MATLAB Answers

Why does my plot say undefined variable or function for X2?

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Ash Maxwell
Ash Maxwell on 30 Apr 2020
Commented: Steven Lord on 30 Apr 2020
Lv = 4.5;
Lh = 2.5;
Pmax = 83;
MaxXcvalue = 0.75;
Xc = 0.25:0.01:0.75;
Areaofvertical = (Lv*X);
Areaofhorizontal = (Lh*X);
Areaofsoil = (4.5)*(1.5);
for X = 1:Xc ;
Ww = (Unitweight)*(((Lv)*(X))+((Lh)*(X)))*1;
Leverarmofvertical = (0.5+(X/2));
Leverarmofhorizontal = (Lh/2);
Leverarmofsoil = (0.5 + (X) + (1.5/2));
Moment = ((Areaofvertical)*(Leverarmofvertical))+((Areaofhorizontal)*(Leverarmofhorizontal))+((Areaofsoil)*(Leverarmofsoil));
Totalarea = (Areaofvertical)+(Areaofhorizontal)+(Areaofsoil);
X1 = (Moment)/(Totalarea);
X2 = X1 - ((Pmax)/(Ww))*((Lv + X)/(3));
end;
plot(X, X2);
grid on;
[SL: Formatted code as code]

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Answers (2)

Jeremy Marcyoniak
Jeremy Marcyoniak on 30 Apr 2020
Your code returns an undefined function or variable X error, because you try to define
Areaofvertical = (Lv*X);
before you have defined X. Maybe you meant Areaofvertical to use Xc?

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Ash Maxwell
Ash Maxwell on 30 Apr 2020
I have tried that but it still does not seem to like it. When I put it in the while loop it comes up with the graph but with no values plotted.
Jeremy Marcyoniak
Jeremy Marcyoniak on 30 Apr 2020
well, I think this is your problem:
Xc = 0.25:0.01:0.75;
for X = 1:Xc
I do not believe the line X = 1:Xc is doing what you think it's doing, since Xc is an array of values, not a single value. Perhaps instead, you should try
for X = Xc

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Steven Lord
Steven Lord on 30 Apr 2020
There are several problems with your code, but the main one is that your for loop body never executes.
for X = 1:Xc
Since Xc is a non-scalar, MATLAB will only use the first element of that vector when constructing the vector of values over which X iterates. So the loop will run once per element of this vector:
iterates = 1:0.25
You probably want to have X iterate from 1 to the numel of Xc, operating on element X of Xc in turn and assigning into element X of X2.

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Ash Maxwell
Ash Maxwell on 30 Apr 2020
I understand how it iterates one, but I don't quite understand what you mean for the solution. Could you perhaps explain it in Layman's terms? Thank you so much for the suggestion!
Steven Lord
Steven Lord on 30 Apr 2020
x = randperm(10, 5); % random selection (without replacement) of 5 elements in 1:10
y = x.^2;
for k = 1:numel(x)
fprintf("%d squared is %d.\n", x(k), y(k))
end
If I'd iterated over x, I could have asked for say the 10th element of x and y, but they'll only have 5 elements.

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