 0-by-1
syms v1 v2 v3
 eqn1 = 5*v1 -2*v2 -3*v3 == 24;
 eqn2 = 4*v1 +5*v2 == 0;
 eqn3 = -6*v1 +12*v2 +8*v3 == 0;
 eqns = [eqn1,eqn2,eqn3];
 vars = [v1 v2 v3];
 answer = solve(eqns, vars);
 vpa(answer.v1)
 vpa(answer.v2)
 vpa(answer.v3)
ans =
 32.0
 ans =
 -25.6
 ans =
 62.4

Can someone share with me your knowledge and experience?

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2 Answers

  1. Best Answer
    Expert

    The cause:

    The reason of this error is from the following line:

    eqns = [5*v1 -2*v2 -3*v3 == 24, ...
4*v1 +5*v2 == 0, ...
-6*v1 +12*v2 +8*v3 == 0];

    If you run this line and write equations in the console, you will receive:

    >> eqns
eqns =
[ 5*v1, -2*v2, -3*v3 == 24, 4*v1, 5*v2 == 0, -6*v1, 12*v2, 8*v3 == 0]

    It says that there are actually 8 equations, not 3 equations. This is the reason why you don’t find any answers and the error happens.

    Solution:

    To solve this error, you need to add several spaces. It seems that the sequence space->plus or minus sign->digit is interpreted as a new equation, which is a bizarre behavior. As a consequence, the program:

    syms v1 v2 v3
eqns = [5*v1 - 2*v2 - 3*v3 == 24,...
4*v1 + 5*v2 == 0,...
-6*v1 + 12*v2 + 8*v3 == 0];
vars = [v1 v2 v3];
answer = solve(eqns);
vpa(answer.v1)
vpa(answer.v2)
vpa(answer.v3)

    will give the accurate answer as below:

    ans =
32.0
ans =
-25.6
ans =
62.4
    • 0

  2. Line is the problem

    eqns = [5*v1 -2*v2 -3*v3 == 24, ...
 4*v1 +5*v2 == 0, ...
 -6*v1 +12*v2 +8*v3 == 0];

    This line can be executed and then entered eqns into console to get:

    >> eqns
eqns =
[ 5*v1, -2*v2, -3*v3 == 24, 4*v1, 5*v2 == 0, -6*v1, 12*v2, 8*v3 == 0]

    This is 8 equations rather than 3. This is why there is no solution. If you add spaces, the problem disappears. Strange behavior. It appears that the sequence space->plus/minus sign->digit can be understood as a new equation. Hence,

    syms v1 v2 v3
eqns = [5*v1 - 2*v2 - 3*v3 == 24,...
 4*v1 + 5*v2 == 0,...
 -6*v1 + 12*v2 + 8*v3 == 0];
vars = [v1 v2 v3];
answer = solve(eqns);
vpa(answer.v1)
vpa(answer.v2)
vpa(answer.v3)

    The correct answer will be returned:

    ans =
32.0
ans =
-25.6
ans =
62.4
    • 16

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