Re-ordering a vector... (MATLAB)

mjlaz · #1 $Old$ September 14th, 2009, 05:55 PM

My assignment is to create a function that takes a vector of the form

v = [3 9 1 8 5 4]

and puts it in order from smallest to largest as in

v = [1 3 4 5 8 9]

So far, I've got this, but it only works on a vector of the form v = [a b].

Code:

function x = putinorder(p)

a = length(p); 

for i=1:a-1
    if p(a) < p(a-i)
        ex = p(a-i);
        p(a-i) = p(a);
        p(a) = ex;
    end
x = p;
end

end

>> putinorder([5,1])

ans =

     1     5

I am required to use a for loop. Any suggestions or tips? Thanks,

Marc

CaptainBlack · #2 $Old$ September 14th, 2009, 09:34 PM

Quote:

Originally Posted by mjlaz $View Post$

My assignment is to create a function that takes a vector of the form

v = [3 9 1 8 5 4]

and puts it in order from smallest to largest as in

v = [1 3 4 5 8 9]

So far, I've got this, but it only works on a vector of the form v = [a b].

Code:

function x = putinorder(p)

a = length(p); 

for i=1:a-1
    if p(a) < p(a-i)
        ex = p(a-i);
        p(a-i) = p(a);
        p(a) = ex;
    end
x = p;
end

end

>> putinorder([5,1])

ans =

     1     5

I am required to use a for loop. Any suggestions or tips? Thanks,

Marc

Consider a function that finds the smallest element and puts it into the first position in the array (this has not been run so is almost certianly full of errors which you will have to elliminate):

Code:

function rv=smallstep(array)

  ll=length(array);
  sml=array(1);smidx=1;

  for idx=2 to ll
    if (array(idx)<sml)
       sml=array(idx);
       smidx=idx;
    end
  end
  iidxs=find(array~=sml);
  rv=[sml,array(iidxs)];

Once you have this working use it to sort your array.

CB

elbarto · #3 $Old$ September 15th, 2009, 12:39 AM

This solution is not exactly what you were chasing (I missed the bit about the for loop before I got started), but it sorts through a array using recursion rather than looping.

Code:

function A = mhfQuickSort(myarray)
%http://en.wikipedia.org/wiki/Sorting_algorithm
%http://en.wikipedia.org/wiki/Quicksort
Less = [];Greater = [];
if length(myarray) <= 1
    A = myarray;
    return
else
    Pivot = myarray(1);
    myarray(1) = [];
    for x = myarray
        if x <= Pivot
            Less = [Less x];
        else
            Greater = [Greater x];
        end
    end
    A   = [mhfQuickSort(Less) Pivot mhfQuickSort(Greater)];
    return
end

Example:

Code:

EDU>> A = round(10*rand(1,10))

A =

     7     1     0     6     6    10     7     7     5     3

EDU>> mhfQuickSort(A)

ans =

     0     1     3     5     6     6     7     7     7    10

EDU>>

Regards Elbarto.

mjlaz · #4 $Old$ September 15th, 2009, 12:32 PM

Thanks for your input. Unfortunately, my class is pretty much beginner level in MATLAB programming so I don't know if I would have the courage to turn in such intricate functions that you guys wrote!

At any rate, I figured it out!

Code:

function x = reordervector(v)

a = length(v);

for i=1:a-1
    for k=0:a-1
        if v(a-k) < v(a-i)
            ex = v(a-i);
            v(a-i) = v(a-k);
            v(a-k) = ex;
        end
    end
end 
x = v;
end

>> reordervector([9 6 8 4 5 3 2 1])

ans =

     1     2     3     4     5     6     8     9

>> reordervector([12:-1:0])

ans =

     0     1     2     3     4     5     6     7     8     9    10    11    12

>> reordervector([9 -1 3 3 -2 -4 8 9])

ans =

    -4    -2    -1     3     3     8     9     9

I went nuts when it worked.... people thought I was crazy. Thanks!