SAS Arrays and DO Loop Made Easy

SAS Arrays : Introduction

SAS arrays provides a simple, efficient way to process a list of variables in a SAS DATA step. In other words, arrays are useful when you need to perform similar operations on multiple variables or when you want to avoid writing similar code for multiple variables.

Syntax of SAS Arrays

The syntax of SAS arrays is as follows:

Array array-name {number-of-elements} list-of-variables;

Note: You can use [ ] or { } or ( ) for defining number of elements in the ARRAY statement.

• ARRAY ABC[5] a b c d e;: ABC is an array-name, 5 implies the number of variables in the array, and "a b c d e" are the fields that make up the array.
• ARRAY ABC[*] a b c d e;: In this example, SAS would automatically calculate the number of variables in the array.
• ARRAY ABC[*] X1-X10;: Where the X1 variable contains the X1 value, X2 contains the X2 value, etc.
• ARRAY ABC[*] $ X1-X10;: If the variables are of character type then use $ sign before specifying list of variables.

SAS DO Loop : Introduction

The DO loop allows you to perform iterative processing. You can use the DO loop to repeat a set of statements for a specific number of iterations or for a particular range of values.

Syntax of SAS DO Loop

The syntax of SAS DO Loop is as follows:

DO index-variable = start-value TO end-value;
  /* Statements to be executed inside the loop */
END;

• index-variable: This is a temporary variable used to keep track of the loop iterations. It is usually a numeric variable.
• start-value: This is the initial value for the index variable.
• end-value: This is the final value for the index variable.

Sample Dataset

Let's create a sample SAS dataset for demonstration purposes.

SAS Array : Example

data temp;
input x1 x2 x3 x4$ x5$;
cards;
1 2 3 AA BB
2 3 4 AB CC
3 4 5 AC DD
4 5 6 AD EE
5 6 7 AE FF
6 7 8 AF GG
;
run;

Example 1 : Replace values greater than 3 in numeric variables with missing data

The following code reads data from the "temp" dataset and then goes through each observation and checks if the values of the variables "x1," "x2," and "x3" are greater than 3. If any of these values are greater than 3, the value is replaced with a missing value. It stores the updated values in a new dataset "test".

data test;
set temp;
array nvars {3} x1-x3;
do i = 1 to 3;
if nvars{i} > 3 then nvars{i} =.;
end;
run;

Output : Array Statement

Why i is 4 in the output data set?

The first time the loop processes, the value of "i" is 1; the second time, 2; and the third time, 3. At the beginning of the fourth iteration, the value of "i" is 4, which is found to be greater than the stop value of 3 so the loop stops. The value of i is now 4 and not 3, the last value before it would be greater than 3 as the stop value.

Note : We can drop variable "i" with drop statement or drop data set option.

Efficient version of the above code

data test;
set temp;
array nvars (*) _numeric_;
do i = 1 to dim(nvars);
if nvars{i} > 3 then nvars{i} =.;
end;
drop i;
run;

Notes -

1. The "_numeric_" is used to specify all the numeric variables.
2. The DIM function returns the number of elements (variables).

Example 2 : Extract first letter of all the character variables

The following code reads data from the "temp" dataset and then goes through each character variable and keeps only the first letter, discarding the rest. The result is a new dataset "test" with character variables truncated to their first letters.

data test;
set temp;
array cvars (*) _character_;
do i = 1 to dim(cvars);
cvars{i} = substr(cvars{i},1,1);
end;
drop i;
run;

Note - The "_character_" is used to specify all the character variables.

Example 3 : Extract first letter and assign to new variables

The following code reads data from the "temp" dataset and then goes through each character variable and extracts the first letter from each variable, and assigns it to the corresponding "X6" or "X7" variable in the "test" dataset. The result is a new dataset "test" with the first letter of each character variable stored in "X6" and "X7" variables.

data test;
set temp;
array cvars (*) _character_;
array dvars (*) $ X6 X7;
do i = 1 to dim(cvars);
dvars{i} = substr(cvars{i},1,1) ;
end;
drop i;
run;

Example 4 : Assign Initial Values in a SAS Array

The following code reads data from the "temp" dataset and then goes through each numeric variable in "temp" and multiplies it by a corresponding percentage increase factor from the temporary "pctinc" array. The results are assigned to the "px1", "px2", and "px3" variables in the "abcd" dataset. The temporary array "pctinc" is used for the calculation but is not included in the final dataset.

data abcd;
set temp;
array nvars (*) _numeric_;
array pvars (*) px1 px2 px3;
array pctinc {3} _temporary_ (1.1 , 1.2 ,1.3); 
do i = 1 to dim(nvars);
pvars{i} = nvars{i} * pctinc{i};
end;
drop i;
run;

Notes -

1. In the above example, we are multiplying values of variables with different numbers.
2. When the key word _TEMPORARY_ is used in a ARRAY statement, data elements are created but are not stored in the data file.

Example 5 : Calculate Percentage Growth

The following code reads data from the "temp" dataset and then calculates the difference and percentage difference between consecutive numeric variables in "temp" and stores the results in the temporary "diff" and "percent" arrays. The final "abcd" dataset will not include the temporary "diff" array but will have the percentage differences stored in the "percent" array.

data abcd;
set temp;
array nvars(*) _numeric_;
array diff{2} _temporary_;
array percent{2};
do i = 1 to 2;
diff{i} = nvars{i +1} - nvars{i};
percent{i} = diff{i} / nvars{i} ;
end;
drop i;
run;

Using the OF Operator in a SAS Array

The following two codes are equivalent :

array gnp (*) x y z;
sumgnp = sum(of gnp(*));

OR

sumgnp = sum(x,y,z);

Calculate the mean : mean_score = mean(of gnp(*));
Calculate the minimum : min_score = min(of gnp(*));

Suppose you are asked to create a flag in cases wherein sum of variables x1,x2 and x3 is greater than 10.

data test;
set temp;
array nvars (*) x1-x3;
if sum(of nvars(*)) > 10 then flag =1;
else flag=0;
run;

DO OVER LOOP

The DO OVER loop is one of the most useful DO loops. It can be used with an array when indexing of the array is not needed.

In the example below, the "do over" statement iterates over each numeric variable in the array "nvars". For each variable, the code checks if its value is greater than 3. If it is, then the value is set to missing (represented by a dot "."). This effectively replaces any numeric value greater than 3 with a missing value.

data test;
set temp;
array nvars _numeric_;
do over nvars;
if nvars > 3 then nvars = .;
end;
run;

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About Author:

Deepanshu founded ListenData with a simple objective - Make analytics easy to understand and follow. He has over 10 years of experience in data science. During his tenure, he worked with global clients in various domains like Banking, Insurance, Private Equity, Telecom and HR.

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