DSCI 325: Handout 3 Creating and Redefining Variable in SAS Spring 2017

Transcription

1 DSCI 325: Handout 3 Creating and Redefining Variable in SAS Spring 2017 Content Source: The Little SAS Book, Chapter 3, by L. Delwiche and S. Slaughter. CREATING NEW VARIABLES OR REDEFINING VARIABLES In SAS, you can create new variables using an assignment statement with this general form: variable = expression; You name the variable on the left-hand side of the equal sign and tell SAS what value you want this variable to assume on the right-hand side. If this is a new variable, SAS will add it to your data set; if the variable already exists, SAS replaces the old value with the new. For example, consider the following statements. DATA test; INPUT y; Value = 1; Value2 = '1'; Value3 = y + 1; DATALINES; ; PROC PRINT; PROC CONTENTS; SAS returns the following: From PROC PRINT From PROC CONTENTS 1

2 For a more realistic example, consider the NYC_Trees data set. If you haven t already done so, create your own local library. Then, use the Import Wizard to place the NYC_Trees data set in your local library. The following PROC IMPORT code did this for me. PROC IMPORT OUT= Hooks.NYC_Trees DATAFILE= '/folders/myfolders/nyc_trees.csv' DBMS=CSV REPLACE; GETNAMES=YES; DATAROW=2; A print-out of the first few rows of this data set is given below. The variables Diameter, Height, and CanopyArea are all expressed using the metric system (i.e., they are measured in cm, m, and m 2, respectively). Suppose your goal is to convert these measurements into the standard measurement system used in the U.S. Before we write the SAS code to convert these measurements, let s compute the diameter in inches, the height in feet, and the canopy area in ft 2 for the first observation. Diameter (inches): = Height (feet): = Canopy Area (ft 2 ): = The following code can be used to convert the measurement units of these variables. DATA NYC_Trees2; SET Hooks.NYC_Trees; Diameter = *Diameter; Height = *Height; CanopyArea = *CanopyArea; PROC PRINT DATA=NYC_Trees2; 2

3 A portion of the SAS print-out of the converted variables is shown below. Note that SAS replaced the values of the original variables Diameter, Height, and CanopyArea. Suppose you wanted to instead keep the original diameter measurements in metric units and to simply add a new variable, say DiameterInch, to the dataset for the converted values. Consider the following code. DATA NYC_Trees2; SET Hooks.NYC_Trees; Diameter = *Diameter; DiameterInch = *Diameter; PROC PRINT data=nyc_trees2; VAR Native Condition Diameter DiameterInch; Note: The VAR statement in the PROC PRINT tells SAS to print only the variable(s) listed. Question: Note that the values for DiameterInch are NOT correct; the value for the first observation should be Why did this happen? 2. How would you fix this? 3

4 USING SAS FUNCTIONS SAS has the ability to do much more than simple arithmetic in its DATA step. There are many SAS functions available (e.g., string functions, mathematical functions, date functions, probability functions, etc.) that can be used to manipulate data and create new variables. The general form of a function is as follows: function_name(argument1, argument2, ) To introduce some commonly used SAS functions, we will consider the Grades.csv data set. Use PROC IMPORT to read in this data set and store it in your own library (recall that you can select PROC IMPORT code from the Snippets tab). First, we will discuss the SUM function which can be used to add up quiz scores. Consider the following programming statements: DATA Grades2; SET Hooks.Grades; TotalQuiz = SUM(Quiz1,Quiz2,Quiz3,Quiz4,Quiz5,Quiz6,Quiz7,Quiz8,Quiz9,Quiz10,Quiz11,Quiz12); PROC PRINT; Note that this program creates a new data set named Grades2. The SET command within the DATA step tells SAS that you want to work with an existing data set (specifically, the Grades data set which is located in the Hooks library). The Grades2 data set will be stored in the temporary WORK library. The Grades2 data set will contain ALL information (i.e., all variables and all observations) from Hooks.Grades in addition to any new variables that are created in this DATA step (e.g., TotalQuiz). A portion of the Grades2 dataset containing the new variable TotalQuiz is shown below. 4

