Problem plotting a function

2018 4 23
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更新时间：2018 4 24
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I am having problem plotting the function. When I try to run the code it says "Error using plot Data must be numeric, datetime, duration or an array convertible to double."
syms w;
X = (2*w + 3).*(heaviside(w+6)-heaviside(w+4));
X = X + ((-2*w - 1).*(heaviside(w+4)-heaviside(w+2)));
w = -10:10;
plot(w, X)

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Walter Roberson
Walter Roberson 2018-4-23

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plot(w, subs(X))

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Erbaz Khan
Erbaz Khan 2018-4-23
Can you please explain why did you use subs(X) instead of X?
Walter Roberson
Walter Roberson 2018-4-24

When you use

syms w

and then construct a formula using w, then the w in the resulting formula is not a reference to "the current value of the variable w if it has been assigned a value"

Just like if you do

A = 2
B = A * 3
A = 5

then B copied the current value of A at the time of execution, rather than making B a formula that has a reference to the current value of A with B intended to update as A updates.

The situation is the same with the symbolic engine: when you refer to w the current value of w is copied, rather than a reference being made to whatever value of w is current. Therefore when you assign a numeric value to w after constructing a formula with symbolic w, the formula does not change to evaluate according to the new w value. You need to instruct the symbolic engine to look through the workspace to find values for the symbolic variables in the expression and incorporate them into the expression; subs() tells it to do that.

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