In Python, if I were to make a function which would return a list whose length varies depending on the input, if I couldn’t fit the code into a list comprehension, I’d usually use a yield statement to yield each array element I want in the output. Is there a recommended pattern for doing this in Julia?

One could always create an empty list and then append each wanted entry to that list but I wonder if there’s a better way to do that? Speed is a very crucial factor here as this function will need to be applied to every element in a very large matrix.

In case anyone needs an example of what I actually mean, I’m trying to convert something similar to this function from Python to Julia:

```
def hop(x, n):
for i in range(n-1):
mask = 3 << i
if x & mask != 0 and x & mask != mask:
yield x ^ mask
mask = (1 << (n-1)) | 1
if x & mask != 0 and x & mask != mask:
yield x ^ mask
```

What’s going on here is that you give this function two integers `x`

and `n`

, where you care about the binary pattern of x mod n. Ie. if I am concerned with the binary pattern `1010`

then `x`

should be `10`

in base `10`

and `n`

should be `4`

because I’m treating `x`

as a `4`

bit integer.

The function is making a mask, initially 3, ie. `0011`

in base `2`

and its checking checking if `x`

shares exactly one `1`

bit with `mask`

and if so, will yield an integer whose binary pattern is the same those of `x`

in the spots where `mask`

has zeros and will yield the complement of `x`

’s digits in the positions where `mask`

has `1`

s. Finally, it does the same thing with a `mask`

that has `1`

s at both ends of the digit and `0`

s everywhere else.

From the amount of control structure here, I don’t think this is a good candidate for list comprehensions but I also suspect that creating an empty list and appending to it isn’t ideal.

Any ideas?