greedy algorithm / insertion sort / quick sort

always makes the choice that seems to be the best at that moment. 

Example #1:

@function:  scheduling

// You are given an array A of integers, where each element indicates the time
// thing takes for completion. You want to calculate the maximum number of things
// that you can do in the limited time that you have.

//
//  main.cpp
//  greedy

#include <iostream>

using std::cout;
using std::cin;
using std::string;

#define SIZEOF_ARRAY(a) (sizeof(a)/sizeof(a[0]))

template<typename T>
void insertion_sort(T *a, size_t n) {
    T tmp;
    size_t j, p;
    for (p = 1; p < n; p++) {
        tmp = a[p];
        for (j = p; 0 < j && tmp < a[j-1]; j--) {
            a[j] = a[j-1];
        }
        a[j] = tmp;
    }
}

template<typename T>
void swap(T *a, T *b) {
    T tmp = *a;
    *a = *b;
    *b = tmp;
}

// Return median of "left", "center", and "right"
// Order these and hide the pivot
template<typename T>
T median3(T *a, size_t left, size_t right) {
    size_t center = (left+right)/2;
    if (a[left] > a[center])
        swap(&a[left], &a[center]);
    if (a[left] > a[right])
        swap(&a[left], &a[right]);
    if (a[center] > a[right])
        swap(&a[center], &a[right]);
    
    /* Invariant: a[left]<=a[center]<=a[right] */
    swap(&a[center], &a[right-1]); /* HIde pivot */
    return a[right-1]; /* return pivot */
}

#define cutoff (3)
template<typename T>
void quicksort(T *a, size_t left, size_t right) {
    size_t i, j;
    T pivot;
    if (left + cutoff <= right) {
        pivot = median3(a, left, right);
        i = left;
        j = right-1;
        for (;;) {
            while (a[++i] < pivot) {}
            while (a[--j] > pivot) {}
            if (i < j)
                swap(&a[i], &a[j]);
            else
                break;
        }
        swap(&a[i], &a[right-1]); /* restore pivot */
        
        quicksort(a, left, i-1);
        quicksort(a, i+1, right);
    } else {
        insertion_sort(a+left, right-left+1);
    }
}

template<typename T>
void sort(T *a, size_t n) {
    quicksort(a, 0, n-1);
}

template<typename T>
void print_array(T *a, size_t n) {
    size_t i;
    cout << "[";
    for (i = 0; i < n-1; i++) {
        cout << a[i] << ",";
    }
    cout << a[i] << "]\n";
}

int scheduling(int *a, size_t n, int t) {
    int numberOfThings = 0, currentTime = 0;
    
    sort<int>(a, n);
    print_array<int>(a, n);
    
    for (int i = 0; i < n; i++) {
        currentTime += a[i];
        if (currentTime > t) {
            break;
        }
        numberOfThings++;
    }
    return numberOfThings;
}

int main(int argc, const char * argv[]) {
    
    int a[] = {15, 17, 15, 18, 10};
    int t = 40, n = SIZEOF_ARRAY(a);
    
    cout << "the Scheduling problem output: "<< scheduling(a, n, t) << "\n";
    
    return 0;
}

output:

the Scheduling problem output: [10,15,15,17,18]

3

Program ended with exit code: 0