Module 7: Do Loops and Arrays

Question 1

What does an array do?

Answer 1

Provides a way to reference a group of columns for processing in the DATA setp. It groups columns together to processes repetitive calculations efficiently

Question 2

What is an element in an array?

Answer 2

How columns are referred. Each specific column is a different element, which makes it easier to reference

Question 3

What are the two steps needed to create an array?

Answer 3

1) Define the array.
ARRAY array-name [number-of-elements] <array-elements>;
2) Reference the array specifying the column that is desired.

Ex. of defined array: array scores[12] game01 – game12;

Question 4

T/F: When you define an array in one dataset, you can also use it in another.

Answer 4

False: Arrays can only be accessed in the dataset they are defined. Need to redefine in a seperate data set.

Question 5

How do you handle an unknown number of array elements?

Answer 5

Use an asterisk within the brackets when defining an array.

Ex. array name[*] var1 – varn

Question 6

When you have an unknown number of variables, how do you execute a do loop properly?

Answer 6

Can use a DIM function to return the number of elements in the array

Ex. i = 1 to dim(array-name);

Question 7

If you want to assign initial values to array elements, how would you code it?

Answer 7

General: array array-name [# elements] <elements> (initial values);
Ex. array var[4] (1, 2, 3, 4) => var1 assigned 1, var2 assigned 2, etc.</elements>

Question 8

Write the general code for a do loop.

Answer 8

data new; set new;
do;
end;
run;

Question 9

T/F: Every “Do” loop requires an end.

Answer 9

True: else it will won’t end.

Question 10

What are iteravtive do loops?

Answer 10

Do loops that process a group of varaibles repeatedly, rather than once, based on the value of an index variable.

Question 11

Write the general syntax for an interative do loop?

Answer 11

do indexvar = start_val to indexvar = stop_value;
statements;
end;

Ex. do 1 = 2 to 8

Question 12

How do you increment by a number other than 1 in an iterative do loop?

Answer 12

Use a BY option.
Ex. do 1 = 2 to 8 by 2.

This increments by 2 instead of 1

Question 13

Write a general accumulative summing statement.

Answer 13

var1+var2

Question 14

For the do loop below, how many times is the loop executed?

do i = 1 to 20;

Answer 14

It executes 21 times. It increments to 21 and because it is over 20, it stops and processing and continues to the next DATA step statement.

Question 15

What does an explicit output stament do in a do loop?

Answer 15

It overrides automatic output, causing SAS to add an observation to the data set only when the explicit OUTPUT statement is executed.

Question 16

What is a nested do loop?

Answer 16

A do loop inside of a do loop.

Question 17

How many observations will this do loop output?

data earn;
do year = 1 to 20;
do month = 1 to 12;
output;
end;
end;
run;

Answer 17

12*20 = 240; displays EOM reports

Question 18

How many observations will this do loop output?

data new;
do year = 1 to 20;
do month = 1 to 12;
end;
output;
end;
run;

Answer 18

20 times; only displays EOY reports

Question 19

What does DO UNTIL do?

Answer 19

Executes a DO loop until the expression becomes true

Question 20

What does DO WHILE do?

Answer 20

Executes a DO loop until the expression becomes false

Question 21

What is the major difference between DO UNTIL and DO WHILE?

Answer 21

It is how SAS evaluates the loop. UNTIL evaluates at the end and WHILE evaluates at the beginning