5 Next, consider the task of finding a final percentage grade for each student. Assume final percentage grades are determined using the following formula: Points Earned / Points Possible, where Points Earned = (Quiz Total Lowest Quiz) + (Exam Total 0.5*Lowest Exam) + Final + EC Points Possible = 570 Tasks: 1. In the table below, write the additional statements necessary to compute the final percentage grade. Name this variable FinalPercent. Use as many lines as necessary. DATA Grades2; SET Hooks.Grades; TotalQuiz = SUM(Quiz1,Quiz2,Quiz3,Quiz4,Quiz5,Quiz6,Quiz7,Quiz8,Quiz9,Quiz10,Quiz11,Quiz12); 2. Type your code into your SAS program and make sure it works. Use the following SAS code to print out these variables to verify your answer (the values for the first four observations are shown below). PROC PRINT data=grades2; VAR FIRSTNAME LASTNAME FinalPercent; 5

6 3. Add the following statement to your DATA step: LastMonth = MONTH(LastDay); What does the MONTH function do? How is this value stored? 4. Create a new variable called Name which concatenates the students first and last names (search the web or use SAS Help to learn how to concatenate strings in SAS). Write the SAS code used to do this below, and verify that it works. DATA Grades2; SET Hooks.Grades; 6

7 Next, we will discuss a few character functions available in SAS that are useful for working with string data. LENGTH(arg) returns the length of an argument (not counting trailing blanks) SUBSTR(arg, p, n) extracts a substring from an argument arg starting at position p for n characters (or until the end if no n is specified). FIND(string, substring, optional modifiers, optional start position) locates a substring within a string for character variables. This function also allows for some modifiers such as i' to ignore case and t to ignore trailing blanks (see the example below). It also allows you to specify the position at which the search should start (by default, it starts at the beginning of the string). Consider the following statements: DATA test; STRING1 = 'Tisha tisha Hooks'; STRING2 = 'tisha'; TEST1 = LENGTH(STRING1); TEST2 = LENGTH(STRING2 ' ' 'hooks'); TEST3 = SUBSTR(STRING1,13,4); TEST4 = FIND(STRING1,STRING2); TEST5 = FIND(STRING1,STRING2, 'i'); TEST6 = FIND(STRING2, a ); PROC PRINT; Fill in the output window below with appropriate values, and then run the program in SAS to check your answers. 7

8 Tasks: Use the Grades data set for the following. 1. Use the SUBSTR character function to create a new variable called ZipCode. The zip code for each observation is the first 5 characters of the existing ZipTownAreaCode variable. 2. Create a new variable called Town that contains the Town name. Be careful to not include any unnecessary spaces (you can check the length of your new variable with the LENGTH function). Here is my output: 3. Note that you could use the FIND function to find the location of the character ( in the ZipTownAreaCode variable. Look up information on the INDEX function and find the location of this character using INDEX, instead. What advantage(s) does the FIND function have over the INDEX function? 4. Look up information on the ZIPCITYDISTANCE function in SAS and use it to compute the distance between the zip code for each student and (Winona s zip code). What does the value SAS returns represent? Your output should match the following: 8

9 Tasks: Consider the data in the ContactInfo.csv file. 1. Read this data file into SAS. Be sure to check that the data is read in properly for all observations! 2. Create a new variable named DomainName that contains only the domain names from each address (e.g., verizon.net). Write your SAS program used to do this below: 3. Note that once you have created this new variable, you can use the following code to calculate the total counts and percentage of observations that fall under each domain name. PROC FREQ; TABLE DomainName; A portion of the output is shown below. What is the problem? Find a SAS function that can be used to fix this problem. 9

10 This handout has introduced a few functions that are available in SAS. For more information, see the following resources from support.sas.com: More information on Character Functions: More information on SAS Functions: r.htm#p0w6napahk6x0an0z2dzozh2ouzm.htm 10