# What am I doing wrong in this array comprehension?
**URL:** https://discourse.julialang.org/t/what-am-i-doing-wrong-in-this-array-comprehension/62016
**Category:** General Usage
**Tags:** question, array
**Created:** [May 28, 2021, 8:36pm UTC](https://discourse.julialang.org/t/what-am-i-doing-wrong-in-this-array-comprehension/62016 "2021-05-28T20:36:40Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@stephenjfox](https://discourse.julialang.org/u/stephenjfox)
#### Post date: [May 28, 2021, 8:36pm UTC](https://discourse.julialang.org/t/what-am-i-doing-wrong-in-this-array-comprehension/62016/1 "2021-05-28T20:36:40Z")
In the midst of writing some ring arithmetic code, I found my Julia session producing very strange results for an element-wise modulo operation.
```julia
v = collect(1:20)
[(i, n) for i=1:20, n=(v .^2 .% 17) if i == n] # produces 19 tuples of (n, n)
"""
test_that_above_is_incorrect()
Show that the tuple isn't incrementing the index, or is throwing out the
value assigned to `i`. Whatever it is doing, applying the same logic in
a loop shows the only found element is unity.
"""
function test_that_above_is_incorrect()
out = []
for (counter, n) in enumerate(1:20)
if (n ^2) % 17 == counter
push!(out, n)
end
end
out # outputs a vector of a single element
end
```
I know the logical equivalent with `enumerate` work correctly i.e.
```julia
[(i,n) for (i,n) in enumerate(v) if i == ((n ^ 2) % 17)]
```
is just fine.
So where did I go astray in the initial comprehension?
Thanks in advance
---
### Author:  [@contradict](https://discourse.julialang.org/u/contradict)
#### Post date: [May 28, 2021, 9:44pm UTC](https://discourse.julialang.org/t/what-am-i-doing-wrong-in-this-array-comprehension/62016/2 "2021-05-28T21:44:42Z")
The comprehension generates the outer product of the two iterators.
```julia
julia> [(i,j) for i=1:3, j=1:3]
3×3 Matrix{Tuple{Int64, Int64}}:
(1, 1) (1, 2) (1, 3)
(2, 1) (2, 2) (2, 3)
(3, 1) (3, 2) (3, 3)
```
---
### Author:  [@rafael.guerra](https://discourse.julialang.org/u/rafael.guerra)
#### Post date: [May 28, 2021, 9:57pm UTC](https://discourse.julialang.org/t/what-am-i-doing-wrong-in-this-array-comprehension/62016/3 "2021-05-28T21:57:24Z")
For equivalence with enumerate you should zip (or use `pairs`):
`[(i, n) for (i,n) in zip(1:20,v) if i==(n^2)%17]`
what you have is a double loop instead.
---
### Author:  [@stephenjfox](https://discourse.julialang.org/u/stephenjfox)
#### Post date: [May 28, 2021, 9:58pm UTC](https://discourse.julialang.org/t/what-am-i-doing-wrong-in-this-array-comprehension/62016/4 "2021-05-28T21:58:34Z")
That’s interesting. In additional testing, it seems the `if` just selects a column of that outer product. Thanks for clarifying
---
### Author:  [@stephenjfox](https://discourse.julialang.org/u/stephenjfox)
#### Post date: [May 28, 2021, 10:01pm UTC](https://discourse.julialang.org/t/what-am-i-doing-wrong-in-this-array-comprehension/62016/5 "2021-05-28T22:01:41Z")
I was under the impression the double loop was a bit more explicit, a la
```julia
[(i, n) for i in 1:20 for n in (v .^ 2 % 17) if i==n]
```
---
### Author:  [@cjdoris](https://discourse.julialang.org/u/cjdoris)
#### Post date: [May 30, 2021, 9:06pm UTC](https://discourse.julialang.org/t/what-am-i-doing-wrong-in-this-array-comprehension/62016/6 "2021-05-30T21:06:37Z")
You get a product iterator either way, but using commas will get you a matrix (or higher dim tensor with more components).