Reading and Writing Data in SAS - Study Guide | ECO 725, Study notes of Economics

Download Reading and Writing Data in SAS - Study Guide | ECO 725 and more Study notes Economics in PDF only on Docsity! 1   Reading and Writing Data in SAS 1) Basic input/output (i/o) commands: • in the introduction, we went over basic i/o commands • basic input: − to read a SAS data set, you would use the SET command − to read a space-delimited ASCII data set, you would use the INFILE and simple (variable list only) INPUT commands • basic output: − to create a SAS data set, you would use the DATA statement − to create a space-delimited ASCII data set, you would use the OUTPUT and simple PUT commands • this class will cover more advanced operations, including − creating and reading permanent SAS data sets − options for creating and reading SAS data sets − different types of formatted ASCII data sets − working with SAS transport files 2) What does a DATA step normally do?1 • Compilation Phase − checks syntax of submitted statements − creates input buffer, program data vector, and descriptor information • Execution Phase—implicit loop over observations being read; observations processed − starting from DATA statement − a record is either read into the input buffer (if INPUT is used) or directly into the program data vector if a SAS data set is read (through a SET or MERGE) − additional statements in the DATA step are executed on that observation − at end of DATA step, ◊ observation is output ◊ processing returns to the top of that step ◊ all information in the program data vector is reset − iteration counter (_N_) is updated − flow continues until the end of the input file is reached • processing is best-suited for “rectangular” data sets but can accommodate other structures • lots of variations possible 3) the OUTPUT and DELETE commands • unless told otherwise, SAS outputs one record to the SAS data set specified in the DATA statement at the end of the DATA step for each record read − if you do not want SAS to output a record to the SAS data set, you would type the DELETE command                                                              1 See http://support.sas.com/documentation/cdl/en/lrcon/59522/HTML/default/a000985872.htm. 2   ◊ when this command is used, the current observation is not output ◊ also, SAS immediately returns to the top of the DATA step to begin a new iteration (the commands following a DELETE statement are not executed) ◊ the DELETE command shortens the length of a SAS data set (recall that the DROP and KEEP commands reduced the “width” of a data set) − if you want to write an observation to a SAS data set somewhere other than the end of a DATA step, you would use the OUTPUT command 4) Temporary and permanent SAS data sets • SAS distinguishes between temporary and permanent SAS data sets • temporary SAS data sets − only last as long as a SAS session; they are automatically deleted when the SAS session closes − have a one-part name, e.g., <temp_data_set_nm> − to read a temporary data set, you would use the command: SET <temp_data_set_nm>; − to create a temporary data set, you would begin the DATA step with the statement: DATA <temp_data_set_nm>; • permanent SAS data sets − are stored in libraries and can “outlive” a SAS session − have a two-part name, <lib_ref>.<perm_data_set_nm>, where ◊ the first part, <lib_ref>, is the internal library name (defined as the first argument in a LIBNAME statement), and ◊ the second part, <perm_data_set_nm>, is the permanent data set name − in Windows, the permanent data set would be stored as <perm_data_set_nm>.sas7bdat in the directory defined in the second argument of the LIBNAME statement − to read a permanent data set, you would use the command: SET <lib_ref>.<perm_data_set_nm>; − to create a permanent data set, you would begin the DATA step with the statement: DATA <lib_ref>.<perm_data_set_nm>; 5) SAS data set options2 • options can be used when a SAS data set is created or read • most options can be used for either operation, but some are specific to creating or reading • the options would appear in parentheses after the SAS data set name                                                              2 See http://support.sas.com/documentation/cdl/en/lrdict/59540/HTML/default/a002295655.htm. 5   would read variable1 from one line and variable2 from the next line − line holds – normally at the end of an INPUT or PUT command, SAS advances to the next line; line hold commands prevent this; these commands are useful if the formatting of data (the record lay-out) varies across lines; you can determine what type of record you are reading with one INPUT command and then specify the correct format for the rest of the record using a subsequent command ◊ to hold a line within the current DATA step iteration, you would leave an @ at the end of the line; for example, INPUT variable1 @; IF variable1 EQ 1 THEN INPUT variable2; ELSE INPUT variable3; would read variable1 from the beginning of a record line; depending on the value of variable1, the code would read and interpret the next piece of data on the same line as either variable2 or variable3 ◊ to hold a line across DATA step iterations, you would leave an @@ at the end of the line • informat specifications − informat specifications immediate follow either a variable or a list of variables − to specify the input or output as character data, you would use $ as the informat specification; to specify something as a character of a particular length, you would use $n. as the informat, where n gives the length of the character − to specify the input or output as an integer of a particular length, you would use n. − to specify the input or output as a real number of a particular length and precision, you would use n.d, where n is the total length of the number (including the decimal point) and d is the number of decimal places in the number INPUT variable1 $8. variable2 5. variable3 8.3; would read variable1 as a character variable of length 8, variable2 as an integer of length 5, and variable3 as a real variable of length 8 and with 3 decimal places − SAS has MANY other informat specifications, see http://support.sas.com/documentation/cdl/en/lrdict/59540/HTML/default/a000309877.htm for a complete list − for lists of variables with the same informat, you would group the variables and informat into parentheses; for example INPUT (variable1 variable2 variable3) (5.); • column specifications − another way to read or write data is to specify the beginning and ending column positions 6   − these specifications would follow the variable name in an INPUT or PUT command PUT variable1 12-15 variable2 16-18; writes variable1 in the 12th – 15th column positions and variable2 in the 16th – 18th positions 8) SAS transport files • the structure of SAS files varies across computer platforms; a SAS file created for a Windows computer will not be readable on a UNIX computer • SAS does have a facility for creating files that can be moved across systems; these are called transport files • transport files cannot be read directly in a DATA step; you need to first create a copy of the file in your system’s format • suppose that you had a SAS transport file called newdata.xpt in a directory C:\mysasdata, and suppose that you wanted to create a SAS permanent data based on this to the same directory; you could do this by typing the code LIBNAME outlib "C:\mysasdata"; LIBNAME trandata XPORT "C:\mysasdata\newdata.xpt"; PROC COPY IN=trandata OUT=outlib; RUN; • transport files can also be read (and in some cases must be read) using the CIMPORT procedure; if the COPY procedure does not work, you might try the CIMPORT • to perform the same steps using the CIMPORT procedure, you would type LIBNAME outlib "C:\mysasdata"; FILENAME tran_in "C:\mysasdata\newdata.xpt"; PROC CIMPORT LIBRARY=outlib INFILE=tran_in; RUN; • note: transport files can be tricky to use, and some of the specifications for these files can vary across versions of SAS