Use awk to create index of words from file - shell

Question

I'm learning UNIX for school and I'm supposed to create a command line that takes a text file and generates a dictionary index showing the words (exluding articles and prepositions) and the lines where it appears in the file.
I found a similar problem as mine in: https://unix.stackexchange.com/questions/169159/how-do-i-use-awk-to-create-an-index-of-words-in-file?newreg=a75eebee28fb4a3eadeef5a53c74b9a8 The problem is that when I run the solution
$ awk '
{
gsub(/[^[:alpha:] ]/,"");
for(i=1;i<=NF;i++) {
a[$i] = a[$i] ? a[$i]", "FNR : FNR;
}
}
END {
for (i in a) {
print i": "a[i];
}
}' file | sort
The output contains special characters (which I don't want) like:
-Quiero: 21
Sancho,: 2, 4, 8
How can I remove all the special characters and excluding articles and prepositions?

Answer 1

$ echo This is this test. | # some test text
awk '
BEGIN{
x["a"];x["an"];x["the"];x["on"] # the stop words
OFS=", " # list separator to a
}
{
for(i=1;i<=NF;i++) # list words in a line
if($i in x==0) { # if word is not a stop word
$i=tolower($i) # lowercase it
gsub(/^[^a-z]|[^a-z]$/,"",$i) # remove leading and trailing non-alphabets
a[$i]=a[$i] (a[$i]==""?"":OFS) NR # add record number to list
}
}
END { # after file is processed
for(i in a) # in no particular order
print i ": " a[i] # ... print elements in a
}'
this: 1, 1
test: 1
is: 1