Basic function in MATLAB

Question

I am starting to work with MATLAB and I still don't know how to do a lot of things. I have to create a simple plot function, let's supose it is:

f(x)=3x+1, defined for example from -5 < x < 5

What I need to do is to create a function that transforms the independent variable (it's an input parameter). For example, if my function is named plotFunction, if execute the command plotFunction(2x+3) the final plotted graphic is the function:

f(x)=6x+10

Answer 1

Functions can be defined by the function environment. You want a function that has no output variable, just the plot apparently.

function [] = plotFunction(f,minRange,maxRange)
    range = minRange:0.01:maxRange; % Create plot range, change 0.01 to w/e precision you want to attain
    figure; % Create figure
    PlotFunc = 3.*f(range)+1; % Your function
    plot(range,PlotFunc)
end

There's two things you need to know when trying to use other functions as 'base function': I have now set it to take steps of 0.01 this might need to be set smaller (e.g. when plotting on [-1e-4,1e-4]). You can do this by hand or use a precision switch as @efirvida used. The other thing: I call 3*f(range)+1 as PlotFunc. If you want to use other functions, do it there, e.g. if you want to use cos(x)+1/3*sin(pi*x)*e^(-x)) set all the x to f(x): PlotFunc = cos(f(x))+1/3.*sin(pi.*f(x))*exp(-f(x)))

Now you have to take care to call x as a function handle, like so: f = @(x)(2*x+3). The @ makes it a function handle, the argument directly behind it defines the variable in the defined function, here that's (x). The second set of brackets contains the actual function. Then define your range, i.e. minRange = -5 maxRange = 5 and call your function:

plotFunction(f,minRange,maxRange)

resulting in:

Answer 2

My solution

file plotFunction.m

function plotFunction(f, lb, ub, precision)
    f = str2func(['@(x) ', f]);
    plot(lb:precision:ub, f(lb:precision:ub))
end

where f is string representation of function, and lb,ub the bound of the independent variable, and precision the precision of the plot

run function

>>> plotFunction('2*x+3', -5, 5, 0.01)

>>> plotFunction('6*x+10 ', -5, 5, 0.01)