Nested do loop in sas

[SAS_inquisitive]

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;

    end;

run;

[Reeza]

No. It's repeated 3 times 

[KachiM]

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

[Reeza]

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;

[SAS_inquisitive]

Reeza, I mean the entire inner do loop block.

[LinusH]

Both @Reeza...?

[Reeza]

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

[Cynthia_sas · Accepted solution]

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

[Astounding]

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;

end;

end;

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

[Cynthia_sas]

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

[SAS_inquisitive]

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