Download Proc Sort, Random Number Generators, If

BMTRY 789 Lecture 6: Proc Sort,
Random Number Generators,
and Do Loops
Readings – Chapters 5 & 6
Lab Problem - Brain Teaser
Homework Due – HW 2
Homework for next week - HW 3
(Libname/Cards Quiz) NO CLASS
Random Number Generators
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Proc Sort
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Primarily used to sort the observation of your
data by a certain variable or collection of
variables.
However, it can also be used to create a new
data set, subset your data, rename, drop, or
keep variables, and format or label variables.
An additional very important feature is to
select out duplicate records
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Proc Sort Options
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Almost always a good idea to use the OUT=
option when using proc sort to do anything
except for a simple sort.
Why?
Because Proc Sort automatically writes over
your data set!
Go to SAS user group paper to take a look at
features and examples…
NODUPKEY
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Do-Loops

Do-loops are one of the main tools of
SAS programming. They exist in several
forms, always terminated by an END;
statement
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Do-Loops (cont.)
DO; -groups blocks of statements together
 DO OVER arrayname; -process array elements
 DO VAR=start TO end <by inc>; - range of
numeric values
 DO VAR= list-of-values;
 DO WHILE (expression); (expression evaluated
before loop)
 DO UNTIL (expression); (expression evaluated
after
loop) – guaranteed to be executed
at least once
*Some of these forms can be combined.
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Iterative Do-loop
Do loops can be nested. The following example calculates how long it would
take for an investment with interest compounded monthly to double:
Data interest;
do rate = 4, 4.5, 5, 7, 9, 20;
mrate = rate / 1200; *converts from percentage;
months = 0;
start = 1;
Do While (start <2);
start = start * (1 + mrate);
months = months + 1;
End;
years = months / 12;
output;
End;
Keep rate years;
Run;
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Random Number Functions
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SAS can generate random observations
from discrete and continuous distributions.
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Binomial (n,p) - ranbin(seed,n,p)
Exponential (~=1) - ranexp(seed)
Standard Normal (µ=0; sigmaSquared=1) rannor(seed)
Poisson (mean > 0) - ranpoi(seed, mean)
Uniform (interval (0,1) ) - ranuni(seed)
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Seeds
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A SEED - is a number used by the random
number generator to start the algorithm
They can be any POSITIVE NUMBER or
Zero
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0 seed = a different series of numbers each time
you run the program.
Any positive seed = a repeatable series of
numbers each time you run the program.
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Practical Ex: Randomly Assign
Subject to Study Groups
Data Assign;
Do Subj = 1 to 20;
If RANUNI(123) LE .5 Then Group = 1;
Else Group = 2;
Output;
End;
Run;
Proc Print Data = Assign;
Run;
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Practical Ex: Randomly Assign Subject
to Study Groups (of equal size)
Data Random;
Do Subj = 1 to 20;
Group = RANUNI(0) ;
Output;
End;
Run;
Proc Rank Data = Random Groups=2 Out=Split;
Var Group;
Run;
Proc Print Data = Split Noobs;
Run;
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Here is your brain teaser, inclass assignment…
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Practical Do-Loop Example
You have a SAS data set DIET which contains variables ID, DATE, and
WEIGHT. There are multiple records per ID, and the records are
sorted by DATE within ID. The task is to create a new SAS data set
DIET2 from DIET which contains only one record per subject, with
each record containing the subject ID and the mean weight. (Could
use Proc Means here but for the purpose of this exercise, we want to
perform this task within the Data Step).
Hints: Include a By ID statement after a SET statement in the DATA step,
and then use First. and Last. variables.
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Example Data
Data Set DIET
ID DATE
1 10/01/92
1 10/08/92
1 10/15/92
2 09/02/92
2 09/09/92
2 09/16/92
2 09/23/92
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WEIGHT
155
158
158
200
198
196
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