This module demonstrates how to select variables – using the keep and drop statements – more efficiently. Sometimes data files contain information that is superfluous to a particular analysis, in which case we might want to change the data file to contain only variables of interest. Programs will run more quickly and occupy less storage space if files contain only necessary variables, and you can use the keep and drop statements in such a way to make your program run more efficiently. The following program builds a SAS file called auto.
DATA auto ;
LENGTH make $ 20 ;
INPUT make $ 1-17 price mpg rep78 hdroom trunk weight length turn
displ gratio foreign ;
CARDS;
AMC Concord 4099 22 3 2.5 11 2930 186 40 121 3.58 0
AMC Pacer 4749 17 3 3.0 11 3350 173 40 258 2.53 0
AMC Spirit 3799 22 . 3.0 12 2640 168 35 121 3.08 0
Audi 5000 9690 17 5 3.0 15 2830 189 37 131 3.20 1
Audi Fox 6295 23 3 2.5 11 2070 174 36 97 3.70 1
BMW 320i 9735 25 4 2.5 12 2650 177 34 121 3.64 1
Buick Century 4816 20 3 4.5 16 3250 196 40 196 2.93 0
Buick Electra 7827 15 4 4.0 20 4080 222 43 350 2.41 0
Buick LeSabre 5788 18 3 4.0 21 3670 218 43 231 2.73 0
Buick Opel 4453 26 . 3.0 10 2230 170 34 304 2.87 0
Buick Regal 5189 20 3 2.0 16 3280 200 42 196 2.93 0
Buick Riviera 10372 16 3 3.5 17 3880 207 43 231 2.93 0
Buick Skylark 4082 19 3 3.5 13 3400 200 42 231 3.08 0
Cad. Deville 11385 14 3 4.0 20 4330 221 44 425 2.28 0
Cad. Eldorado 14500 14 2 3.5 16 3900 204 43 350 2.19 0
Cad. Seville 15906 21 3 3.0 13 4290 204 45 350 2.24 0
Chev. Chevette 3299 29 3 2.5 9 2110 163 34 231 2.93 0
Chev. Impala 5705 16 4 4.0 20 3690 212 43 250 2.56 0
Chev. Malibu 4504 22 3 3.5 17 3180 193 31 200 2.73 0
Chev. Monte Carlo 5104 22 2 2.0 16 3220 200 41 200 2.73 0
Chev. Monza 3667 24 2 2.0 7 2750 179 40 151 2.73 0
Chev. Nova 3955 19 3 3.5 13 3430 197 43 250 2.56 0
Datsun 200 6229 23 4 1.5 6 2370 170 35 119 3.89 1
Datsun 210 4589 35 5 2.0 8 2020 165 32 85 3.70 1
Datsun 510 5079 24 4 2.5 8 2280 170 34 119 3.54 1
Datsun 810 8129 21 4 2.5 8 2750 184 38 146 3.55 1
Dodge Colt 3984 30 5 2.0 8 2120 163 35 98 3.54 0
Dodge Diplomat 4010 18 2 4.0 17 3600 206 46 318 2.47 0
Dodge Magnum 5886 16 2 4.0 17 3600 206 46 318 2.47 0
Dodge St. Regis 6342 17 2 4.5 21 3740 220 46 225 2.94 0
Fiat Strada 4296 21 3 2.5 16 2130 161 36 105 3.37 1
Ford Fiesta 4389 28 4 1.5 9 1800 147 33 98 3.15 0
Ford Mustang 4187 21 3 2.0 10 2650 179 43 140 3.08 0
Honda Accord 5799 25 5 3.0 10 2240 172 36 107 3.05 1
Honda Civic 4499 28 4 2.5 5 1760 149 34 91 3.30 1
Linc. Continental 11497 12 3 3.5 22 4840 233 51 400 2.47 0
Linc. Mark V 13594 12 3 2.5 18 4720 230 48 400 2.47 0
Linc. Versailles 13466 14 3 3.5 15 3830 201 41 302 2.47 0
Mazda GLC 3995 30 4 3.5 11 1980 154 33 86 3.73 1
Merc. Bobcat 3829 22 4 3.0 9 2580 169 39 140 2.73 0
Merc. Cougar 5379 14 4 3.5 16 4060 221 48 302 2.75 0
Merc. Marquis 6165 15 3 3.5 23 3720 212 44 302 2.26 0
Merc. Monarch 4516 18 3 3.0 15 3370 198 41 250 2.43 0
Merc. XR-7 6303 14 4 3.0 16 4130 217 45 302 2.75 0
Merc. Zephyr 3291 20 3 3.5 17 2830 195 43 140 3.08 0
Olds 98 8814 21 4 4.0 20 4060 220 43 350 2.41 0
Olds Cutl Supr 5172 19 3 2.0 16 3310 198 42 231 2.93 0
Olds Cutlass 4733 19 3 4.5 16 3300 198 42 231 2.93 0
Olds Delta 88 4890 18 4 4.0 20 3690 218 42 231 2.73 0
Olds Omega 4181 19 3 4.5 14 3370 200 43 231 3.08 0
Olds Starfire 4195 24 1 2.0 10 2730 180 40 151 2.73 0
Olds Toronado 10371 16 3 3.5 17 4030 206 43 350 2.41 0
Peugeot 604 12990 14 . 3.5 14 3420 192 38 163 3.58 1
Plym. Arrow 4647 28 3 2.0 11 3260 170 37 156 3.05 0
Plym. Champ 4425 34 5 2.5 11 1800 157 37 86 2.97 0
Plym. Horizon 4482 25 3 4.0 17 2200 165 36 105 3.37 0
Plym. Sapporo 6486 26 . 1.5 8 2520 182 38 119 3.54 0
Plym. Volare 4060 18 2 5.0 16 3330 201 44 225 3.23 0
Pont. Catalina 5798 18 4 4.0 20 3700 214 42 231 2.73 0
Pont. Firebird 4934 18 1 1.5 7 3470 198 42 231 3.08 0
Pont. Grand Prix 5222 19 3 2.0 16 3210 201 45 231 2.93 0
Pont. Le Mans 4723 19 3 3.5 17 3200 199 40 231 2.93 0
Pont. Phoenix 4424 19 . 3.5 13 3420 203 43 231 3.08 0
Pont. Sunbird 4172 24 2 2.0 7 2690 179 41 151 2.73 0
Renault Le Car 3895 26 3 3.0 10 1830 142 34 79 3.72 1
Subaru 3798 35 5 2.5 11 2050 164 36 97 3.81 1
Toyota Celica 5899 18 5 2.5 14 2410 174 36 134 3.06 1
Toyota Corolla 3748 31 5 3.0 9 2200 165 35 97 3.21 1
Toyota Corona 5719 18 5 2.0 11 2670 175 36 134 3.05 1
Volvo 260 11995 17 5 2.5 14 3170 193 37 163 2.98 1
