Using awk to extract data and count - linux

Question

How do I use awk on a file that looks like this:
abcd Z
efdg Z
aqbs F
edf F
aasd A
I want to extract the number of times each letter of the alphabet occurs in the second column, so output should be:
Z 2
F 2
A 1

Answer 1

try: If you want the order of output same as Input_file then following may help you.
awk 'FNR==NR{A[$2]++;next} A[$2]{print $2,A[$2];delete A[$2]}' Input_file Input_file
if you don't bother of order of $2 then following may help you.
awk '{A[$2]++} END{for(i in A){print i,A[i]}}' Input_file
In first solution reading the Input_file twice and creating an array A whose index is $2 with it's incrementing value. then when second Input_file is being read then printing the $2 and it's count.
In Second solution creating an array A whose index $2 and incrementing value of it. Then in end section go through the array A and print it's index and array A's value.

Answer 2

I would use sort | uniq for this purpose as these two utils are designed specifically for this kind of task:
cat <<END |
abcd Z
efdg Z
aqbs F
edf F
aasd A
END
awk '{print $2}' | sort -r | uniq -c | awk '{printf "%s %d\n", $2, $1}'
Would produce exactly the desired output
Z 2
F 2
A 1
Here awk '{print $2}' is used to get the second column from a document with fields separated by one or more whitespace characters. If we knew the width of the columns is fixed, we could use a faster cut utility instead.
sort -r | uniq -c is doing the main algorithmic part of the task - sort the letters in reverse order and count the number of occurrences of each letter.
awk '{printf "%s %d\n", $2, $1}' does some reformatting of the uniq -c output to match the required format exactly.
Update: AWK has powerful array support so this can be done with awk alone:
cat <<END |
abcd Z
efdg Z
aqbs F
edf F
aasd A
END
awk '{a[$2]++}
END {n=asorti(a,b,"#ind_str_desc");
for (k=1;k<=n;k++) {printf b[k], a[b[k]]} }'
We use the array a that is indexed with letters found in the input stream, and on each line the element indexed by the corresponding letter gets incremented.
In the END clause we reverse the order of indices and output the array.