Thread: Still Needing Help : selection sorting

I am still needing understanding of why my largest_element function is not working properly.

What I gather is that 30 elements are randomly generated in an array. The values of which are between 1 and 200. (DONE)

Then another function is called to find the largest element. Now I don't know if I need to return the index of the largest element of the array (that is how it seems) and use this in the selection sorting (ascend_sort and descend_sort) functions or not.

Can someone explain???
finding largest_element
sorting functions

PHP Code:

#include <stdio.h> 
#include <time.h> 
#include <stdlib.h> 

#define NUMELEM 30 
#define MAXELEM 200 

void ascend_sort(int a[],int n); 
void descend_sort(int a[], int n); 
void random_array(int a[], int n); 
int largest_element (int a[], int f, int l); 

main() 
{ 
int array[NUMELEM]; 
int last=0; 

/* create array */ 

random_array(array,NUMELEM); 

/* need to call function largest_element and assign value*/ 
/* returned to a variable to be used in the selection_sort */
/* functions named ascend_sort and descend_sort          */

last=largest_element(array,array[0],array[NUMELEM-1]); 

printf("%d",last); 
} 
/* find largest element */ 

/* FUNCTION: largest_element : my resource text says to */ 
/* make i and j point to the first element of array (but it */ 
/* does not assume a[0] is the first element for some */ 
/* reason; f and l are representing first and last elements */ 
/* of array passed */ 

int largest_element(int a[], int f, int l) 
{ 
        int i=f; 
        int j=f; 

         do { 
                    i++; 
                    if (a[i] > a[j]) { 
                              j=i; 
                    } 
         } while (i != l); 

         return j; 
} 

/* sort in ascending order using selection sorting */ 

void ascend_sort(int a[],int n) 

{ 
int largest; 
int temp; 




} 

/* sort in descending order using selection sorting */ 

void descend_sort(int a[], int n) 
{ 


} 

void random_array(int a[], int n) 
{ 
    int i; 

    for (i=0; i < NUMELEM ; i++) 

         { 
               a[i] = rand()%MAXELEM+1; 
                   printf("%3d ",a[i]); 

                  if ((i+1)%10==0) 
                  printf("\n"); 
         } 
} 


Code:

//call
random_array(array,NUMELEM); 

//definition
void random_array(int a[], int n) 
{ 
    int i; 

    for (i=0; i < NUMELEM ; i++) 

         { 
               a[i] = rand()%MAXELEM+1; 
                   printf("%3d ",a[i]); 

                  if ((i+1)%10==0) 
                  printf("\n"); 
         } 
}

Why would you have an arguement NUMELEM dictated by the parameter n and not use it in your function. n is NUMELEM. Either use it instead of NUMELEM or get rid of it. One way to eliminate your #define would be to pass the size like this: sizeof(array)/sizeof(array[0])

The largest element is found by circualting throgh the array. Just pass the array arguement and size.

Code:

last=largest_element(array, sizeof(array)/sizeof(array[0]);

int largest_element(char a[], int size)
{
   int largest = 0;
   int i;

   for(i=0; i < size; i++)
   {
      if(array[i] > largest) largest = array[i];
   }
    return largest;
}

I'd have to look up selection sort. Don't feel like doing that right now. I only know bubble sort by heart.

last=largest_element(array,array[0],array[NUMELEM-1]);
I think you meant
largest_element(array,0,NUMELEM-1);


also another thing , the way largest_element is implemented may run into trouble

int largest_element(int a[], int f, int l)
{
int i=f;
int j=f;
do {
i++;
if (a[i] > a[j]) {
j=i;
}
} while (i != l);
return j;
}

you seem to implicitly assume f < l
suppose I want to find the maximum element of an array with 1 element ,
suppose a is defined in main as
int a[]={0,2,3};
so , suppose I call it as
last=largest_element(a,0,0);
your function will return 1 , not 0 .
of course you can just add a line after the declarations to solve this problem , if you dont want to change the do{...}while(); loop to a while loop or a for loop for some reason.

int largest_element(int a[], int f, int l)
{
int i=f;
int j=f;
if(l==f) return l;
do {
i++;
if (a[i] > a[j]) {
j=i;
}
} while (i != l);
return j;
}

I was wrong , in case of an array with one element , your function wont return wrong value , it will become an infinite loop as the condition (i==l) will never be true , but the change I suggested should work .

Originally posted by pinko_liberal
last=largest_element(array,array[0],array[NUMELEM-1]);
I think you meant
largest_element(array,0,NUMELEM-1);


also another thing , the way largest_element is implemented may run into trouble

int largest_element(int a[], int f, int l)
{
int i=f;
int j=f;
do {
i++;
if (a[i] > a[j]) {
j=i;
}
} while (i != l);
return j;
}

you seem to implicitly assume f < l
suppose I want to find the maximum element of an array with 1 element ,
suppose a is defined in main as
int a[]={0,2,3};
so , suppose I call it as
last=largest_element(a,0,0);
your function will return 1 , not 0 .
of course you can just add a line after the declarations to solve this problem , if you dont want to change the do{...}while(); loop to a while loop or a for loop for some reason.

int largest_element(int a[], int f, int l)
{
int i=f;
int j=f;
if(l==f) return l;
do {
i++;
if (a[i] > a[j]) {
j=i;
}
} while (i != l);
return j;
}

hi there... in case you want to see...
this is tutorial about selection sort:
http://www.cpp-home.com/tutorial.php?113_1

This website is way cool, but I don't know how to convert
C++ code into C. I am new to programming in any language. Took C as first language, having many troubles with pointers, structures and recursive functions.

Can you explain how to convert to C source code???
Just a few pointers would be a start, I suppose.

I was trying to set a separate function to find largest element but this C++ code has that function in a loop in the main() to and just a separate function to do the selection sorting.

I think you could put a loop in your main function to
check to see if an element is larger than the next

similar to bubble sorting

review Troll King's code, it makes sense !!