转换前1
ctr year A B C
A 1995 0 1 2
B 1995 -1 0 3
C 1995 -2 -3 0
A 1996 0 4 5
B 1996 -4 0 6
C 1996 -5 -6 0
A 1997 0 7 8
B 1997 -7 0 9
C 1997 -8 -9 0
转换后1
ctr year 双边贸易额
AB 1995 1
AB 1996 4
AB 1997 7
AC 1995 2
AC 1996 5
AC 1997 8
BA 1995 -1
BA 1996 -4
BA 1997 -7
BC 1995 3
BC 1996 6
BC 1997 9
CA 1995 -2
CA 1996 -5
CA 1997 -8
CB 1995 -3
CB 1996 -6
CB 1997 -9