Array syntax with AWK

Question

I am baffled at the response of these scripts.

Trial 1:

awk '{line_arr[$0] } END {for (line in line_arr) print line}' text_file

Output:

Trial 2:

awk '{line_arr[$0] } END {for (line in line_arr) print line}' text_file

Output:

is some text.
Hello. Here
Lots of random
text.

File content:

$ cat text_file
Hello. Here
is some text.
Lots of random
text.

Why is the index of the array storing as an actual value?

Why is the second trial out of order?

I am not sure why I would put line as the index. line is just a iteration of array from the for loop. Someone else mentioned this to me and I tried it. It was just printing 4 blank lines, nothing else. I am confused by the array sytax mostly. The array order was unkown to me. Thanks. — Michael Bruce
– Michael Bruce, Commented Aug 29, 2016 at 4:03
What is your output in the first case? Is it output line number or the line content? — cuonglm
– cuonglm, Commented Aug 29, 2016 at 4:11
It is line number. On another file it is also a climbing pattern. — Michael Bruce
– Michael Bruce, Commented Aug 29, 2016 at 4:14

J. Chomel · Accepted Answer · 2016-08-29 06:33:30Z

3

With this rare syntax, awk stores your lines in an Associative Array. Thus it has no defined order. You reached an undefined behavior.

Here is how to get convinced: print the line number from the associative array itself:

$ awk '{line_arr[$0]=NR } END \
       {for (line in line_arr) print line_arr[line]"=>"line } \
      ' text_file

3=>Lots of random
1=>Hello. Here
4=>text.
2=>is some text.

answered Aug 29, 2016 at 6:33

J. Chomel

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