Setting up and plotting functions

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Dan_LDS 2020-1-31

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/502965-setting-up-and-plotting-functions

评论： Dan_LDS 2020-1-31

采纳的回答： edward holt

I'm quite new to matlab and I'm trying to grasp how to write functions and plot them as well.

One of my excercises in class asks me to:

"Make a function that plots the equation ax^2+bx+c=0, where a,b,c and x are used as input."

Does that mean I need to define values for a,b,c and x first, in order to plot anything?

Not sure how to best approach this in matlab.

My code so far looks like this:

function F(x,a,b,c);
y = a*x.^2+b*x+c;
plot(x,y)
end

But that doesn't plot anything as it is though..

I was hoping on some input on how to setup a good solution here, or maybe just a simple way by assigning values to the variables.. - Again, my coding skills are completely beginner, so any help is appreciated.

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Adam 2020-1-31

That should work, for a simple case (i.e. it will plot on whatever the current axis is or create one if none exists), if you call it as e.g.

a = 3;
b = 4;
c = 5;
x = 0:0.1:10;
F( x, a, b, c )

I haven't really put any thought into appropriate values for a, b, c and x there, but that would be the general idea. Don't just hit the big green 'run' button as it is a function that requires you to pass in 4 arguments.

Dan_LDS 2020-1-31

Thanks!

function F(x,a,b,c);
a=2
b=3
c=4
x = 0:0.1:10;
y = a*x.^2+b*x+c;
plot(x,y)
end

..But it doesn't plot anything still? I am missing something very basic here i figure, sorry

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Answer 1

edward holt 2020-1-31

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It's rough, but I'm also quite new to Matlab.

You answered your own question - you do need to define x, a, b and c.

x needs to be a vector (a series of values over which your function evaluates).

% xmin and xmax define the range over which your function plots
% xres defines how often a point is plotted (low number - smooth curve,
% high number, rough)
inputs = inputdlg({'x min','x max','x resolution','a','b','c'},'input)')
xmin = str2num(inputs{1})
xmax = str2num(inputs{2})
xres = str2num(inputs{3})
a = str2num(inputs{4})
b = str2num(inputs{5})
c = str2num(inputs{6})
x = xmin:xres:xmax
f(x,a,b,c)
function f(x,a,b,c)
    y = a*x.^2+b*x+c;
    plot(x,y)
end

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Dan_LDS 2020-1-31

Aah! This makes sense, I was missing out some very basic stuff here. Very nice setup btw, that's pretty advanced for me though (yet).

Thanks for the input!

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Answer 2

Jakob B. Nielsen 2020-1-31

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It does, indeed, need to be given some inputs. Matlab has no way of knowing what a, b, c and x are.

function F(x,a,b,c);
y = a*x.^2+b*x+c;
plot(x,y)
end

It will work, but you need to tell it inputs. E.g. in the command window try making an x vector for example x=1:0.01:100; and then call your function with some variables for a, b and c that you can pretty much make up as you want. For example

F(x,1,2,3)

That will execute the funtion y=1*x.^2+2*x+3 and then plot x,y.