Breakdown of one line of code involving awk - awk

Question

I am currently working on a project which involves comparing data from two different files. I had been looking for a command which would compare one line of file1 with each line of file2, and print out a '1' if there is a match and a '0' if not matched. Then, it would repeat this command for the second line of file1 for each line of file1.
I found this bit of code online which seems to work for my project, but I was hoping someone would help to break it down for me and provide more of an explanation.
awk 'FNR==NR{a[$1]; next} {print $1, ($1 in a) ? "1":"0"}' file1.txt file2.txt
Additionally, I'm new to this so any resources which may guide me to my answer would be really helpful. Thank you.

Answer 1

Here's what this awk is saying:
awk 'FNR==NR{a[$1]; next} {print $1, ($1 in a) ? "1":"0"}' file1.txt file2.txt
IF the record number of this specific file being processed FNR is the same as the overall record number being processed NR execute {a[$1]; next} block. We can safely assume that if this condition is true that we are processing the first file.
{a[$1]; next} Add the first column as a key in the array named a and then go to the next line without processing anymore of the awk script. Once the first file is fully processed here we will have an array with a key for every distinct value found in the first column of the first file.
{print $1, ($1 in a) ? "1":"0"} Since we must now be on the second file we are printing every line/record we encounter. Here we print the first column, then if that column's value is in the array as a key, then we print 1 otherwise we print 0.
In short this is printing every first column of the second file and stating if that column also exists in the first file printing a 1 or 0.

Answer 2

Repeated here for locality of reference, and in case the question gets edited:
awk 'FNR==NR{a[$1]; next} {print $1, ($1 in a) ? "1":"0"}' file1.txt file2.txt
You should really read a basic awk primer first. Basically the clause FNR==NR is a common idiom to check if we're reading the first file. NR is the overall record number (the line number), while FNR is the record number in the current file, so you're still processing the first file when these are equal. The action then stores the first column (not the entire line) into an array. So the first thing this program does is read the first column of the first file into the array named a. Then it starts reading the second file, and prints the first column of each line, followed by "1" or "0" depending on if the value in the first column is in the array.