The number tests work, but not the string one.

void main() {
	assert([1,2,3,4,5,6,7,8,9,10,11].binarySearch(6));
assert(! [1,2,3,4,5,7,8,9,10,11].binarySearch(6));
assert("abcdefghijklmnopqrstuvwxyz".binarySearch('j')); // not work
	import std.stdio;
	writeln("Assert tests passed!");
}

bool binarySearch(T)(T[] arr, T n) {
	while(arr.length) {
		auto i = arr.length/2;
		if (arr[i] == n)
			return true;
		else
			if (arr[i]  > n)
				arr = arr[i + 1 .. $];
			else
				arr = arr[0 .. i];
	}
	return false;
}

On 02/25/2018 10:18 PM, Joel wrote:
>              if (arr[i]  > n)
>                  arr = arr[i + 1 .. $];

When `arr[i]` is greater than `n`, then the values in `arr[i + 1 .. $]` will only be even greater. You're picking the wrong half of the array.

On Sunday, 25 February 2018 at 21:18:55 UTC, Joel wrote:
> The number tests work, but not the string one.
>
> void main() {
> 	assert([1,2,3,4,5,6,7,8,9,10,11].binarySearch(6));
> assert(! [1,2,3,4,5,7,8,9,10,11].binarySearch(6));
> assert("abcdefghijklmnopqrstuvwxyz".binarySearch('j')); // not work
> 	import std.stdio;
> 	writeln("Assert tests passed!");
> }
>
> bool binarySearch(T)(T[] arr, T n) {
> 	while(arr.length) {
> 		auto i = arr.length/2;
> 		if (arr[i] == n)
> 			return true;
> 		else
> 			if (arr[i]  > n)
> 				arr = arr[i + 1 .. $];
> 			else
> 				arr = arr[0 .. i];
> 	}
> 	return false;
> }



Your cases are wrong:

---
if (arr[i]  > n) // 'n' > 'j'
// The current element is higher than the needle -> you need to go to the left, not right
--


-> Swap them.


Also note that Phobos comes with binary search built-in:

---
assert([1,2,3,4,5,6,7,8,9,10,11].assumeSorted.canFind(6));
---

https://run.dlang.io/is/bfpBpA

On 2/25/18 4:32 PM, Seb wrote:
> 
> Also note that Phobos comes with binary search built-in:
> 
> ---
> assert([1,2,3,4,5,6,7,8,9,10,11].assumeSorted.canFind(6));
> ---
> 
> https://run.dlang.io/is/bfpBpA

canFind (and find) works even on non-sorted ranges, so it's not the greatest proof. But it's good to know that it does work and uses a binary search!

You can see that it only does a few comparisons with something like:

https://run.dlang.io/is/lax6YP

Also, strings are not doing what you think:

"abcd".find('c'); // OK, linear search
"abcd".assumeSorted.find('c'); // Error
"abcd".assumeSorted.find("c"); // OK, but does NOT do a binary search!
[1,2,3,4].assumeSorted.find([3]); // OK, but also does not do a binary search!

My knee-jerk reaction is to blame auto-decoding ;) But maybe it's just a bug.

If you want to guarantee a binary search (i.e. compiler error when it cannot do it), you need to use SortedRange.lowerBound.

-Steve

On Sunday, 25 February 2018 at 21:18:55 UTC, Joel wrote:
> The number tests work, but not the string one.

Thanks guys. I worked it out, I thought my search code was right, since the first asserts worked.