Using awk to get next fields

Question

I have this input file (space is the separator for the two elements in the line otherwise there is just one element)

a:1
a:1 123
b:2 345 
c:3 456 
d:4 
d:4 456
..
..

I am interested in the output to be

a:1 123
d:4 456

i.e lines which have the preceding field to have just one field.

Answer 1

Try this:

 { if (NF == 1) { getline; print; next; } }