C++: Function Template to Handle Ints and Strings
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08-06-2021 - |
Domanda
I'm working on a programming assignment, to make a function template that can handle ints and doubles. I've done that, but for fun, I wanted to make it able to handle strings, as well. Here is the function below. How would I go about making it handle strings?
// This program demonstrates the QuickSort Algorithm.
#include <iostream>
#include <algorithm>
#include <ctype.h> //needed for string handling?
using namespace std;
//**********************************************************
// partition selects the value in the middle of the *
// array set as the pivot. The list is rearranged so *
// all the values less than the pivot are on its left *
// and all the values greater than pivot are on its right. *
//**********************************************************
template <class T>
int partition(T set[], int start, int end)
{
int pivotValue, pivotIndex, mid;
mid = (start + end) / 2;
swap(set[start], set[mid]);
pivotIndex = start;
pivotValue = set[start]; // main.cpp:28: error: cannot convert 'std::basic_string<char, std::char_traits<char>, std::allocator<char> >' to 'int' in assignment
for (int scan = start + 1; scan <= end; scan++)
{
if (set[scan] < pivotValue) // main.cpp:31: error: no match for 'operator<' in '*(((std::string*)(((long unsigned int)scan) * 8ul)) + set) < pivotValue'
{
pivotIndex++;
swap(set[pivotIndex], set[scan]);
}
}
swap(set[start], set[pivotIndex]);
return pivotIndex;
}
//************************************************
// quickSort uses the quicksort algorithm to *
// sort set, from set[start] through set[end]. *
//************************************************
template <class T>
void quickSort(T set[], int start, int end)
{
T pivotPoint;
if (start < end)
{
// Get the pivot point.
pivotPoint = partition(set, start, end);
// Sort the first sub list.
quickSort(set, start, pivotPoint - 1); // main.cpp:56: error: no match for 'operator-' in 'pivotPoint - 1'
// Sort the second sub list.
quickSort(set, pivotPoint + 1, end); // main.cpp:58: error: no match for 'operator+' in 'pivotPoint + 1'
}
}
int main()
{
const int SIZE = 10; // Array size
int count; // Loop counter
// create arrays of various data types
int array[SIZE] = {7, 3, 9, 2, 0, 1, 8, 4, 6, 5};
// string array[SIZE] = {"7", "3", "9", "2","7", "3", "9", "2","a","r"};
double array2[SIZE] = {7.1, 3.3, 9.0, 2.7, 0.2, 1.5, 8.9, 4.5, 6.9, 5.45};
// Display the int array contents.
cout << "Displaying the int array before sorting" << endl;
for (count = 0; count < SIZE; count++)
cout << array[count] << " ";
cout << endl;
// Sort the int array.
quickSort(array, 0, SIZE - 1);
// Display the int array contents.
cout << "Displaying the int array after sorting" << endl;
for (count = 0; count < SIZE; count++)
cout << array[count] << " ";
cout << endl << endl;
// Display the double array contents.
cout << "Diplaying the double array before sorting" << endl;
for (count = 0; count < SIZE; count++)
cout << array2[count] << " ";
cout << endl;
// Sort the double array.
quickSort(array2, 0, SIZE - 1);
// Display the int array contents.
cout << "Displaying the double array after sorting" << endl;
for (count = 0; count < SIZE; count++)
cout << array2[count] << " ";
cout << endl;
return 0;
}
Thanks in advance,
Adam
Soluzione
If you use std::string
for T
, you're probably pretty close to working already.
If you use char*
, you'll need to supply a comparison functor as a template parameter (or have some other way of specifying the comparison method for T
, like a type-trait class).
Also, you shouldn't implement your own swap
. std::swap
already exists and will do smart things for certain types (e.g. swapping two vector
s is constant-time instead of copying every object in the vector).
Altri suggerimenti
Works in MSVC, which is a bit permissive so let me know if you run into problems with your compiler.
This solution uses functors (a class/struct with operator()
) which means the object can be called as if it were a function. It also uses template specialization - take a look at what happens if you remove the template < >
versions of LessThanCompare
- char*
will fall back to comparing the pointer value (giving random results).
In a better implementation you would use a class to put your quicksort and pivot functions - then you could use default templates and avoid having to call like quickSort<char*, LessThanCompare<char*> >
- you could just say quicksort
but that's getting a bit beyond the scope of the question!
#include <iostream>
#include <cstring>
#include <string>
using namespace std;
template <class T>
struct LessThanCompare
{
bool operator()(T lhs, T rhs)
{
return lhs < rhs;
}
};
template < >
struct LessThanCompare<char*>
{
bool operator()(char* lhs, char* rhs)
{
return strcmp(lhs, rhs) == -1; // Note strcmp returns -1 if lhs < rhs
}
};
template <class T, class Comparator>
int partition(T set[], int start, int end)
{
Comparator CompareLessThan; // Declare an instance of the Comparator
T pivotValue;
int pivotIndex, mid; // Is mid an index or a value - use descriptive names!
mid = (start + end) / 2;
swap(set[start], set[mid]);
pivotIndex = start;
pivotValue = set[start];
for (int scan = start + 1; scan <= end; scan++)
{
if (CompareLessThan(set[scan], pivotValue))
{
pivotIndex++;
swap(set[pivotIndex], set[scan]);
}
}
swap(set[start], set[pivotIndex]);
return pivotIndex;
}
//************************************************
// quickSort uses the quicksort algorithm to *
// sort set, from set[start] through set[end]. *
//************************************************
template <class T, class Comparator>
void quickSort(T set[], int start, int end)
{
int pivotPoint;
if (start < end)
{
// Get the pivot point.
pivotPoint = partition<T, Comparator >(set, start, end);
// Sort the first sub list.
quickSort<T, Comparator>(set, start, pivotPoint - 1); // main.cpp:56: error: no match for 'operator-' in 'pivotPoint - 1'
// Sort the second sub list.
quickSort<T, Comparator>(set, pivotPoint + 1, end); // main.cpp:58: error: no match for 'operator+' in 'pivotPoint + 1'
}
}
int main()
{
const int SIZE = 10; // Array size
// Create arrays of strings
char* cstrArr[SIZE] = {
"one", "two", "three", "four", "five", "six", "seven", "eight", "nine", "ten"};
std::string strArr[SIZE];
for (int i = 0; i < SIZE; ++i)
{
strArr[i] = std::string(cstrArr[i]);
}
quickSort<char*, LessThanCompare<char*> >(cstrArr, 0, SIZE-1);
quickSort<std::string, LessThanCompare<std::string> >(strArr, 0, SIZE-1);
for (int i = 0; i < SIZE; ++i)
{
cout << cstrArr[i] << "\t\t" << strArr[i] << '\n';
}
return 0;
}