What makes this F# so slow in VS2012? - visual-studio

Question

In Visual Studio 2012, why would this code execute so quickly in the interactive mode and so slowly when run as a console application? I have a fast computer, but I can count to 4 before the function completes at run-time and not even to 1 in the interactive studio window.
The other part that irks me about this is that when I test other people's F# code for Project Euler #4, they all run fine. So it leaves me to believe that there is something about this code that is not optimal. (and it was so neat and clean too >:P)
let reverse(digits:string) =
digits.ToCharArray() |> Array.rev |> System.String.Concat
let isPalindrome(number:int) =
let text = number.ToString()
if text.Length % 2 = 0 then
text = reverse(text)
else
false
let palindromes(floor:int, ceiling:int) =
seq {
for i1 = floor to ceiling do
for i2 = floor to ceiling do
let result = i1 * i2
if isPalindrome result then
yield result
}
let run =
palindromes(100, 999)
|> Seq.max
SUMMARY
For the sake of posterity, I'll list the most effective performance changes.
Removing Concat and creating a new string instead.
Retaining only the largest palindrome instead of collecting of all of them
Replacing the string version of reverse with a computational reverse
It still doesn't explain my original issue. But it's is so negligible now, it's hard to convince myself to spend any more time on it. I appreciate everyone's input. Thanks!

Answer 1

If you compile the project on Release mode (Optimize code option turned on), you can hardly see the difference between running the program and executing in F# Interactive.
If you have read other people's versions, you can see that checking isPalindrome can be done directly on numbers. However, a quick fix still using String:
let reverse(digits:string) =
System.String (digits.ToCharArray() |> Array.rev)
Note that string concatenation is slow compared to a String constructor call.
As #wsanville said, odd-length numbers could be palindrome too:
let isPalindrome(number:int) =
let text = number.ToString()
text = reverse(text)
In palindromes function, execution time can be cut down by half by iterating i2 starting from i1:
let palindromes(floor:int, ceiling:int) =
seq {
for i1 = floor to ceiling do
for i2 = i1 to ceiling do
let result = i1 * i2
if isPalindrome result then
yield result
}
let run =
palindromes(100, 999)
|> Seq.max
With these simple optimizations, your code run 5x faster on my machine.

Answer 2

The Question is quite old, and I have the same problem before
Console apps in F#(or any F# apps), loads the "Fsharp.Core.dll" file during execution,
So in order to have a faster exe file do the following.
1.)Build using "Release" mode, (say Hello.exe)
2.)Merge the file "Fsharp.Core.dll" to your exe file
Using ILMERGE(http://www.microsoft.com/en-us/download/details.aspx?id=17630)
-Open Command Prompt and CD to "Release Folder"
ILMERGE Hello.exe Fsharp.Core.dll /out:Hello2.exe
Try to run Hello2.exe, and Check if it loads faster.