Small bug in simple array sorting I can't find.

[skysurf76]

I'm working on a homework assignment (we are allowed to get help), and I am 99% done, but I can't seem to get a simple array to sort from least to most. Its the last bug and I'm done with this thing.

The assignment is to do several things with an array that a user enters. I know my program is capturing the array just fine. However when I use the following functions to try to sort the entries from least to most I get weird outcomes. If the array is -2, -1, 0, 1, 2, 3, 4, 5, 6, 7 it will sort the array fine but it will show 7 as the least number. Then I tried entering 4, 2, -1, 5, -4, 3, 8, and it sorted it into -1, -4, 3, 2, 4, 8, 5

-array is x
-len is length of array

Array gets passed to the selection function which is...

void selection(double x[], int len)
{
for(int i=len-1; i>=0; i--)
     {
     int j;
     j=findmax(x,i);
     swap(x[j],x[i]);
     }
}

Explanation= Because array index values start at 0, and len is how many entries the array has, the len-1 is the top index number of the array. So the selection function will start with the full array, and the findmax function will find the biggest entry in the entry, and call the swap function to put that entry on top. So now the biggest element in the array is on top. Now the selection function passes findmax the array again only it ignores the top entry because we already know that was the largest, and the second largest entry is found and put into index len -2, and so on and so forth.

And then here are the findmax and swap functions...

void swap(double& x, double& y)
{
double temp=x;
x=y;
y=temp;
}

int findmax(double x[], int len)
{
int current=0;
for (int i=1; i<len; i++)
     {
     if (x[i] > x[current])
          {
          current=i;
          }
     }
return current;
}

Thats it. Somewhere in that little bit of code something is going terribly wrong and I'm going cross eyed from looking at it so long.

I'm sure some experienced eyes will be able to set me straight pretty quick :) Thank you so much for any help in advance.

[Akshit]

In findmax() function,use

for (int i=1; i<=len; i++)

[Akshit]

actually you got confused by using same variable names.When you first passes i in findmax function len get value of i that is len-1.

Use different names next time.

Also you can use bubble sort or selection sort for this.

[skysurf76]

lol, I wrote out a reply trying to argue with you. It then dawned on me that I probably shouldn't argue with someone on here. So I sat down and looked at it really hard and I see exactly what you're saying.

The i that is getting passed to findmax was based on len-1, but i is getting passed to findmax as len again, so I thought I needed to not pay attention to the top index again and I took another one off the top even though the slection function already did that.

And I just changed it and ran it and that fixed it.

Thank you so very very much. I could have sat there for days and never seen that. Man you gotta have the precision of a Swiss watch maker to be a programmer.

Thank you for saving my night. I now have some time to play some video games. :)

[Akshit]

int findmax(double x[], int l)
{
int current=0;
for (int i=1; i<=l; i++)
     {
     if (x[i] > x[current])
          {
          current=i;
          }
     }
return current;
}

See this one.here initial l=len-1.and i also get to len-1.
So it compares x[len-1]>x[current] which is true for your -2, -1, 0, 1, 2, 3, 4, 5, 6, 7 input.so current=9.
But in your function(in first post) it will check only x[len-2]>x[current]

Hope you got this.

[skysurf76]

Yep, got it. Just changed my previous reply. Didn't get it at first, tried to argue, then sat and looked for awhile and realized what you were saying. Thanks again. :)

[Akshit]

So its working now?