Why is the output my function disregard the IF condition?

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.

[Steven Lord]

KSSV has already given you an alternative. As for the reason why your if statement body was not executed, that's due to how if handles non-scalar expressions. From 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.

The expression x >= d in your if statement contains only nonzero elements only if all the elements in the vector x are greater than the scalar d. The first element of the W vector you use to compute x is 0, thus the first element of x is also 0. The result of (0 >= 0.1) is false so the result of the expression has at least one zero element.

Instead of using a loop as KSSV did, you could use logical indexing. See this documentation page for more examples.

Answer 1

[KSSV]

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)

Answer 2

[LuqmanF]

Thank you so much for helping me with the problem. I did what you suggested and it ran perfectly. However I have 1 small question left. Why can't the lines that you wrote be in the same script as my function. Somehow it gave an error when I did that. From my understanding the computings are processed linewise. Which means it should call the function when prompted to do so. No?

[Walter Roberson]

Because you are not using MATLAB. You are using Octave.

[LuqmanF]

Thanks for the clarification