MATLAB Answers

Setting up and plotting functions

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Dan_LDS
Dan_LDS on 31 Jan 2020
Commented: Dan_LDS on 31 Jan 2020
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.

  2 Comments

Adam
Adam on 31 Jan 2020
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
Dan_LDS on 31 Jan 2020
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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Accepted Answer

edward holt
edward holt on 31 Jan 2020
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

  1 Comment

Dan_LDS
Dan_LDS on 31 Jan 2020
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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More Answers (1)

Jakob B. Nielsen
Jakob B. Nielsen on 31 Jan 2020
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.

  1 Comment

Dan_LDS
Dan_LDS on 31 Jan 2020
Yes, I see that now. Pretty basic stuff, but it's still valuable information (for a noob)!
Thanks for the input

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