Thread: Selection Sort

Last night while you were helping me my code was working (i realize this isn't a selection sort but it's not really a big deal)

here is the output

Code:

 Spots in array:
 5

 Spot 1 in array : 5

 Spot 2 in array : 4

 Spot 3 in array : 3

 Spot 4 in array : 2

 Spot 5 in array : 1
 if 5 > 5
 if 5 > 4
then swap them if 5 > 3
then swap them if 5 > 2
then swap them if 5 > 1
then swap them if 4 > 3
then swap them if 4 > 2
then swap them if 4 > 1
then swap them if 4 > 5
 if 2 > 1
then swap them if 2 > 4
 if 4 > 5
 if 2 > 4
 if 4 > 5
 if 4 > 5
 
 
 iSort
3
1
2
4
5
Press any key to continue . . .

Why did she skip 3? For the life of me I can not figure this out.

Nevermind, I have been playing with it and changed the inner loop to go other way..

Code:

for (j=i; j >= 0; j--)

It works guys!

What i don't understand is why this works. I can't visually see our j loop starting at i (which is zero) and moving downwards (j--) to sort. If you guys wouldn't mind giving me a metaphor I would really appreciate it. Thank you.

sorry to keep posting it up but I understand now! (Programming is so awesome)

Bubblesort?
Selection sort?
What it is?
Well it seems to be something you made up, which is good from the point of view that you thought how it worked.But in more difficult problems it is good to see the pseudocode of an already existing algorithm.This will help you do your work and moreover , do it efficiently

Your algorithm is taking time complexity of O(n²) ,which can be reduced enough

You should know that sorting is one of the most common computer tasks, and studies have shown computers spend a lot of their time, doing it. For that reason, it has been studied in depth.

Each sorting algorithm is different: some are fast with random data, (Quicksort), some are fast with nearly sorted data (Insertion), some sorts don't care (Heapsort). Some make very few swaps (Selection), one makes the fewest possible number of sorts (Perfect), some are in place (Quicksort, Shell sort), others use a large amount of extra memory (Merge sort).

For small sets of data (and "small" changes with computer hardware):
Bubble, Insertion, Substitution - all easy to code

For medium sets of data:
Insertion, Shell, Bucket, QuickShort, qsort()

For large sets of data:
Heapsort, Quicksort, Mergesort, Radix, Bucket, qsort()

qsort() is part of the C standard.
This is the shortest version of Quicksort - called QuickShort.

Code:

void quickShort(int A[], int l, int u) {
  int i, m;
  if(l >= u) return;
  m = l;
  for(i = l+1; i<=u; i++) {
    if(A[i] < A[l])
      swap(A,++m, i);
  }
  swap(A, l, m);
  quickShort(A, l, m-1);
  quickShort(A, m+1, u);
}

QuickShort isn't a speed demon, but it is a solid mid-performer.

It's a good idea to become familiar with one or two sorting algorithms from at least two of the categories. Bubble or Substitution sort are very easy for the smallest sorting jobs, but I'm going to recommend Insertion sort - it's clearly faster, and no harder to code.

For large jobs, I tend to favor my fastest sorter, an Optimized Quicksort (Quicksort + Insertion for the smaller sub arrays), and Mergesort. They're real workhorses for huge sorting jobs.

I guess this is more or less a bubble sort that moves int's backwards..i realize its not a selection sort, why its not a selection sort, and have learned a lot so its a success, now I will be trying to incorporate taking data out of a file, sorting it, and writing it into a different file from the command line.

Why you call it like this?

Code:

sort(&a[i],x);

Just pass the array as first argument.

Code:

sort(a,x);

Aha! Back in business!

So i was telling it to sort the locations not the values?

