MATLAB: Is the output the function disregard the IF condition

functionif statement

Hi I've created a function,calcDist to calculate the distance travelled by a weight, W when put on springs, k1 and k2. If the distance travelled by the weight on the first spring, x is greater than the height difference of spring1(k1) and spring 2(k2), d then, the second equation should come in effect. The first equation only serves as a determiner of which case.

function x = calcDist(W,k1,k2,d)
    x = W/k1 % equation 1
  if x >= d
    x = (W+2*k2*d)/(k1+2*k2) % equation 2
  else
    x = W/k1;
  endif
endfunction

The function works for a single value of W.

>> calcDist(500,10^4,1.5*10^4,0.1)
ans =  0.050000
>>
>> calcDist(2000,10^4,1.5*10^4,0.1)
ans =  0.12500
>>

However since I need to plot a graph of x vs W for 0<W<3000N, I tried putting in multiple values at once in the command window

Try = calcDist(0:3000,10^4,1.5*10^4,0.1)

The output given was totally wrong. Somehow it skips the IF condition. I'm totally baffled by this situation. Any help and advice are greatly appreciated.

Best Answer

YOu cannot use if condition like that.....you better run a loop with your function calcDist as shown below and plot the required.

W = 0:3000 ;
k1 = 10^4 ;
k2 = 1.5*10^4 ;
d = 0.1 ;
x = zeros(size(W)) ;
for i = 1:length(W)
    x(i) = calcDist(W(i),k1,k2,d)
end
plot(x,W)

Related Solutions

MATLAB: For loop overwriting previous answers rather than storing

if W < k1*d

How does if behave if the condition is not a scalar (in your second case it is a vector.) Let's check the documentation.

if expression, statements, end evaluates an expression, and executes a group of statements when the expression is true. An expression is true when its result is nonempty and contains only nonzero elements (logical or real numeric). Otherwise, the expression is false.

In your second case, W is not empty and neither is k1*d so the expression (W < k1*d) is nonempty. But does it contain only nonzero elements? Or is at least one element of W not less than k1*d?

You could try removing the for loop and using logical indexing or you could have the body of your for loop operate on each element of W in turn rather than the whole array each iteration.

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.

Best Answer

Related Solutions

MATLAB: For loop overwriting previous answers rather than storing

MATLAB: Writing a Function for Plotting

Related Question