SAS讲义-第九课
一、Do循环
1、 大家回看第四课的例11，可以发现Do循环应该要和End搭配使用。下面都是可行的Do语句。
do while语句怎么用 do i=5;
do i=2,3,5,7;
do i=1 to 100;
do i=1 to 100 by 2;
do i=100 to 1 by -1;
do i=1 to 5,7 to 9;
do i=’01jan99’d,’25feb99’d;
do i=’01jan99’d to ‘01jan2000’d by 1;
例1 产生1,2,9,8 的序列。
data a;
do i=1,2,9,8;
output;
end;
run;
思考：若output放在end之后，或者去掉output，那会怎样呢？
例2 产生1-20的奇数序列。
data a;
do i=1 to 20 by 2;
output;
end;
run;
例3 求1-100的自然数之和。
data a;
do i=1 to 100 ;
n+i;
output;
end;
run;
例4 求1-100的自然数的平方和。
data a;
do i=1 to 100 ;
n+i**2;
output;
end;
run;
例5用do循环处理数组。（下课还会深入说数组）
data a(drop=i);
array day{7} d1-d7;
do i=1 to 7;
day{i}=i+1;
end;
run;
2、do while语句。先判断while表达式，若成立则执行，否则推测循环。
例6
data a;
n=0;
do while (n<5);
n+1;
output;
end;
run;
例7 计算1加到100的过程中，第一个大于等于2000的数。
data a;
do i=1 to 100 while (n<2000) ;
n+i;
output;
end;
run;
3、do until 语句。先执行，直到until的表达式为真，推出循环。
4、do over 语句。我们到下课再说。
二、select语句。
Select-when 相当于一般编程语言里面的swich-case语句。直接看例子。
例8 data a;
set resdat.class;
x=0;
obs=_n_;
select(obs);
when(2) x=2;
when(3,7)x=5;
otherwise x=3;
end;
run;
三、return语句。
Return语句可以让系统返回到data步开头。
例 9return语句与if-then共用
data a;
input x y z;
if x=y then return;
s=x+y;
cards;
1 2 3
2 2 3
;
run;
分析:默认情况下，系统将每条观测读入到数据集a，可是当x=y时，return语句被执行，也就是s=x+y没有被执行，那么s即为空值。思考，如果在s=x+y语句后面增加一条output，又会怎样呢？此时要考虑一旦有了output，run本身的输出功能消失了。
例10 return语句与Do循环语句共同使用。
data a;
input a b c @@;
do x=1 to 5;
ax=a*x;
if ax>b then return;
output;
end;
cards;
1 2 3 2 6 8
;
run;
分析：由于Do循环包含了output，一个输入数据行本来可以生产5条观测，可是在循环中增加了return。也就是，当ax>b，系统执行return语句，没有执行output语句。
练习10某人将500元存入在某银行的存款账户，这个账户利率为7%，每年计息一次。用累加语句和循环语句计算三年末此人能得多少钱。
下面的练习仅供大家参考学习，不作练习。
练习11 given the SAS data set SASDATA.TWO:
X Y
-- --
5 2
3 1
5 6
The following SAS program is submitted:
data SASUSER.ONE SASUSER.TWO OTHER;
set SASDATA.TWO;
if X eq 5 then output SASUSER.ONE;
if Y lt 5 then output SASUSER.TWO;
output;
run;
What is the result?
A.
data set SASUSER.ONE has 5 observations
data set SASUSER.TWO has 5 observations
data set WORK.OTHER has 3 observations
B.
data set SASUSER.ONE has 2 observations
data set SASUSER.TWO has 2 observations
data set WORK.OTHER has 1 observations
C.
data set SASUSER.ONE has 2 observations
data set SASUSER.TWO has 2 observations
data set WORK.OTHER has 5 observations
D. No data sets are output. The DATA step fails execution due to syntax errors.
Answer: A
练习12Consider the following data step:
data WORK.NEW;
set WORK.OLD(keep=X);
if X < 10 then X=1;
else if X >= 10 AND X LT 20 then X=2;
else X=3;
run;
In filtering the values of the variable X in data set WORK.OLD, what new value would be assigned to X if its original value was a missing value?
A. X would get a value of 1.
B. X would get a value of 3.
C. X would retain its original value of missing.
D. This step does not run because of syntax errors.
Answer: A
练习 13 The following SAS program is submitted:
data WORK.SALES;
do Year=1 to 5;
do Month=1 to 12;
X + 1;
end;
end;
run;
How many observations are written to the WORK.SALES data set?
A. 0
B. 1
C. 5
D. 60
Answer: B /*假如在X+1后添加output，那又选什么？*/
练习14 The following SAS program is submitted:
data WORK.OUTDS;
do until(Prod GT 6);
Prod + 1;
end;
run;
What is the value of the variable Prod in the output data set?
A. . (missing)
B. 6
C. 7
D. Undetermined, infinite loop.
Answer: C
练习15 Given the SAS data set WORK.PRODUCTS:
ProdId Price ProductType Sales Returns
------ ----- ----------- ----- -------
K12S 95.50 OUTDOOR 15 2
B132S 2.99 CLOTHING 300 10
R18KY2 51.99 EQUIPMENT 25 5
3KL8BY 6.39 OUTDOOR 125 15
DY65DW 5.60 OUTDOOR 45 5
DGTY23 34.55 EQUIPMENT 67 2
The following SAS program is submitted:
data WORK.REVENUE(drop=Sales Returns Price);
set WORK.PRODUCTS(keep=ProdId Price Sales Returns);
Revenue=Price*(Sales-Returns);
run;
How many variables does the WORK.REVENUE data set contain?
A. 2
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