SAS FAQ
How do I use keep and drop efficiently?

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

How to cite this page

Report an error on this page or leave a comment

The content of this web site should not be construed as an endorsement of any particular web site, book, or software product by the University of California.