Functions and CALL Routines |
Category: | Combinatorial |
Interaction: | When invoked by the %SYSCALL macro statement, CALL LEXPERM removes the quotation marks from its arguments. For more information, see Using CALL Routines and the %SYSCALL Macro Statement. |
Syntax |
CALL LEXPERM(count, variable-1 <, ..., variable-N>); |
specifies a numeric variable that has an integer value that ranges from 1 to the number of permutations.
specifies either all numeric variables, or all character variables that have the same length. The values of these variables are permuted by LEXPERM.
Requirement: | Initialize these variables before you call the LEXPERM routine. |
Details |
These variables are defined for use in the equation that follows:
N |
specifies the number of variables that are being permuted--that is, the number of arguments minus one. |
M |
specifies the number of missing values among the variables that are being permuted. |
d |
specifies the number of distinct non-missing values among the arguments. |
N_{i} |
for i=1 through i=d, N_{i} specifies the number of instances of the ith distinct value. |
The number of distinct permutations of non-missing values of the arguments is expressed as follows:
Use the CALL LEXPERM routine in a loop where the argument count accepts each integral value from 1 to P. You do not need to compute P provided you exit the loop when CALL LEXPERM returns a value that is less than zero.
For 1=count<P, the following actions occur:
The argument types and lengths are checked for consistency.
The M missing values are assigned to the last M arguments.
The N-M non-missing values are assigned in ascending order to the first N-M arguments following count.
CALL LEXPERM returns 1.
For 1<count<=P, the following actions occur:
The next distinct permutation of the non-missing values is generated in lexicographic order.
If variable-1 through variable-I did not change, but variable-J did change, where J=I+1, then CALL LEXPERM returns J.
For count>P, CALL LEXPERM returns -1.
If the CALL LEXPERM routine is executed with the first argument out of sequence, the results might not be useful. In particular, if you initialize the variables and then immediately execute CALL LEXPERM with a first argument of K, you will not get the Kth permutation (except when K is 1). To get the Kth permutation, you must execute CALL LEXPERM K times, with the first argument accepting values from 1 through K in that exact order.
You can call the LEXPERM routine when you use the %SYSCALL macro. In this case, the variable arguments are not required to be the same length, but they must be the same type. If %SYSCALL identifies an argument as numeric, then %SYSCALL reformats the returned value.
If an error occurs during the execution of the CALL LEXPERM routine, then both of the following values are set:
&SYSERR is assigned a value that is greater than 4.
&SYSINFO is assigned a value that is less than -100.
If there are no errors, then &SYSERR is set to zero, and &SYSINFO is set to one of the following values:
1 if 1=count<P
1 if 1<count<=P and the value of variable-1 changed
J if 1<count<=P and variable-1 through variable-I did not change, but variable-J did change, where J=I+1
-1 if count>P
Comparisons |
SAS provides three functions or CALL routines for generating all permutations:
ALLPERM generates all possible permutations of the values, missing or non-missing, of several variables. Each permutation is formed from the previous permutation by interchanging two consecutive values.
LEXPERM generates all distinct permutations of the non-missing values of several variables. The permutations are generated in lexicographic order.
LEXPERK generates all distinct permutations of K of the non-missing values of N variables. The permutations are generated in lexicographic order.
ALLPERM is the fastest of these functions and CALL routines. LEXPERK is the slowest.
Examples |
The following example uses the DATA step to generate all distinct permutations of the non-missing values of several variables in lexicographic order.
data _null_; array x[4] $3 ('ant' 'bee' 'cat' 'dog'); n=dim(x); nfact=fact(n); do i=1 to nfact; call lexperm(i, of x[*]); put i 5. +2 x[*]; end; run;
SAS writes the following output to the log:
1 ant bee cat dog 2 ant bee dog cat 3 ant cat bee dog 4 ant cat dog bee 5 ant dog bee cat 6 ant dog cat bee 7 bee ant cat dog 8 bee ant dog cat 9 bee cat ant dog 10 bee cat dog ant 11 bee dog ant cat 12 bee dog cat ant 13 cat ant bee dog 14 cat ant dog bee 15 cat bee ant dog 16 cat bee dog ant 17 cat dog ant bee 18 cat dog bee ant 19 dog ant bee cat 20 dog ant cat bee 21 dog bee ant cat 22 dog bee cat ant 23 dog cat ant bee 24 dog cat bee ant
The following is an example of the CALL LEXPERM routine that is used with macros. The output includes values for the %SYSINFO macro.
%macro test; %let x1=ant; %let x2=baboon; %let x3=baboon; %let x4=hippopotamus; %let n=4; %let nperm=%sysfunc(perm(4)); %do j=1 %to &nperm; %syscall lexperm(j,x1,x2,x3,x4); %let jfmt=%qsysfunc(putn(&j,5.)); %put &jfmt: &x1 &x2 &x3 &x4 sysinfo=&sysinfo; %if &sysinfo<0 %then %let j=%eval(&nperm+1); %end; %mend; %test;
SAS writes the following output to the log:
1: ant baboon baboon hippopotamus sysinfo=1 2: ant baboon hippopotamus baboon sysinfo=3 3: ant hippopotamus baboon baboon sysinfo=2 4: baboon ant baboon hippopotamus sysinfo=1 5: baboon ant hippopotamus baboon sysinfo=3 6: baboon baboon ant hippopotamus sysinfo=2 7: baboon baboon hippopotamus ant sysinfo=3 8: baboon hippopotamus ant baboon sysinfo=2 9: baboon hippopotamus baboon ant sysinfo=3 10: hippopotamus ant baboon baboon sysinfo=1 11: hippopotamus baboon ant baboon sysinfo=2 12: hippopotamus baboon baboon ant sysinfo=3 13: hippopotamus baboon baboon ant sysinfo=-1
See Also |
Functions and CALL Routines: |
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