Modifying one line of an array
I just started learning J and there is something I don’t know how to work correctly
Suppose I want to print a 2 character checkerboard like
baba
abab
baba
To do this, I assumed that you can simply create an array
baba
baba
baba
and flip the second line.
Creating an array of very simple 3 4 $ 'ba'
. But reversing the second line is where I am struggling.
I can get a back reference to the second line executing |. 1 { 3 4 $ 'ba'
, but that only gives me the second line, not the whole array. I can't see how to use the J syntax, I can actually keep the top and bottom line and only apply |.
to the middle line.
More generally, how would you apply to |.
just every other line?
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What did you ask
To apply |.
to a single line, try something like:
x =: 3 4 $ 'ba'
(|. 1{x) 1} x
baba
abab
baba
To undo all other lines, I don't know if there is something simpler:
,/ 1 (]`(|."1))\ i. 5 4
0 1 2 3
7 6 5 4
8 9 10 11
15 14 13 12
16 17 18 19
This makes use of a relatively obscure feature in the dyad \ (Infix)
:
x m\ y
applies sequential gerund verbsm
to infixesy
, expanding cyclicallym
as needed.
Here x
is 1, so our infix is just 1 × 4 matrices; we loop the gerund ( ] ` (|."1)
) to alternate doing nothing ( ]
) and changing one row of the submatrix ( |."1
). We then flatten the resulting 5 × 1 × 4 array back to a 5 × 4 matrix with ,/
.
Instead, you might want
An easier way to achieve a "checkerboard" is as follows: first, use +/
on two ranges to create a "padding table", for example:
(i.3) +/ (i.4)
0 1 2 3
1 2 3 4
2 3 4 5
Then take all of these mod 2 values to get a checkerboard pattern of 0s and 1s:
2 | (i.3) +/ (i.4)
0 1 0 1
1 0 1 0
0 1 0 1
Then index from the select string with {
:
(2 | (i.3) +/ (i.4)) { 'ba'
baba
abab
baba
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Method 1: change }
Replace the second line with the modified line:
( 4 $ 'ab') (1 }) m =: 3 4 $ 'ba'
or in general, replace with a pattern a =: 4 $ 'ab'
for indices i =: +:i.5
:
a i } 10 4 $ 'ba'
Path 2: Gerund Cycling and Cutting ;.
You can cycle verbs by binding them to `. For every other line (rank "1
) you want to either do nothing ]
or reverse |.
:
(]"1)`(|."1) ;.1 m
Method 3: using a different template
You can see your template as 4 $ 'ba'
, followed by its inverse:
3 $ (,:|.) 4 $ 'ba'
By the way,
odd sized (3) even patterned ('ba') lets you simplify |: 4 3 $ 'ba'
.
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Well, you already have many answers, but none of them were the first that popped into my head, so I'll add the following:
0 1 0 |."(0 1) 3 4$'ba'
This takes advantage of the fact that rotating the middle row by 1 looks the same as flipping. You can generalize this by calculating the longest list of 0s and 1s as needed based on the number of lines in your chessboard.
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The approach I would try would not require reversing the rows of the array, but it works by rephrasing the situation in a friendly way.
I would add a column to the array to have columns of an odd number (5) with an even number of elements ('ba'), then remove the last element in each row.
4 5$'ba'
babab
ababa
babab
ababa
}:"1 (4 5$'ba')
baba
abab
baba
abab
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