Unknown Errors with Proc Transpose - sas

Question

Trying to utilize proc transpose to a dataset of the form:
ID_Variable Target_Variable String_Variable_1 ... String_Variable_100
1 0 The End
2 0 Don't Stop
to the form:
ID_Variable Target_Variable String_Variable
1 0 The
. . .
. . .
1 0 End
2 0 Don't
. . .
. . .
2 0 Stop
However, when I run the code:
proc transpose data=input_data out=output_data;
by ID_Variable Target_Variable;
var String_Variable_1-String_Variable_100;
run;
The change in file size from input to output balloons from 33.6GB to over 14TB, and instead of the output described above we have that output with many additional completely null string variables (41 of them). There are no other columns on the input dataset so I'm unsure why the resulting output occurs. I already have a work around using macros to create my own proxy transposing procedure, but any information around why the situation above is being encountered would be extremely appreciated.

Answer 1

In addition to the suggestion of compression (which is nearly always a good one when dealing with even medium sized datasets!), I'll make a suggestion for a simple solution without PROC TRANSPOSE, and hazard a few guesses as to what's going on.
First off, wide-to-narrow transpose is usually just as easy in a data step, and sometimes can be faster (not always). You don't need a macro to do it, unless you really like typing ampersands and percent signs, in which case feel free.
data want;
set have;
array transvars string_Variable_1-string_Variable_100;
do _t = 1 to dim(transvars);
string_variable = transvars[_t];
if not missing(String_variable) then output; *unless you want the missing ones;
end;
keep id_variable target_variable string_Variable;
run;
Nice short code, and if you want you can throw in a call to vname to get the name of the transposed variable (or not). PROC TRANSPOSE is shorter, but this is short enough that I often just use it instead.
Second, my guess. 41 extra string variables tells me that you very likely have some duplicates by your BY group. If PROC TRANSPOSE sees duplicates, it will create that many columns. For EVERY ROW, since that's how columns work. It will look like they're empty, and who knows, maybe they are empty - but SAS still transposes empty things if it sees them.
To verify this, run a PROC SORT NODUPKEY before the transpose. If that doesn't delete at least 40 rows (maybe blank rows - if this data originated from excel or something I wouldn't be shocked to learn you had 41 blank rows at the end) I'll be surprised. If it doesn't fix it, and you don't like the datastep solution, then you'll need to provide a reproducible example (ie, provide some data that has a similar expansion of variables).

Answer 2

Without seeing a working example, it's hard to say exactly what's going on here with regards to the extra variables generated by proc transpose.
However, I can see three things that might be contributing towards the increased file size after transposing:
If you have option compress = no; set, proc transpose creates an uncompressed dataset by default. Also, if some of your character variables are different lengths, they will all be transposed into one variable with the longest length of any of them, further increasing the file size if compression is disabled in the output dataset.
I suspect that some of the increase in file size may be coming from the automatic _NAME_ column generated by proc transpose, which contains an extra ~100 * max_var_name_length bytes for every ID-target combination in the input dataset.
If you are using option compress = BINARY; (i.e. compressing all output datasets that way by default), the SAS compression algorithm may be less effective after transposing. This is because SAS only compresses one record at a time, and this type of compression is much less effective with shorter records. There isn't much you can do about this, unfortunately.
Here's an example of how you can avoid both of these potential issues.
/*Start with a compressed dataset*/
data have(compress = binary);
length String_variable_1 $ 10 String_variable_2 $20; /*These are transposed into 1 var with length 20*/
input ID_Variable Target_Variable String_Variable_1 $ String_Variable_2 $;
cards;
1 0 The End
2 0 Don't Stop
;
run;
/*By default, proc transpose creates an uncompressed output dataset*/
proc transpose data = have out = want_default prefix = string_variable;
by ID_variable Target_variable;
var String_Variable_1 String_Variable_2;
run;
/*Transposing with compression enabled and without the _NAME_ column*/
proc transpose data = have out = want(drop = _NAME_ compress = binary) prefix = string_variable;
by ID_variable Target_variable;
var String_Variable_1 String_Variable_2;
run;