How can I match a field in two different files and append together in output file?

Question

I need to match two unequal files using $1 of file 1 and $2 of file 2 and print $1 of file 2 on file 1.

Input file 1

101 2
101 5
101 7
103 2
103 3
103 4
105 3
105 2

Input file 2

24 101
23 103
26 105

Desired output

101 2 24
101 5 24
101 7 24
103 2 23
103 3 23
103 4 23
105 3 26
105 2 26

I have tried the following code but it gave me incorrect output.

awk 'FNR==NR{a[$2]=$0;next};{print a[$2]}' file2 file1

Answer 1

A classic job for join:

join -1 1 -2 2 file1 file2

• -1 1 specifies the field in the first file.
• -2 2 specifies the field in the second file.

Answer 2

Starting from your code, I also post an awk solution because you were almost there:

awk 'FNR==NR { a[$2]=$1; next; } { print $0,a[$1] } ' file2 file1

Output:

101 2 24
101 5 24
101 7 24
103 2 23
103 3 23
103 4 23
105 3 26
105 2 26