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I used this snippet of code to start a new column after every 20th row and each of the columns is separated by tabs. I took the code from this post and then tweaked it a bit: How to start a new column after every nth row?

awk '{a[NR%20] = a[NR%20] (NR<=20 ? "" : "\t") $0} END {for (i = 1; i <= 20; i++) print a[i%20]}' 

It does exactly what I want it to do. However, I don't really understand how it works. Can someone please explain it to me? I know that $0 will read in the entire record (line) of a file, and that the condition before the question mark is evaluated, and if true, the first statement is executed, and if false, the second is executed. So in this case, if NR<=20 then nothing is printed because we're on the first column, but if NR>20 then a tab is printed to start a new column. I also know that the for loop prints out the elements of an array, starting from a[1%20] which is a[1] and so on, to a[19%20] which is a[19], and finally a[20%20] which is a[0]. But what does a[NR%20] = a[NR%20] do? Why is it set equal to itself? I see that when I omit a[NR%20] = a[NR%20], 20 blank lines are printed out.

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In awk, expressions that are separated by spaces get joined together. This concatenation is described in the POSIX awk manual in a table of expressions (the formatting on that page isn't very clear, it's easier to read via man 1p awk). a[NR%20] is being joined together with its current value + ""/"\t" + the current record. For the first twenty records, both the array value and the ternary ?: expression will be empty strings. Brackets might make it more clear:

a[NR%n] = (a[NR%n] (NR<=n ? "" : "\t") $0)
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  • Ah ok, makes sense now. Thanks. – rplee Jun 3 at 14:55
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(NR<=20 ? "" : "\t") is modifying the second a[20%20], so it is not set equal to itself but gets appended with a tabulation if NR > 20.

Then the END script (the part inside the right set of braces) prints the result.

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