从unix命令行连接两个文件的最简单方法，为缺少的键插入零条目

Question

I'm trying to join two files each of which contains rows of the form <key> <count> . Each file contains a few lines that are missing from the other, and I would like to have zero inserted for all such values rather than omitting these lines (I've seen -a, but this isn't quite what I'm looking for). Is there a simple way to accomplish this?

Here is some sample input:

a.txt

apple 5
banana 7

b.txt

apple 6
cherry 4

expected output:

apple 5 6
banana 7 0
cherry 0 4

Answer 1

join -o 0,1.2,2.2 -e 0 -a1 -a2 a.txt b.txt

• -o 0,1.2,2.2 → output join field, then 2nd field of 1st file, then 2nd field of 2nd file.
• -e 0 → Output 0 on empty input fields.
• -a1 -a2 → Show all values from file 1 and file 2.

Answer 2

Write a script, whatever language you want. You will parse both files using a map/hashtable/dictionary data structure (lets just say dictionary). Each dictionary will have the first word as the key and the count (or even a string of counts) as the value. Here is some pseudocode of the algorithm:

Dict fileA, fileB; //Already parsed
while(!fileA.isEmpty()) {
      string check = fileA.top().key();
      int val1 = fileA.top().value();
      if(fileB.contains(check)) {
          printToFile(check + " " + val1 + " " + fileB.getValue(check));
          fileB.remove(check);
      }
      else {
          printToFile(check + " " + val1 + " 0");
      }
      fileA.pop();
}
while(!fileB.isEmpty()) {      //Know key does not exist in FileA
     string check = fileB.top().key();
     int val1 = fileB.top().value();
     printToFile(check + " 0 " + val1);
     fileB.pop();
}

You can use any type of iterator to go through the data structure instead of pop and top. Obviously you may need to access the data a different way depending on what language/data structure you need to use.

Answer 3

@ninjalj's answer is much saner, but here's a shell script implementation just for fun:

exec 8< a.txt
exec 9< b.txt

while true; do
   if [ -z "$k1" ]; then
    read k1 v1 <& 8
   fi
   if [ -z "$k2" ]; then
    read k2 v2 <& 9
   fi
   if [ -z "$k1$k2" ]; then break; fi
   if [ "$k1" == "$k2" ]; then
    echo $k1 $v1 $v2 
    k1=
    k2=
   elif [ -n "$k1" -a "$k1" '<' "$k2" ]; then
    echo $k1 $v1 0 
    k1=
   else 
    echo $k2 0 $v2
    k2=
   fi
done