what is the difference between dsd and dlm="," in SAS? - sas

Question

Let me sum up what I have got from this website. https://communities.sas.com/t5/General-SAS-Programming/Please-explain-DSD-and-DLM-differences/td-p/146773
(1): Without dsd, the cursor passes all the delimiters before reading the next field, while on the other hand, with dsd, the cursor only pass one delimiter.
(2): If you use dsd, the informat should use a colon somehow?
Do you know any differences between the two? Many thanks for your time and attention.

Answer 1

The most obvious difference is how DSD treats consecutive delimiters. From the docs:
When you specify DSD, SAS treats two consecutive delimiters as a
missing value and removes quotation marks from character values.
Whereas the default functionality of DLM=',' is to treat consecutive commas as a single comma, DSD will assign missing values between consecutive commas. Here's an example:
data work.dlm_test;
infile datalines dlm=','; /* using dlm */
input var1 var2 var3;
datalines; /* note how the consecutive commas are treated! */
1,2,3
1,,3
,2,3
;
data work.dsd_test;
infile datalines dsd; /* using dsd */
input var1 var2 var3;
datalines;
1,2,3
1,,3
,2,3
;
proc print data=dlm_test;
/* this will print something like:
OBS | var1 | var2 | var3
-----+------+------+------ Note only 2 observations b/c of
1 | 1 | 2 | 3 default FLOWOVER functionality.
2 | 1 | 3 | 2 <--- Also, final '3' is ignored because there
is no variable to store it.
*/
run;
proc print data=dsd_test;
/* this will print something like:
OBS | var1 | var2 | var3
-----+------+------+------
1 | 1 | 2 | 3
2 | 1 | . | 3 <-- note the missing value in var2
3 | . | 2 | 3 <-- note the third observation, with missing val
*/
run;
Also, DSD will be able to tell that a comma found inside quotation marks is actually not a delimiter, but part of a character string. In contrast, if you use only DLM=',', then it will ignore quotation marks and treat every comma-cluster as a delimiter.
TIP: By default, DSD drops the quotes around character strings, but you can keep the quotes by using the ~format identifier in the INPUT statement.
It is useful to note that DSD and DLM can also be used together to get the behavior of DSD, but change the default delmiiter from a comma to something else, like a semicolon (;). Example:
infile (filename) dsd dlm=';';
I found this documentation page to be the most instructive.
Remember: DSD stands for "delimiter-sensitive data" because it is more deliberate about processing delimiters!

Answer 2

The real issue how the input statement behaves when it sees a delimiter when it starts to read a variable. With the DSD option it will set the value to missing and move the pointer past the delimiter. Without the DSD option it will skip over the delimiter (or multiple adjacent delimiters) before reading the value. You can confirm this by reading a line that starts with a delimiter.
The colon modifier helps when the actual value is shorter than the informat's width, but it also helps by moving the pointer PAST the delimiter so that the NEXT variable is read correctly. This is what makes it important when using formatted input statements with the DSD infile option.
You can avoid the need to worry about the : modifiers by using an INFORMAT statement instead of listing the informats in the input statement.