On Friday, 29 November 2019 at 15:30:22 UTC, realhet wrote:
Hi,
I have an input range.
I use the map! on it to transform using a function.
Finally I use filter! on the transformed data.
When I do a foreach on this, i noticed, that the transform
function is called twice for each element.
If I remove the filter! it does only one transform function
call per element.
Is this because the function is NOT PURE, so Phobos thinks that
the function can give different results for the same input?
If I modify the function to be pure, will the transform
function be called only once?
It's actually a much simpler reason: filter calls .front twice
for each element in its input (once to check if the value
satisfies the predicate, and then again to return the value if it
does), and the range returned by map doesn't save the value of
.front between calls, so it has to re-compute it each time by
calling the transform function.
You can work around this using std.algorithm.iteration.cache, a
range wrapper that calls .front on the underlying range exactly
once, and then saves the value and reuses it for subsequent calls:
myRange
.map!transform
.cache
.filter!pred
.each!doSomething;
