C++ 数组与向量排序（在我的情况下，向量比数组慢约 2.5 倍（无优化））

Question

I am doing an insertion sort on of 100000 elements. I have written two functions.

1- It is in which I copy the vector given to sort into an array then I apply sort and then I copy the array back to a vector for return.

2- It is in which I apply the sort on the given vector and then returns it.

Now according to my knowledge vectors are also dynamically created an array and the difference of C++ Vector [] operator speed should be nonexistent or atleast not this much. So the method 2 should be faster than method 1 . But to my surprise It is opposite. I am trying to find a concrete answer rather than just array is faster. :)

Compiler version => gcc version 6.2.0 (Rev2, Built by MSYS2 project) g++ main.cpp -o main

#include <time.h>
#include <iostream>
#include <cstdlib>
#include <vector>

using namespace std;

vector<long> InsertionSort1(vector<long> nums){
    int vsize = nums.size();
    long* arr = new long[vsize];
    int swap, j;

    // Coping the vector to an array
    for(int i=0;i<vsize;i++){
        arr[i] = nums[i];
    }

    //sorting
    for(int i=1;i<vsize;i++){
        swap = arr[i];
        j = i-1;
        while(j>=0 && arr[j] > swap){
            arr[j+1] = arr[j];
            j--; 
        }

        arr[j+1] = swap;   

    }

    // Coping the array back to vector
    for(int i=0;i<vsize;i++){
        nums[i] = arr[i];
    }

    return nums;
}

vector<long> InsertionSort2(vector<long> nums){
    int vsize = nums.size();
    int swap, j;

    for(int i=1;i<vsize;i++){
        swap = nums[i];
        j = i-1;
        while(j>=0 && nums[j] > swap){
            nums[j+1] = nums[j];
            j--; 
        }

        nums[j+1] = swap;   

    }

    return nums;
}

int main(){

    vector<long> entries;
    for(int i=0;i<100000;i++){
        entries.push_back(rand()%100000);
    }

    double start = time(0);
    InsertionSort1(entries);
    double end = time(0);

    cout<<"With Array => "<<end-start<<endl;

    start = time(0);
    InsertionSort2(entries);
    end = time(0);

    cout<<"With Vector => "<<end-start<<endl;
}

The result of above Code is:

With Array 100000 => 8
With Vector 100000 => 19

Answer 1

I think it is a good thing to measure performance! std::chrono::high_resolution_clock or google benchmark are better suited tools for the job.

You need to enable a proper optimisation level, say -O3 in order to have meaningful results.

Why do you like to implement a sorting algorithm. I compared your approch with std::sort ? For academic purposes this is fine, otherwise you need to have a good reason and a measured proof, that you did a better job.

With Array => 2.55608

With Vector => 1.74857

With std::sort => 0.00657156

#include <time.h>
#include <iostream>
#include <cstdlib>
#include <vector>

using namespace std;

vector<long> InsertionSort1(vector<long> nums){
    int vsize = nums.size();
    long* arr = new long[vsize];
    int swap, j;

    // Coping the vector to an array
    for(int i=0;i<vsize;i++){
        arr[i] = nums[i];
    }

    //sorting
    for(int i=1;i<vsize;i++){
        swap = arr[i];
        j = i-1;
        while(j>=0 && arr[j] > swap){
            arr[j+1] = arr[j];
            j--; 
        }

        arr[j+1] = swap;   

    }

    // Coping the array back to vector
    for(int i=0;i<vsize;i++){
        nums[i] = arr[i];
    }
    // delete of is missing arr;
    return nums;
}

vector<long> InsertionSort2(vector<long> nums){
    int vsize = nums.size();
    int swap, j;

    for(int i=1;i<vsize;i++){
        swap = nums[i];
        j = i-1;
        while(j>=0 && nums[j] > swap){
            nums[j+1] = nums[j];
            j--; 
        }

        nums[j+1] = swap;   

    }

    return nums;
}

int main(){

    vector<long> entries;
    for(int i=0;i<100000;i++){
        entries.push_back(rand()%100000);
    }

    double start = time(0);
    InsertionSort1(entries);
    double end = time(0);

    cout<<"With Array => "<<end-start<<endl;

    start = time(0);
    InsertionSort2(entries);
    end = time(0);

    cout<<"With Vector => "<<end-start<<endl;
}