Code:

#include <stdio.h>
#include <conio.h>
 
 
void swap (int *a, int *b) 
{
int temp;
temp = *a;
*a = *b;
*b = temp;
}
int sort(int a[], int x)
{
int i, j;
 for (i = -1; i < x-1; i++)
 {
  
 
  for(j=i; j >= 0; j--)
 
  { 
        printf(" if %d > %d \n",a[j], a[j+1]);
   if (a[j] > a[j+1])
   {
        printf("\n\nthen swap them\n");
   swap(&a[j], &a[j+1]);
   }
  }
 }
 
}
int main()  
  {  
  
      int a[100], x, i;  
  
        
      printf(" Spots in array: \n ");  
      scanf("%d",&x);  
  
      for( i=0;i<x;i++)  
  { 
      printf("\n\n Spot %d in array : ",i+1);  
      
      scanf("%d",&a[i]);  
        }  
  
       sort(a ,x);
  
       printf("\n\n\n\n\n iSort \n\n");  
       for( i=0;i<x;i++)
       {
       printf("%d at %d\n",a[i], &a[i]);  
       }
       
       FILE *fp;
       fp=fopen("c:\\test.txt", "wb");
       fwrite(a, sizeof(a[0]), sizeof(a)/sizeof(a[0]), fp);

  
  system("Pause");
  }

I am now trying to send my array into a file on C:

Using fwrite in this manner should write from memory into the file according the the I/O tutorial.

this however is my output:

Code:

                  h            I
 H % 
   ˆ P
       P
 s     P
 ›  ›  
›  ›   Øþ( Õq)w   þÿÿÿ   î<%w   I
 H   p  %        î<%w       P
 `     h ˆ ð             
         

¼ý( Ä } øþ( Õq)wÎ
1þÿÿÿš8%w’4%w    h h         ` ÿ( ͘Óu       Ú˜Óu÷•õ            ÿ^n€þÿÿÿù­ÓuC®ÓuÐþ(    Äÿ( ÕŒÕuhn€þÿÿÿÚ˜Óuú Ôuh x 8ÿ( ŸÔu        pÿ( ä   Ü   @-} 7   

ok.The b specifier is for binary.You do not want that.
Open the file with "w"
Then see the ref for fwrite
Set as third argument 100.More clear for sure.
Then set as second argument sizeof(int), again for readability.

Judging from what you've said and the fwrite source I tried these:

Code:

FILE *fp;
       fp=fopen("c:\\test.txt", "w");
       fwrite(a, sizeof(100), 100, fp);
       fclose (fp);

Code:

FILE *fp;
      fp=fopen("c:\\test.txt", "w");
      fwrite(a, 100, sizeof(a[100]), fp);
      fclose (fp);

I'm quite confused as to where I'm wrong, it seems as if this should work.

Originally Posted by std10093

Then set as second argument sizeof(int)

I have to write something in order to let me post.

fwrite sucks < fprintf rocks

Code:

#include <stdio.h>
#include <conio.h>
 
 
void swap (int *a, int *b) 
{
int temp;
temp = *a;
*a = *b;
*b = temp;
}
int sort(int a[], int x)
{
int i, j;
 for (i = -1; i < x-1; i++)
 {
  
 
  for(j=i; j >= 0; j--)
 
  { 
        printf(" if %d > %d \n",a[j], a[j+1]);
   if (a[j] > a[j+1])
   {
        printf("\n\nthen swap them\n");
   swap(&a[j], &a[j+1]);
   }
  }
 }
 
}
int main()  
  {  
  
      int a[100], x, i;  
      FILE *
      
      fp=fopen("c:\\users\\hello\\desktop\\test.txt", "a+");
  
        
      printf(" Spots in array: \n ");  
      scanf("%d",&x);  
  
      for( i=0;i<x;i++)  
  { 
      printf("\n\n Spot %d in array : ",i+1);  
      
      scanf("%d",&a[i]);  
        }  
  
       sort(a ,x);
  
       printf("\n\n\n\n\n iSort \n\n");  
       for( i=0;i<x;i++)
       {
       printf("%d at %d\n",a[i], &a[i]);  
       fprintf (fp, "%d, ", a[i]);
       }
       
  
     
      
       
       
      
       system("Pause");
       
       
  }