Quicksort C - A little different version - c

Question

The code does run. This is a little different version of quicksort I am working on. I am running into some major issues with it. First off It prints out the first element in the array as n: for example(if you set n = 3, even if you make the first element in the array 1 lets say, it will still print out 3 as the first element). Also when you print out the sorted version it doesn't actually change anything.
Example input with n = 3,
Set values = 8 , 7 , 6
Initial output will equal 3 , 7 , 6
Final output will equal 3 , 7 , 6
(The output SHOULD be 6 , 7 , 8)
I haven't been able to find any code online similar to my code, so this may be something new! Thanks.
//preprocessor directives and header files
#include <stdio.h>
#define MAX_ARRAY_SIZE 50
//function prototypes separated by data types
void print_array( int array[], int n ); // Print out the array values
void swap( int array[], int index1, int index2 ); // Swap two array elements.
void quicksort( int array[], int low, int high ); // Sorting algorithm
int populate_array( int array[] ); // Fill array with values from user.
int partition( int array[], int low, int high ); // Find the partition point (pivot)
//the main function
int main(void)
{
int array[MAX_ARRAY_SIZE];
//set n = to size of user created size of array
int n = populate_array(&array[MAX_ARRAY_SIZE]);
//print the original array to the screen
print_array(&array[MAX_ARRAY_SIZE], n );
//perform the algorithm
quicksort(array, 0, n-1);
printf("The array is now sorted:\n");
print_array(&array[MAX_ARRAY_SIZE], n);
return 0;
}
// *array and array[] are the same...
int populate_array(int array[])
{
int n = -1;
printf("Enter the value of n > ");
scanf("%d", &n);
if(n > MAX_ARRAY_SIZE)
{
printf("%d exceeds the maximum array size. Please try again.\n\n", n);
populate_array( &array[MAX_ARRAY_SIZE]);
}
else if(n < 0)
{
printf("%d is less than zero. Please try again.\n\n", n);
populate_array( &array[MAX_ARRAY_SIZE]);
}
else if(n == 0)
{
printf("%d Array of size 0? Please don't try this, and... Please try again.\n\n", n);
populate_array( &array[MAX_ARRAY_SIZE]);
}
else
{
for(int i = 0; i < n; i++)
scanf("%d", &array[i]);
}
printf("The initial array contains: \n");
return n;
}
void print_array(int array[], int n)
{
for(int i = 0; i < n; i++)
printf("%+5d\n", array[i]);
}
void quicksort(int array[], int low, int high)
{
if (low < high)
{
/* pivot is partitioning index, array[p] is now
at right place */
int pivot = partition(array, low, high);
// Separately sort elements before
// partition and after partition
quicksort(array, low, pivot - 1);
quicksort(array, pivot + 1, high);
}
}
int partition(int array[], int low, int high)
{
int pivot = array[high];
int i = low;
for (int j = low; j <= high- 1; j++)
{
// If current element is smaller than or
// equal to pivot
if (array[j] <= pivot)
{
swap(array, i, j);
i = i +1;
}
}
swap(array, i, high);
return i;
}
void swap(int array[], int index1, int index2)
{
int temp = array[index1];
array[index1] = array[index2];
array[index2] = temp;
}

Answer 1

Here is a heavily commented answer. I changed the code quite a bit.
This is now a fully functional quicksort array for user input.
The problem I was having before was with the &array[MAX_ARRAY_SIZE]. This needed to be changed to just "array" instead. The &array[MAX_ARRAY_SIZE] was trying to access a memory location past the actual size of the array.
Changing it to just "array" means that it is accessing the first element in the array.(Correct if wrong)
I also changed the populate array function to be a robust do-while loop. And instead of trying to re-call the function inside itself. The do-while loop will only allow you to change the value of 'n'.
/*
Author: Zachary Alberda
*/
//preprocessor directives and header files
#include <stdio.h>
#define MAX_ARRAY_SIZE 50
//function prototypes separated by data types
void print_array( int array[], int n ); // Print out the array values
void swap( int array[], int index1, int index2 ); // Swap two array elements.
void quicksort( int array[], int low, int high ); // Sorting algorithm
int populate_array( int array[] ); // Fill array with values from user.
int partition( int array[], int low, int high ); // Find the partition point (pivot)
//the main function
int main(void)
{
int array[MAX_ARRAY_SIZE]; //set n = to size of user created size of array
int n = populate_array(array); //print the original array to the screen
print_array(array, n ); //print array of size n
quicksort(array, 0, n-1); //perform the algorithm low is 0, high is size of array -1.
printf("The array is now sorted:\n");//Inform user that the array is sorted.
print_array(array, n);//print the sorted array
return 0; // exit without errors.
}
// *array and array[] are the same...
int populate_array(int array[])
{
int n = -1;//initialize variable n(local variable to function populate_array)
printf("Enter the value of n > ");//inform user of what to input
scanf("%d", &n);
/*
CHECK IF N IS VALID
This is a robust do while loop!
1) Performs the if-statements while 'n' is not valid in a do-while loop.
-The reason I do this is because it will cause errors
if the if-statements are individual without the do-while loop.
2)The program will not crash if you try different combinations
of inputs for 'n'. :)
3)Checks if user input is > MAX_ARRAY_SIZE
4)Checks if user input is < 0
5)Checks if user input is == 0
*/
do
{
if(n > MAX_ARRAY_SIZE)
{
printf("%d exceeds the maximum array size. Please try again.\n\n", n);
printf("Enter the value of n > ");
scanf("%d", &n);
}
else if(n < 0)
{
printf("%d is less than zero. Please try again.\n\n", n);
printf("Enter the value of n > ");
scanf("%d", &n);
}
else if(n == 0)
{
printf("%d Array of size 0? Please don't try this, and... Please try again.\n\n", n);
printf("Enter the value of n > ");
scanf("%d", &n);
}
}while(n <= 0 || n > MAX_ARRAY_SIZE);
//scan in array if user input is valid
for(int i = 0; i < n; i++)
scanf("%d", &array[i]);
printf("The initial array contains: \n");//Inform user of initial array
return n;
}
void print_array(int array[], int n)
{
//print array in pre/post order before and after the algorithm.
for(int i = 0; i < n; i++)
printf("%+5d\n", array[i]);
}
void quicksort(int array[], int low, int high)
{
if (low < high)
{
/* pivot is partitioning index, array[pivot] is now
at right place */
int pivot = partition(array, low, high);
// Separately sort elements before
// partition and after partition
quicksort(array, low, pivot - 1);
quicksort(array, pivot + 1, high);
}
}
int partition(int array[], int low, int high)
{
int pivot = array[high];
int i = low;
for (int j = low; j <= high- 1; j++)
{
// If current element is smaller than or
// equal to pivot
if (array[j] <= pivot)
{
swap(array, i, j);
i = i +1;
}
}
swap(array, i, high);
return i;
}
void swap(int array[], int index1, int index2)
{
//swap positions of array index 1 and 2
int temp = array[index1];
array[index1] = array[index2];
array[index2] = temp;
}