What is the most efficient way to execute this code?

Question

My teacher asked us to make a program in the most efficient way possible and to use a switch case for this.

The program asks the user for input, and depending on what the user inputs, the program will have to follow a set of instructions.

If the input is "A" or "a", the array has to be sorted from A to Z.

If the input is "Z" or "z", the array has to be sorted from Z to A.

If the input is "R" or "r", the array has to be reversed.

The array is a string[].

So I'm wondering if it's more effecient to use

switch (choice.ToLower())
{
    case "a":
        Array.Sort(array);
        break;
    case "z":
        Array.Sort(array);
        Array.Reverse(array);
        break;
    case "r":
        Array.Reverse(array);
        break;
}

or

 if (choice.ToLower() == "a" || choice.ToLower() == "z")
 {
     Array.Sort(array);
 }

 if (choice.ToLower() == "r" || choice.ToLower() == "z")
 {
     Array.Reverse(array);
 }

and also if this code can be optimised even further.

So, is the most efficient way to use a switch case, or the if structure as seen above and explain why?

I'm just curious since I always try to optimise all my code to full extent.

first one is more efficient because it calls ToLower once. but they both compile into if-else. in compiled code with switch `choice.ToLower()` is saved in local variable and its value is checked over if-else. however the second version it is possible that ToLower method gets called from 1 time to 4 times. — M.kazem Akhgary, Oct 10 '15 at 17:16
At least sorting directly to reverse order would be faster than sort and reverse, if you also care about that — Sami Kuhmonen, Oct 10 '15 at 17:19
functionally, these two are equivalent; However, if the point of the exercise was to use a switch statement, then that should be your focus. You may want to read http://meta.programmers.stackexchange.com/questions/6166/open-letter-to-students-with-homework-problems. It's an older post, but the sentiment still applies. When learning, performance should be your least important factor. — Claies, Oct 10 '15 at 17:19
So, if I'd store the choice.ToLower in choice before executing the code, then leave out the ToLower in the if structure, would that execute faster? — Jesse Verbruggen, Oct 10 '15 at 17:29
@Claies We already went over the switch case thing, the point of the exercise was displaying arrays, but our teacher hinted towards the use of switch case, but i tried to see if my idea wasn't better. — Jesse Verbruggen, Oct 10 '15 at 17:41
@JesseVerbruggen Please don't change your question's code. This makes the comments you got make less sense. — Lee Taylor, Oct 10 '15 at 19:37

Fabjan · Accepted Answer · 2015-10-10T17:41:27.480

Well, you can check it by yourself :

class Program
{
    static void MyMethod1(int[] array, string choice)
    {            
        switch (choice.ToLower())
        {
            case "a":
                Array.Sort(array);
                break;
            case "z":
                Array.Sort(array);
                Array.Reverse(array);
                break;
            case "r":
                Array.Reverse(array);
                break;
        }
    }

    static void MyMethod2(int[] array, string choice)
    {            
        if (choice.ToLower() == "a" || choice.ToLower() == "z")
        {
            Array.Sort(array);
        }

        if (choice.ToLower() == "r" || choice.ToLower() == "z")
        {
            Array.Reverse(array);
        }
    }

    static int[][] CreateRandomArrays(int num, int length)
    {
        Random rand = new Random();
        int[][] arrays = new int[num][];

        for (int i = 0; i < arrays.Length; i++)
        {
            arrays[i] = new int[length];
            for (int i2 = 0; i2 < length; i2++)
                arrays[i][i2] = rand.Next();
        }

        return arrays;
    }

    static void Main(string[] args)
    {
        int[][] test1 = CreateRandomArrays(50, 200000);
        int[][] test2 = CreateRandomArrays(50, 200000);

        Stopwatch s = new Stopwatch();

        s.Start();

        for (int i = 0; i < test1.Length; i++) MyMethod1(test1[i], "z");

        s.Stop();
        Console.WriteLine(s.ElapsedMilliseconds);

        s.Restart();            

        for (int i = 0; i < test2.Length; i++) MyMethod2(test2[i], "z");

        s.Stop();

        Console.WriteLine(s.ElapsedMilliseconds);
    }
}

As you can see result is almost identical :

1010 ms    vs    1008 ms

This was very helpful code, thank you, My results when changing the choice.ToLower in the if structure so it would only be called once resulted in: 1749 ms vs 1694 ms — Jesse Verbruggen, Oct 10 '15 at 17:38
@JesseVerbruggen By all means. Feel free to experiment with this simple benchmark to test performance... Also as others said `ToUpper()` is better then `ToLower()`. I'd suggest that 99% of time execution for both of these methods is required for `Sort()` and `Reverse()` methods and not `if` or `switch` statements — Fabjan, Oct 10 '15 at 18:20

score 1 · Answer 2 · answered Oct 10 '15 at 17:37

ToUpper is faster + using Linq, ordering things will not be executed till join part...

using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static string[] words = {
        "what", "is", "the", "most", "effecient", "way", "to", "execute", "this", "code"
    };

    static void Main(string[] args)
    {
        IEnumerable<string> result;

        Console.Write("Choose words order (A to Z (A), Z to A (Z), Reversed (R)): ");

        switch (Console.ReadLine().ToUpper())
        {
            case "A": result = words.OrderBy(w => w); break;
            case "Z": result = words.OrderByDescending(w => w); break;
            case "R": result = words.Reverse(); break;
            default: result = words.AsEnumerable(); break;
        }

        Console.WriteLine(string.Join(" ", result));
    }
}

Is adding a default to a switch better than just leaving it out? Because nothing needs to be done when the input isn't a, z or r. — Jesse Verbruggen, Oct 10 '15 at 17:45
It's recommended: http://stackoverflow.com/questions/4649423/should-switch-statements-always-contain-a-default-clause — Matěj Pokorný, Oct 10 '15 at 17:50

CDrosos · Answer 3 · 2015-10-10T17:25:08.483

0

In your case if is slower because every time it check 2 conditions. Check also this article about switch vs if speed. (in your example with the if's 4 conditions are always checked).
If a switch contains more than five elements, it's implemented using a lookup table or a hash list which means that all items get the same access time, compared to a list of if's where the last item takes much more time to reach as it has to evaluate every previous condition first.

edited Oct 10 '15 at 17:25

answered Oct 10 '15 at 17:17

CDrosos

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This is not clear. switch for strings uses a hash table which could be slower. – usr Oct 10 '15 at 17:20
@usr I have include this case in my answer – CDrosos Oct 21 '15 at 19:30
This is true, actually, because his case has less than five conditions. I did not notice that. – usr Oct 21 '15 at 19:52

What is the most efficient way to execute this code?

3 Answers3