If statement speed question

Question

3.00/5 (3 votes)

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I have a question about run speed. Is this:

C#

if( (x1 == x2) && (x2 == x3) && (x3 == x4) && (x4 == x5) ) return true;
else
    return false;

faster, slower or about the same as this:

C#

if( x == x2)
{
    if( x2 == x3)
        if(x3 == x4)
            if(x4 == x5)
                return true;
}
else
    return false;

Posted 29-Jul-11 6:09am

NathanielM

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4 solutions

Solution 2

Try it and see. You could learn something

Posted 29-Jul-11 6:17am

Not Active

Solution 3

Better use Timer yourself in the code to see the result.

Recently a survey(which contains many discussions) in CP similar to your question, see that

Do you wrap single line statements in braces (or begin/end)?[^]

Posted 29-Jul-11 6:21am

thatraja

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YvesDaoust · Accepted Answer · 2011-07-29T07:23:00

Solution 4

Be sure to repeat the timing experiment several times because the measurements will fluctuate.

In compiled code, there is no reason the speeds should be different, they will translate to exactly the same assembly code.

You could also write

return x1 == x2 && x2 == x3 && x3 == x4 && x4 == x5;

same speed.

And possibly slightly different speed but same effect:

C++

return x1 == x2 && x1 == x3 && x1 == x4 && x1 == x5;

If the x's are integers, it is certainly worth to try:

C++

return ((x1 ^ x2) | (x2 ^ x3) | (x3 ^ x4) | (x4 ^ x5)) == 0;

Posted 29-Jul-11 7:23am

YvesDaoust

Updated 29-Jul-11 7:28am

v2

Comments

NathanielM 29-Jul-11 14:53pm

it's actually an array of booleans representing a string of bits. I'm attempting to go through a long string and see if there are 5 bits in a row that are all high (true) or low (false).
I'm using the if > if > if > if version because then if I find 3 in a row, but the 4th is different, I can skip the 3 bits in the else clause instead of incrementing by one and seeing that the next two bits in a row are the same, etc.

YvesDaoust 29-Jul-11 15:52pm

I see. This opens up a number of new possibilities.

On one hand, you can use one of the classical string searching algorithms. Some of them are linear in the string length, regardless the pattern length. See http://en.wikipedia.org/wiki/String_searching_algorithm.

On another hand, you can pack your bits into 32 or 64 integers and use bitwise operations. X & (X<<1) & (X<<2) & (X<<3) & (X<<4) finds all 5 contiguous 1s in a 28/60 bits sequence.

Using SSE instructions, you can even process 128 bits at a time...

YvesDaoust 29-Jul-11 16:03pm

Forget about the string searching stuff, overkill here. Just count the number of equal bits in a row with a two-state automaton.

bool State= Bit[0];
int Count= 1;
for (i= 1; i < N; i++)
{
if (State == Bit[i])
{
Count++;
if (Count == 5)
break;
}
else
{
Count= 1;
State= Bit[i];
}
}

NathanielM 29-Jul-11 16:25pm

Again, sometimes some of the simplest answers are the hardest to think of. Thanks!

YvesDaoust 29-Jul-11 16:16pm

You can spare a test by using a 10 states automaton. The 10 states represent the number of contiguous 0s or 1s that have been seen so far. You can represent the transition matrix as an array.

YvesDaoust 29-Jul-11 16:29pm

Thanks for the inspiring question.

Espen Harlinn 30-Jul-11 7:35am

Good points, my 5

Cloud Hsu · Accepted Answer · 2011-07-29T06:16:00

C#

/// <summary>
/// Performance Counter to calculate function executing time.
/// </summary>
public class Performance
{
    public delegate void CalculateHanlder();
    /// <summary>
    /// Using Performance.CalculFunction(MethodName)
    /// It will print executing time in Console.
    /// </summary>
    /// <param name="method">A none argument method</param>
    public static void CalculateMethod(CalculateHanlder method)
    {
        Stopwatch st = new Stopwatch();
        st.Start();
        method.Invoke();
        st.Stop();
        string msg = string.Format("Elapsed = {0} in [{1}]", st.Elapsed.ToString(), method.Method.ToString());
        Console.WriteLine(msg);
        Debug.WriteLine(msg);
    }
}
bool Test(int x1, int x2, int x3, int x4, int x5)
{
if( (x1 == x2) && (x2 == x3) && (x3 == x4) && (x4 == x5) ) return true;
else
    return false;
}
bool Test2(int x1, int x2, int x3, int x4, int x5)
{
if( x == x2)
{
    if( x2 == x3)
        if(x3 == x4)
            if(x4 == x5)
                return true;
}
else
    return false;
}
void TestIf()
{
    for(int i = 0 ; i < 100000; i ++)
        Test(1,2,3,4,5);
}
Performance.CalculateMethod(TestIf);

You can use these code to test.
Using StopWatch can count the executing time.
This is my library function, you can pass a none parameter function to tick.