Lesson 33:  Headings, Do Loops, and Sampling

More options using the reset command:

reset autoname;

Autoname gives headings for the rows and columns of matrices that are printed.  Default labels are Row1, Col1, etc.  You can define a vector of headings, such as head={"Mon" "Tue" "Wed"};  Then in a print statement you can put

where rowh is another vector of headings for the rows.

The mattrib command provides a more sophisticated way to assign row and column names.

print x;

This association of the row and column headings will last as long as the iml step runs, so you don't have to keep specifying them in the print statement.

The range assignment like

a=1:10;
b=5:2;

actually creates a vector of integers specified by the range.  We saw that in subscripting we can use variables like a and b to specify the rows and columns to be selected.  However, any vector will work, and the numbers need not be in order.  For example, you could have

a={5, 3, 7, 1};

Using this variable to select rows in a subscript notation would pull the 5th, 3rd, 7th, and 1st rows, in that order.

In addition, once a mattrib statement has been used to define row and column headings, those headings can also be used in the subscript specification instead of the numbers.

Video:  Incorporating Headings into a Matrix

You could even make the row and column headings part of a new matrix.  However, if they are character vectors (as usual), the matrix must be a character matrix.  Look at the following:

Visualize the results or try an example.  The Char function may be used to convert a numeric matrix to character form for this purpose.

Video: Using "do" loops and random sampling

We bring in the usedcars data set for the next example.  If work.usedcars doesn't exist, run a program to load the usedcars3.txt file into a sas data set.  Usedcars has both character and numeric data.

use usedcars;
print UC;

This will bring in only the numeric variables (there are three).

read all var _num_ into UC;

is equivalent, because it specifies numeric variables, which is the default.  You can include this to make your program more readable.

To create a character matrix for the character variables, write:

read all var _char_ into UC2;

Now we'll learn something about do loops.  We begin with the J function to create a column of 1's and the following do loop:

Y=J(34,1);
do i=1 to 34;
Y[i,1]=uniform(0);
end;

Note that the editor turns the "do" red but it is not an error, it is some problem in the editor.  Now do the following:

Yr=rank(Y);
UCsample=UC[Yr[1:10,1],];

Look carefully at what is happening here.  Yr contains the rank values of the random numbers in the Y vector.  I want to choose 10 randomly sampled rows from UC.  I can get the indices from the first 10 entries of Yr.  Remember they don't have to be in order.  So that is what "Yr[1:10,1]" gives.  It is in essence a list of 10 index numbers for rows I want to select.  Now, I use that list or vector to subscript the UC matrix, and take all the rows, assigning the result to UCsample.  Notice that Yr is actually a column vector, but the subscript notation accepts either row or column vectors for arguments.

Now here is an alternative way to get the random numbers.

Y=uniform(0);
do i=1 to 33;
Y=Y/uniform(0);
end;

This works by starting with a random scalar, then vertically concatenating 33 more random numbers to it, building up the Y vector that way.

Exercises:

Copyright reserved by Dr.  Dwight Galster, 2006.  Please request permission for reprints (other than for personal use) from dwight.galster@sdstate.edu  .  "SAS" is a registered trade name of SAS Institute, Cary North Carolina.  All other trade names mentioned are the property of their respective owners.  This document is a work in progress and comments are welcome.  Please send an email if you find it useful or if your site links to it.