How to manipulate arrays?

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Shawn Simon
Shawn Simon on 31 Oct 2015
Commented: Star Strider on 1 Nov 2015
Hello! I am trying to create a script in which I sort an array in descending order without using any sort function. So far, I came up with this:
A = [2 1 4 3 6 5 8 7 10 9 12 11 14 13 16 15 18 17 20 19];
n = 1;
while n <= length(A)
if A(n) < A(n+1)
tmp = A(n);
A(n) = A(n+1);
A(n+1) = tmp;
end
n = n +1;
end
I thought it would be this simple, but it apparently isn't. One of the things I do not know how to do is place a value of one array into a new array, which I feel is vital in a script like this, but I did not know how to do it, so I did not implement it. Any help will be appreciated. Thank you in advance!

Accepted Answer

Star Strider
Star Strider on 31 Oct 2015
Assigning one array to another is like assigning any other variable. Consider the B = A assignment here.
It looks as though you want to do a bubble sort, the simplest sorting algorithm. You’re close. This is how I would do it:
A = [2 1 4 3 6 5 8 7 10 9 12 11 14 13 16 15 18 17 20 19];
B = A; % Original Vector
swap = true;
while swap
swap = false;
for n = 1:length(A)-1
if A(n) < A(n+1)
tmp = A(n);
A(n) = A(n+1);
A(n+1) = tmp;
swap = true;
end
end
end
The idea is to iterate the swapping loop until the ‘swap’ test is false ( i.e. no swap was made during a particular iteration of the vector ). Your loop sorts in descending order. You can add a counter to keep track of the number of swaps that were made.
  5 Comments
Shawn Simon
Shawn Simon on 31 Oct 2015
Awesome!
Star Strider
Star Strider on 31 Oct 2015
Thank you!

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More Answers (2)

Walter Roberson
Walter Roberson on 31 Oct 2015
B(m) = A(n);
for some m and n.
However, that is not the reason your code is not working. You have implemented one pass of a "bubble sort". If your input were [3 2 1] then when n=1 it would change to [2 3 1] and then when n=2 it would change to [2 1 3] (and in the next pass it would crash because you would be trying to compare A(3) to the non-existent A(4), but that is trivially fixed.) Notice that the "2" and the "1" never get compared to each other. You will need to re-examine your strategy to fix that.
  1 Comment
Shawn Simon
Shawn Simon on 31 Oct 2015
Edited: Shawn Simon on 31 Oct 2015
So, even though I used a while loop, I would need a second loop (like a for loop used by Star Strider) to make multiple passes through the array to ensure that each element of the array is compared to each other?

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timo
timo on 31 Oct 2015
Dont reinvent the wheel
>> x =py.list([5,4,1,2,3,7])
x =
Python list with no properties.
[5.0, 4.0, 1.0, 2.0, 3.0, 7.0]
>> x.sort
>> x
x =
Python list with no properties.
[1.0, 2.0, 3.0, 4.0, 5.0, 7.0]
  3 Comments
timo
timo on 1 Nov 2015
but why ? Too much free time ?
Star Strider
Star Strider on 1 Nov 2015
It could be just to see how a sort script works, and to learn programming processes and logic in the process. Some people have the sort of insatiable curiosity about such things that will carry them far. Shawn is likely one such person.

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