VW Dasher 7140 23 4 2.5 12 2160 172 36 97 3.74 1
VW Diesel 5397 41 5 3.0 15 2040 155 35 90 3.78 1
VW Rabbit 4697 25 4 3.0 15 1930 155 35 89 3.78 1
VW Scirocco 6850 25 4 2.0 16 1990 156 36 97 3.78 1
;
RUN;
PROC CONTENTS DATA=auto;
RUN;
The proc contents shown below provides information about the file.
CONTENTS PROCEDURE Data Set Name: WORK.AUTO Observations: 74 Member Type: DATA Variables: 12 -----Alphabetic List of Variables and Attributes----- # Variable Type Len Pos ------------------------------------ 10 DISPL Num 8 84 12 FOREIGN Num 8 100 11 GRATIO Num 8 92 5 HDROOM Num 8 44 8 LENGTH Num 8 68 1 MAKE Char 20 0 3 MPG Num 8 28 2 PRICE Num 8 20 4 REP78 Num 8 36 6 TRUNK Num 8 52 9 TURN Num 8 76 7 WEIGHT Num 8 60
If, for example, we wanted to examine the relationship between mpg and price for various makes, but had no interest in the automobile’s dimensions, we could create a smaller file, by keeping only these three variables.
DATA auto2; set auto; keep make mpg price; RUN;
To verify the contents of the new file, run the following program.
PROC CONTENTS DATA=AUTO2; RUN;CONTENTS PROCEDURE Data Set Name: WORK.AUTO2 Observations: 74 Member Type: DATA Variables: 3 -----Alphabetic List of Variables and Attributes----- # Variable Type Len Pos ----------------------------------- 1 MAKE Char 20 0 3 MPG Num 8 28 2 PRICE Num 8 20
Note that the number of observations, or records, remains unchanged. This program creates auto2 from the original file auto. The new file, named auto2 is identical to auto except that it contains only the variables listed in the keep statement.
SAS will read into working memory all the variables on the auto file, deleting the unwanted variables only when it writes out the new file auto2. This means that all the variables on the input file are available for SAS to use during the program. However, it also means that SAS will be working with a larger data set than may be necessary. An alternate way to control the selection of variables is to use SAS data step options, which specifically control the way variables are read from SAS files and/or written out to SAS files, resulting in more efficient use of computer resources.
The following program creates exactly the same file, but is a more efficient program because SAS only reads the desired variables.
DATA auto2; SET auto (KEEP = make mpg price); RUN;
The drop data step option works in a similar way.
DATA AUTO2; SET auto (DROP = rep78 hdroom trunk weight length turn displ gratio foreign); RUN;
The keep data step option can also control which variables are written to the new file.
DATA AUTO2 (keep = make mpg price); SET auto; RUN;
Or, we can use the drop data step option.
DATA AUTO2 (drop = rep78 hdroom trunk weight length turn displ gratio foreign); SET auto; RUN;
In these two examples, all the variables in the auto file are read into working memory. SAS does not, however, include them when it writes out the new file auto2.
The data step option controls the contents of the file whose name it follows in parenthesis. If it modifies the file on the set statement (the file being read) it determines which variables are read. If it modifies the file on the data statement (the file being written) then it controls which variables are written to the new file.
Data step options may be used on both files, as illustrated in the following program.
DATA AUTO2 (drop=weight length); SET auto (keep=weight length); size = weight * length; run;
In this example, SAS reads two variables (weight and length) into working memory, using them to compute a new variable (size). Since weight and length are dropped on the output file, auto2 contains only 1 variable (size).
Be careful that you do not eliminate variables on a keep or drop on the input file, even though you refer to them in the data step.
For more information
- For information on making SAS data files from raw data, see Inputting Data into SAS.
- For more basic information about subsetting variables or subsetting cases in SAS, see the SAS Learning Module on Subsetting data in SAS.
- For information about making permanent SAS data files, see Reading and writing SAS system files.
- For more advanced issues in subsetting, data transformations and data manipulation see Data transformations and manipulation in SAS in the SAS Library- Web Page Resources.