See generated documentation for the most up-to-date information.
One thing missing from default Nim is the ability to do strided slicing.
This is the ability to specify a range with a step parameter to select every nth element in the range, or to reverse the "direction" of the range in case of a negative step.
This package adds the @: operator to do just this:
import pkg/strides
let text = "hello world"
# Regular slice
assert text[ 2 .. ^3 ] == "llo wor"
# Strided slice with a step of 2.
assert text[ 2 .. ^3 @: 2 ] == "lowr"
# Strided slice with reversed direction.
assert text[ ^3 .. 2 @: -1 ] == "row oll"
# Just the stride, like `xs[::s]` in Python:
assert text[ @: -1 ] == "dlrow olleh"
assert text[ @: 2 ] == "hlowrd"
# Additionally, a third form of length+stride is supported:
# Positive stride works like `xs[:a:s]` in Python:
assert text[ 10 @: 1 ] == "hello worl"
assert text[ 10 @: 2 ] == "hlowr"
assert text[ 10 @: 3 ] == "hlwl"
# Negative stride works like `xs[a::s]` in Python:
assert text[ 10 @: -1 ] == "lrow olleh"
assert text[ 10 @: -2 ] == "lo le"
assert text[ 10 @: -3 ] == "lwlh"These strides can also be used as iterators in lieu of countdown() and countup() if
they do not contain any BackwardsIndexes.
let k1 = collect:
for i in 0 ..< 10 @: 2:
i
assert k1 == @[0, 2, 4, 6, 8]
let k2 = collect:
for i in 20 .. -1 @: -7:
i
assert k2 == @[20, 13, 6, -1]Here's how they relate:
| Nim iterator | Python range() |
Python index | Nim + Strides |
|---|---|---|---|
countdown(a, b, s) |
range(a, b-1, -s) |
xs[a:b-1:-s] |
a .. b @: -s |
countup(a, b, s) |
range(a, b+1, s) |
xs[a:b+1:s] |
a .. b @: s |
Note that Nim convention is to be end-point inclusive1.
There's more to this package than just @:.
The resolved type of StridedSlice (made with @:) is a LinearSegment.
Resolved here means when any BackwardsIndex or StrideIndex has been
translated into actual integers by interpreting them in the context of a length. And a LinearSegment is the finite version of a LinearSequence.
Check out the generated documentation for more.
Footnotes
-
Unfortunately. ↩