How to show the variable name instead of its value in a plot's text, when using "syms", "text" and "latex" functions

Question

Sim 2022-8-26

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评论： Sim 2022-8-28

Question: By using both syms, latex and text functions/tools (as in the example here below), how can I show the variable name instead of its value, in a plot's text ?

What I currently have: In this example, if I assign a value to the variable "a", i.e. "a = 0.01", the latex function will show its value, i.e. "0.01" (i.e. "1/100") and not its name, i.e. "a".

syms x
a = 0.01 ;
y = exp(-a*x); % equation to show inside the plot
figure
fplot(y, [0 500])
text(100,0.5, ['$y = ' latex(y) '$'], 'Interpreter','latex', 'FontSize',16)

My desired output: If possible, I would like to still use both syms, latex and text functions/tools (as in my example), and see/show the variable name "a" in the plot's text, instead of its value "0.01" (i.e. "1/100").

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Answer 1

Dyuman Joshi 2022-8-26

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syms y(a,x)

y(a,x) = exp(-a*x); % equation to show inside the plot

figure

fplot(y(0.01,x), [0 500])

text(100,0.5, ['$y = ' latex(y) '$'], 'Interpreter','latex', 'FontSize',16)

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Sim 2022-8-28

thanks a lot @Dyuman Joshi!!

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Answer 2

Walter Roberson 2022-8-26

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https://ww2.mathworks.cn/matlabcentral/answers/1787585-how-to-show-the-variable-name-instead-of-its-value-in-a-plot-s-text-when-using-syms-text-and#answer_1034880

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You cannot do that, not without reconstructing the formula

syms x
a = 0.01 ;
y = exp(-a*x); % equation to show inside the plot
children(y)
ans = 1×1 cell array
    {[-x/100]}
children(ans{1})
ans = 1×2 cell array
    {[x]}    {[-1/100]}

Notice that there is no "a" anywhere in the breakout of the expression.

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Walter Roberson 2022-8-26

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Consider this:

a = 1
b = a * 5
a = 2

What is b now? Does it become 10 because a changed to 2 and b is a * 5 ? Or is it 5? Or is it internally a formula "a * 5" that you would somehow be able to extract if you only knew how?

When you build a = 0.01, y = exp(-a*x), it works the same way as the numeric case.

Sim 2022-8-28