Extracting combined differences based on a single column

[A-V]

Dear All,
I have two sets of files. File 1 can be any number between 1 and 20 followed by a frequency of that number in a give documents... the lines in the file will be dependent to the analysed document. e.g.
file1

1,5
4,1

then I have file two which is basicall same numbers but with frequency of 0
file2

1,0
2,0
3,0
4,0
...
20,0

I want to create a third file which will map the file1 on file2
file3 - outcome

1,5
2,0
3,0
4,1
...
20,0

I have tried

awk -F, 'FNR==NR{X[$1]=$2;next}$1 in X{print $1","X[$1]==$2;next}{print $1","0}' file1 file2

but it does not print what is required and puts 0 for any item from file1
Can you please help?

[blackrageous]

This looks like you're just combining both files and sorting them.
So if file x.x is one of the files and y.y the other, then,...

cat x.x y.y | sort

Please clarify.

[A-V]

Dear Blackrageous, No, I am not combining. I want to fill the gaps in file1 by the data from the file2
my file2 is always the same and has 20 lines while file1 might have any combination. I want to make sure file1 has 20 lines as well and the values that are missing will be extracted from file2