Why is my foreach faster than my for loop?

Question

int[] testArray = new int[200000000];

Stopwatch st2 = new Stopwatch();
st2.Start();

for (var j = 0; j < testArray.Length; j++)
{

}

st2.Stop();

Console.WriteLine("Total milliseconds - FOR LOOP: {0}", st2.Elapsed.TotalSeconds);



Stopwatch st = new Stopwatch();
st.Start();

foreach (var arr in testArray)
{

}

st.Stop();

Console.WriteLine("Total milliseconds - FOREACH LOOP: {0}", st.Elapsed.TotalSeconds);

Results:

for: 0,7046522 seconds
foreach: 0,05508682 seconds

Why is my foreach faster? I thought that my for loop would be faster than the foreach

I get a result of about 0.6 secs for the foreach - not much different from the for-result (0.7 secs) — Hans Keﬆing, Sep 02 '15 at 09:55
I can't replicate these results. I get `Total milliseconds - FOR LOOP: 0.1187888 Total milliseconds - FOREACH LOOP: 0.2583282` in a release build. — Matthew Watson, Sep 02 '15 at 09:55
Read how to properly benchmark JITed code. This is not a valid benchmark. — CodeCaster, Sep 02 '15 at 09:56
put some operation inside both loop, to have a more sensible test — Felice Pollano, Sep 02 '15 at 09:56
You're not doing anything inside the loop, so it might be just a compiler optimization. If I add a statement inside each loop, I get results like 0.18 for the for-loop and 0.95 for the foreach-loop. — Dennis_E, Sep 02 '15 at 09:57
Simply by running this with optimizations, you'll avoid the bounds check on each iteration of the `for` loop, and you'll see it executing faster. — Yuval Itzchakov, Sep 02 '15 at 10:00

score 3 · Accepted Answer · answered Sep 02 '15 at 09:57

The reason is explained in detailed in this article. it says that

In micro-benchmarking, introducing extra local variables with foreach-loops can impact performance. However, if those local variables are reused several times in the loop body, they can lead to a performance improvement.

Thus: The for-loop is faster than the foreach-loop if the array must only be accessed once per iteration.

You can realize this by including some operations in the loop and runs it again.

Why is my foreach faster than my for loop?

1 Answers1