Use SAS do loop and array to create a dataset

[stat34986]

I am trying to create a SAS dataset that looks like the following 

I am attempting to use array and a do loop to accomplish this via the following code

DATA dat;
    array new(20) X1-X20;
    Do i = 1 to 20;
    	new[i] = new[i] + 1;
    	output;
    end;
RUN;

This code gets me close to the desired dataframe. That is, the position of the missing values are correct however it populates the dataset with all 1s (see below for output). I am not sure what I am doing wrong. I don't understand why new[i[ = new[i] + 1 doesn't increment. 

 

[Patrick · Accepted solution]

Below should return what you're after.

data dat;
  array new(20) x1-x20;
  do i = 1 to dim(new);
    do k=1 to i;
      new[k]=sum(new[k],1);
    end;
    output;
  end;
run;

[Ksharp]

/*It is IML thing.*/
proc iml;
have=t(1:20);
want=lag(have,0:19);
create want from want[c=('x1':'x20')];
append from want;
close;
quit;