topic Re: Nested do loop in sas in SAS Programming

Nested do loop in sas

SAS_inquisitive — Mon, 01 Aug 2016 22:57:04 GMT

Hello,

I am trying to understand the nested do loop concept. In this example, can we say inner do loop is repeated 2 times ?

data test;

do i = 1 to 2;

do j = 1 to 3;

output;

end;

run;

Re: Nested do loop in sas

Reeza — Mon, 01 Aug 2016 23:04:30 GMT

No. It's repeated 3 times

Re: Nested do loop in sas

KachiM — Mon, 01 Aug 2016 23:05:33 GMT

The inner loop repeats 3 times and the outer 2 times, making 6 iterations in all.

Re: Nested do loop in sas

Reeza — Mon, 01 Aug 2016 23:18:28 GMT

Also, inner/outer aren't always clear. Is J an inner or outer loop?

data test;
    do i = 1 to 2;
       do j = 1 to 3;
           do k=1 to 5;
                output;
            end;
       end;
    end;
run;

Re: Nested do loop in sas

LinusH — Mon, 01 Aug 2016 23:23:25 GMT

Both @Reeza...?

Re: Nested do loop in sas

Reeza — Mon, 01 Aug 2016 23:32:44 GMT

@LinusH No idea, it probably depends on context. Just that as terminology goes, it's not definitive.

Re: Nested do loop in sas

Cynthia_sas — Tue, 02 Aug 2016 00:31:46 GMT

Hi:

I agree, it depends on context and how you write the code. I think this comparison of the original do loop example and a modified version with i, j, and k loops in a different position explains how the iterations work -- 30 observations each time (because 2*3*5 or 5*2*3 or 3*2*5 will always equal 30), but the values fall into a different pattern.

cynthia

Re: Nested do loop in sas

Astounding — Tue, 02 Aug 2016 01:22:12 GMT

One consideration is worth noting for efficiency purposes ... even though the inner loop executes 30 times in both cases, the total number of loops is different. The left-hand program is the most efficient order and executes a total of 38 loops (2 outer, 6 middle, 30 inner), while the right-hand program executes a total of 45 loops (5 outer, 10 middle, 30 inner). The least efficient order would put the fastest-moving variables on the outside:

do k=1 to 5;

do j=1 to 3;

do i=1 to 2;

end;

Now there are a total of 50 loops (5 outer, 15 middle, 30 inner).

Re: Nested do loop in sas

Cynthia_sas — Tue, 02 Aug 2016 03:13:47 GMT

Hi:
Most of the time, I use nested DO loops for data generation. If I am using the DO loops to generate some data for testing, then my primary consideration would be to use the looping syntax that produces the structure I need. Otherwise, I generally find the data structure of data I'm reading will dictate the form of the nested loops.

cynthia

Re: Nested do loop in sas

SAS_inquisitive — Tue, 02 Aug 2016 15:30:16 GMT

Reeza, I mean the entire inner do loop block.

Re: Nested do loop in sas

SAS_inquisitive — Tue, 02 Aug 2016 15:32:02 GMT

Cynthia_sas, can you give an example of later case? Thanks !