Reading from input with double trailing ## - input

Question

Input:
G0894 x 1 x 3 x 1 k 1
C4458 x 1 k 5
C9057 x 7 x 4 x 4 x 3 x 5
Desired output:
G0894 x 1
G0894 x 3
G0894 x 1
G0894 k 1
C4458 x 1
C4458 k 5
C9057 x 7
C9057 x 4
C9057 x 4
C9057 x 3
C9057 x 5
This is what I came up with:
data want;
infile cards missover;
input id $ #;
do while (1);
input letter $ number #;
if letter EQ ' ' then leave;
output;
end;
cards;
G0894 x 1 x 3 x 1 k 1
C4458 x 1 k 5
C9057 x 7 x 4 x 4 x 3 x 5
;
run;
And it does work but since we've been talking about double trailing ## in class I think I'm supposed to use it. This was my other approach:
data want;
infile cards missover;
input id $ #;
input letter $ number ##;
cards;
G0894 x 1 x 3 x 1 k 1
C4458 x 1 k 5
C9057 x 7 x 4 x 4 x 3 x 5
;
run;
And it generates an error which says something about using missover and ## in an inconsistent manner. What am I doing wrong?

Answer 1

There is no way in your program for the data step to ever advance to the second row of input data. That is what the error message is telling you.
The ## tells SAS that it should keep the line pointer and column pointer the same when it starts the next data step iteration. The MISSOVER option tells SAS not to go to a new line when it cannot find data to meet the current input request. Hence there is no way for the line pointer to ever advance to line two.

Answer 2

The double trailing at sign (##) holds a record across multiple iterations of the DATA step until the end of the record is reached. However, the single trailing at sign (#) releases a record when control returns to the top of the DATA step.
Try this:
data want;
infile cards missover;
input id $ letter $ number #;
do while (letter ne '' or number ne .);
output;
input letter $ number #;
end;
cards;
G0894 x 1 x 3 x 1 k 1
C4458 x 1 k 5
C9057 x 7 x 4 x 4 x 3 x 5
;
run;