AWK Arrays

awk has arrays, but they are only indexed by strings. This can be very useful, but it can also be annoying. For example, we can count the frequency of words in a document (ignoring the icky part about printing them out):

$ awk '{for(i=1;i <=NF;i++) freq[$i]++ }' filename 

The array will hold an integer value for each word that occurred in the file. Unfortunately, this treats "foo", "Foo", and "foo," as different words. Oh well. How do we print out these frequencies? awk has a special "for" construct that loops over the values in an array. This script is longer than most command lines, so it will be expressed as an executable script:

#!/usr/bin/awk -f
{for(i=1;i <=NF;i++) freq[$i]++ }
END{for(word in freq) print word, freq[word]  
}