Simple rank looping calculation with equation

Hello,

I am a novice at Matlab (actually a medical student)

I am trying to write a script that will do the following:

1. Rank an array of a given amount of numbers from least value to highest. (ex. [2,5,7,15,16,19]

2. Using a simple second order equation (ex. x^2-5xy+6y^2), take the first (least value) and last number (highest value) of the array and plug them into an equation to get a new value.

3. Take the second lowest and second highest numbers of the array, run them through the same equation and spit out a new value.

4. Repeat for all numbers in the original array until a new smaller array is obtained. For this example, a 1x6 array would shrink to a 1x3.

5. Repeat this process again and again until only ONE number remains.

I want to be able to do this for any dimension array I start out with.

So far I have been able to do this only once. I am just adding the values instead of using a quadratic equation as I typed above. My script so far is this:

clear
a=[2,5,7,15,16,19];
b=sort(a);
c=size(b);
x=c(1);
y=c(2);
%while y>1
value(1)=b(1)+b(y);
for i=x:(y/2-1);
    j=b(i+1)+b(y-i);
    value(i+1)=j;
end
y=size(value,2);
%end

disp(value);

I'm thinking I need to create a while loop to run it while the size(array,2)>1. However I can't quite seem to do this. Anyone have any suggestions?

Thanks

Answer 1

[Image Analyst]

This is how I'd do it:

a=[2,5,7,15,16,19];
% Sort the input.
sorteda = sort(a);
% Find the middle element.
middleElement = ceil(length(a)/2)
for k = 1 : middleElement % Go half way.
  % Get x and y.
  x = sorteda(k)
  y = sorteda(end-k+1)
  % Compute formula and store in output.
  output(k) = x^2 - 5*x*y + 6 * y ^ 2;
end
% Display output in command window.
output

[Gautam]

Thanks for the help, looks like a much cleaner code.

However, it still leaves me with the problem of repeating the code using a while loop. If I want to repeat the process on the array 'output' using a while loop, say while size(output,2)>1, it is still calling on the original array 'a', when I want it to now use 'output'..and eventually just get one number. Do you think there is a way to do this?

[Image Analyst]

Sure. Make that a function and first pass in a and get a back and then keep going until length (a) = 1

a = whatever...
while length(a) > 1
  a = theFunction(a);
end

[Gautam]

I tried this, but it looks like when you give the original 'a' to the function, the values it passes back as 'a' are a combination of the computed values and the original values. So for a 1x6 array, the new 'a' would give the first three values as the computed values and the last three as the original last three, still a 1x6 array. When i run it as a loop, it will always reach length (a) >1.

Defining a=a(1:y/2) for an even numbered array would fix this problem, but do you think there's a cleaner fix?

Thanks for all your help.

[Image Analyst]

Not for me. the returned a was about half the size of the original a. Show your code so we can figure out what you did wrong.

[Gautam]

Script:

clear
a=[1,2,3,4,5];
while length(a)>1
   a = CaliberFunction(a);
end

CaliberFunction:

function [a] = CaliberFunction(a)
% Sort the input.
sorteda = sort(a);
disp('this is too big')
% Find the middle element.
middleElement = ceil(length(a)/2);
for k = 1 : middleElement % Go half way.
  x = sorteda(k)
  y = sorteda(end-k+1)
  a(k) = x^2 - 5*x*y + 6 * y ^ 2;
end
a
return

[Image Analyst]

You don't understand how functions work. You need to study up on that. See how you changed what I called output to "a"? That's how you broke it. You don't replace a there in CaliberFunction, you replace it when you call the function. Corrected code is below (test2.m):

function test2
clc;
a=[1,2,3,4,5];
iteration = 1;
while length(a)>1
   fprintf('For iteration #%d, length(a) = %d\na = ', iteration, length(a));
   fprintf('%.4f, ', a);
   fprintf('\n');
   a = CaliberFunction(a)
   iteration = iteration + 1;
end

% CaliberFunction:
function output = CaliberFunction(a)
% Sort the input.
sorteda = sort(a)
% Find the middle element.
middleElement = ceil(length(a)/2);
for k = 1 : middleElement % Go half way.
  x = sorteda(k);
  y = sorteda(end-k+1);
  output(k) = x^2 - 5*x*y + 6 * y ^ 2;
end
output
return;