Random Number Generation C It is often useful to generate random numbers to produce simulations or games (or homework problems:) One way to generate these numbers in. Jun 29, 2017 Generating Random number in C: We will use the rand function from the cstdlib library. Rand will generate one random number for each call. We will get one random number between the 0 to RANDMAX for each call. Please look at the following code.
In this article we will discuss how to implement QuickSort using random pivoting. In QuickSort we first partition the array in place such that all elements to the left of the pivot element are smaller, while all elements to the right of the pivot are greater that the pivot. Then we recursively call the same procedure for left and right subarrays.
Unlike merge sort we don't need to merge the two sorted arrays. Thus Quicksort requires lesser auxillary space than Merge Sort, which is why it is often preferred to Merge Sort.Using a randomly generated pivot we can further improve the time complexity of QuickSort.
We have discussed at two popular methods for partioning the arrays-Hoare's vs Lomuto partition scheme
It is advised that the reader has read that article or knows how to implement the QuickSort using either of the two partition schemes.
Algorithm for random pivoting using Lomuto Partitioning
Algorithm for random pivoting using Hoare Partitioning
Below is the CPP implementation of the Algorithms
Lomuto (C++)
/* C++ implementation QuickSort using Lomuto's partition #include usingnamespacestd; /* This function takes last element as pivot, places the pivot element at its correct position in sorted array, and places all smaller (smaller than pivot) to left of pivot and all greater elements to right intpartition(intarr[], intlow, inthigh) intpivot = arr[high]; // pivot inti = (low - 1); // Index of smaller element for(intj = low; j <= high - 1; j++) { // If current element is smaller than or if(arr[j] <= pivot) { i++; // increment index of smaller element } swap(arr[i + 1], arr[high]); } // Generates Random Pivot, swaps pivot with intpartition_r(intarr[], intlow, inthigh) // Generate a random number in between srand(time(NULL)); swap(arr[random], arr[high]); returnpartition(arr, low, high); arr[] --> Array to be sorted, high --> Ending index */{ /* pi is partitioning index, arr[p] is now intpi = partition_r(arr, low, high); // Separately sort elements before quickSort(arr, low, pi - 1); } voidprintArray(intarr[], intsize) inti; printf('%d ', arr[i]); } // Driver program to test above functions { intn = sizeof(arr) / sizeof(arr[0]); printf('Sorted array:'); return0; |
Hoare (C++)
partition scheme. */#include /* This function takes last element as pivot, places the pivot element at its correct position in sorted array, and places all smaller (smaller than pivot) to left of pivot and all greater elements to right intpartition(intarr[], intlow, inthigh) intpivot = arr[low]; // or equal to pivot i++; // or equal to pivot j--; if(i >= j) } // end element and calls the partition function // In Hoare partition the low element is selected intpartition_r(intarr[], intlow, inthigh) // Generate a random number in between srand(time(NULL)); swap(arr[random], arr[low]); returnpartition(arr, low, high); arr[] --> Array to be sorted, high --> Ending index */{ /* pi is partitioning index, arr[p] is now intpi = partition_r(arr, low, high); // Separately sort elements before quickSort(arr, low, pi); } voidprintArray(intarr[], intn) for(inti = 0; i < n; i++) printf('); intmain() intarr[] = { 10, 7, 8, 9, 1, 5 }; quickSort(arr, 0, n - 1); printArray(arr, n); } |
Output:
Notes
Generate Random Number In Dev C 5
- Using random pivoting we improve the expected or average time complexity to O (N log N). The Worst Case complexity is still O ( N^2 ).
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