MATLAB: How to get a function to accept a vector as input

functionsMATLABvector

I'm trying to write a function that takes multiple variables in

function [x] = classproj(W, k1, k2, d)
%UNTITLED4 Summary of this function goes here
%   Detailed explanation goes here
if W.*(1./k1) < d
    x = W.*(1/k1);
elseif W.*(1/k1) >= d
    x = (W + 2*k2*d).*(1./(k1+2*k2));
end
end

This works if all variables are scalar, but if I try to make W a vector it results in an error claiming that x is not assigned during call

Best Answer

Hi,

no if-else is needed, when using logical indexing:

function [x] = classproj(W, k1, k2, d)
x(W.*(1/k1)<d) = W(W.*(1/k1)<d).*(1./k1);
x(W.*(1/k1)>=d) = (W(W.*(1/k1)>=d) + 2*k2*d).*(1./(k1+2*k2));
end

Best regards

Stephan

Related Solutions

MATLAB: Writing a Function for Plotting

Your first part is wrong, please tell me how can you compare x with d without having x defined first?

Please test your functions first so we can help you better, the statement

else (x >=d)

is also wrong, it should be something similar to

elseif (x >=d)

but that condition is irrelevant, it should only be

else

because it's the opposite condition from the if, it takes all the values that the if doesn't "catch".

Your second expression should be

x=(W + 2*d*k2)/(k1 + 2*k2)

I made the plotting but I won't post the code because you must do your own homework and learn with your own mistakes.

Edit: ok here' the code, I'm just posting it because Big Meech shown that he's working on the homework and the use of logical indexing is the way to go (Matt Fig sugestion), the following code works just fine but it's slower than the one using logical indexing.

function x=spring(W,k1,k2,d)
if ((W/k1)<d)
    x= W/k1;
else
    x=(W + 2*d*k2)/(k1 + 2*k2);
end

Script for the plot, this is not one optimized version, there's no pre-allocation of the x and the plot is called too many times (each time creating a new object thus wasting time and memory)

clf
hold on
st=5; %step


for W=0+st:st:3000-st;
x=spring(W,k1,k2,d);
plot(W,x)
end

extra text and lines

line([0 k1*d],[d d],'linestyle','--','color',[1 0 0])
line([k1*d k1*d],[0 d],'linestyle','--','color',[1 0 0])
text(0,d,'d')
text(k1*d,0,'k_1*d')
text(k1*d/2,d/2,'x=W/k_1')
text(k1*d+k1*d/6,d+d/6,'x=(W + 2*d*k_2)/(k_1 + 2*k_2)')

Just for fun I tested both ways to do it, logical indexing got also the pre-allocation of x memory space.

%constant values used in the test
k1 = 10000; % N/m;



k2 = 15000; % N/m;
d = 0.1;
W = 500; %N
st=0.1; %step

Time required for the plot to be done with logical indexing: 0.029902 seconds.

Time required for the plot to be done with if and else: 23.764888 seconds.

It's a big difference!

Last Edit

Here's the version with logic indexing

    function x=spring2(W,k1,k2,d)
    %This time the function is called spring2 not just spring
    %W is now a vector, not just a number
    x=0*W; %create the local x vector with the size of W and full of zeros
    x((W/k1)<d)= W(W<k1*d)/k1;
    x((W/k1)>=d)=(W(W>=k1*d) + 2*d*k2)/(k1 + 2*k2);
    end

Script that calls the function (don't put it in the same file as the function!!!)

%initial values
k1 = 10000; % N/m;
k2 = 15000; % N/m;
d = 0.1;
st=0.1; %step
x=zeros(1,3000/st-2); %pre-allocate memory for the x vector
W=0+st:st:3000-st; %create the W vector
x(1:numel(W))=spring2(W,k1,k2,d); %call the function just once
plot(W,x) %plot the results just once (only creates a plot object)

If you want to compare the time it takes to execute use the tic and toc functions, the version with pre-allocating of memory and logic indexing is the best way to do it.

MATLAB: How can I rewrite line #4 (comp(i) = calcSpringCompression(W,k1,k2,d);) to where I do not get an error when I run the code

function [] = plotSpringCompression(Wmin,Wmax,k1,k2,d)
W = [Wmin:1:Wmax];
for i = 1:length(W)
    comp(i) = calcSpringCompression(W(i),k1,k2,d);
end
hold on
plot(W, comp)
xlabel('Weight(N)');
ylabel('Compression(m)');
end

Best Answer

Related Solutions

MATLAB: Writing a Function for Plotting

MATLAB: How can I rewrite line #4 (comp(i) = calcSpring​Compressio​n(W,k1,k2,​d);) to where I do not get an error when I run the code

Related Question

MATLAB: How can I rewrite line #4 (comp(i) = calcSpringCompression(W,k1,k2,d);) to where I do not get an error when I run the code