Pfthb

I meet a awk array issue, details as below:

[~/temp]$ cat test.txt
1
2
3
4
1
2
3

Then I want to count the frequency of the number.

[~/temp]$ awk 'num[$1]++;ENDfor (i in num)"sort -r -n -k1" ' test.txt
1
2
3
2 3
2 2
2 1
1 4

As you see, why does the output of first 3 line '1 2 3' will come blank value?

Thank for your answer.

An awk statement consists of a pattern and related action. Omitted pattern matches every record of input. Omitted action is an alias to print $0, ie. output the current record, which is what you are getting. Looking at the first part of your program:

$ awk 'num[$1]++' file
1
2
3

Let's change that a bit to understand what happens there:

$ awk 'print "NR:",NR,"num["$1"]++:",num[$1]++' file
NR: 1 num[1]++: 0
NR: 2 num[2]++: 0
NR: 3 num[3]++: 0
NR: 4 num[4]++: 0
NR: 5 num[1]++: 1
NR: 6 num[2]++: 1
NR: 7 num[3]++: 1

Since you are using postfix operator num[$1]++ in the pattern, on records 1-4 it gets evaluated to 0 before it's value is incremented. Output would be different if you used the prefix operator ++num[$1] which would first increment the value of the variable after which it would get evaluated and would lead to outputing every record of input, not just the last three, which you were getting.

Correct way would've been to use num[$1]++ as an action, not as a pattern:

$ awk 'num[$1]++' file

Put your "per line" part in i.e. num[$1]++;

awk programs a a collection of [pattern] actions (the pattern is optional, the is not). Seems that in your case your line is being treated as the pattern.