标题: 能帮我看看我写的code到底哪里错了 为什么y不累计呢 ？ [打印本页] 作者: shiyiming 时间: 2009-9-13 12:28 标题: 能帮我看看我写的code到底哪里错了 为什么y不累计呢 ？ During World War II German submarines (U-boats) patrolled the Atlantic ocean looking for enemy ships to sink. Suppose a U-boat has 16 torpedoes (underwater missiles) to start off with when they left their U-boat pens in occupied France.
Suppose that each U-boat attack on an enemy ship uses 1 torpedo with probability 0.7, 2 torpedoes with probability 0.2, and 3 torpedoes with probability 0.1. (This is because big targets may require more hits and some torpedoes miss their target etc.)
We wish to estimate Y = the total number of enemy ships that will be attacked until the U-boat has run out of torpedoes. This includes the last ship even if there aren‟t enough torpedoes to sink it.
(a) Write a SAS program to simulate the distribution of Y. Perform 1000 simulations. If you wish to write a SAS macro then you may assume that the code below has been given to you and your code to be inserted at the position of the /*******/. Alternatively, you could attempt to do the simulation entirely within a data step.
%MACRO uboat(numiters);
%DO iter=1 %TO &numiters;
/**********/
/* Append results to dataset simout */ DATA simout; %IF &iter=1 %THEN %DO; SET sim; %END; %ELSE %DO; SET simout sim; %END; RUN; %END; /* End of %DO loop */
PROC MEANS DATA=simout; VAR Y; TITLE "Summary statistics of the ships attacked; &numiters iterations"; RUN;
PROC FREQ DATA=simout;
TABLES Y; RUN;
%MEND(uboat);
%uboat(1000);
[color=#FF0080:1pdevxd7]%macro uboat(numiters);
%do iter=1 %to &numiters;
data sim;
num=0;
Y=0;
keep Y;
%do %WHILE (num LE 16);
ship=ranuni(0);
if 0 LE ship LE 0.7 then num=num+1 ;
else if ship GE 0.9 then num=num+3 ;
else num=num+2;
Y=Y+1;
%end;
run;
data simout;
%if &iter=1 %then %do;
set sim;
%end;
%else %do;
set simout sim;
%end;
run;
%end;
proc means data=simout;
var Y;
run;
proc freq data=simout;
tables Y;
run;
%mend uboat;
%uboat(1000);[/color:1pdevxd7]
[/color]作者: shiyiming 时间: 2009-9-13 14:50 标题: Re: 能帮我看看我写的code到底哪里错了 为什么y不累计呢 ？ 应该使用do while语句而不是%do %while语句,代码可以改写一下,可参考jingju11的帖子
[code:2c079e3k]%macro uboat(numiters);
%do iter=1 %to &numiters;
data sim;
num=0;
Y=0;
keep Y;
do WHILE (num LE 16);
ship=ranuni(0);
if 0 LE ship LE 0.7 then num=num+1 ;
else if ship GE 0.9 then num=num+3 ;
else num=num+2;
Y=Y+1;
end;
run;
data simout;
%if &iter=1 %then %do;
set sim;
%end;
%else %do;
set simout sim;
%end;
run;
%end;
proc means data=simout;
var Y;
run;
proc freq data=simout;
tables Y;
run;
%mend uboat;
这个应该到１６就停了阿 怎么还有１７呢！！！！作者: shiyiming 时间: 2009-9-13 16:48 标题: Re: 能帮我看看我写的code到底哪里错了 为什么y不累计呢 ？ if语句的条件写的有问题呗
[code:10wpr2on]if 0 LE ship LE 0.7 then num=num+1 ;
else if ship LE 0.8 then num=num+2 ;
else num=num+3;[/code:10wpr2on]作者: shiyiming 时间: 2009-9-13 17:29 标题: Re: 能帮我看看我写的code到底哪里错了 为什么y不累计呢 ？ ５５５５５５ 还是错的 ｙ还是有１７ 作者: shiyiming 时间: 2009-9-13 18:31 标题: Re: 能帮我看看我写的code到底哪里错了 为什么y不累计呢 ？ 奥,是DO WHILE循环的条件不对
[code:2i7lezp5]do while(num lt 16);
or
do until(num ge 16);[/code:2i7lezp5]作者: shiyiming 时间: 2009-9-14 09:17 标题: Re: 能帮我看看我写的code到底哪里错了 为什么y不累计呢 ？ missiles 数量可以（假设性的）大于16。想一想如下情形：uboat只剩下一个missile（用了15）。但是此时它仍然可以攻击需要两个missiles的船只：“This includes the last ship even if there aren‟t enough torpedoes to sink it.”所以只要已用的missiles小于16，最后的攻击必须做出，而不管你遇到了什么样的敌船（这样问题简单些）。
另外你的结果不必要和给定的相同。除非他用的是固定seed random number generator。如果他是，你用同样的seed，结果应该是相同的。作者: shiyiming 时间: 2009-9-14 20:50 标题: Re: 能帮我看看我写的code到底哪里错了 为什么y不累计呢 ？ [quote:2q9qwhbk]do while(num lt 16);
or
do until(num ge 16);[/quote:2q9qwhbk]
这个条件语句， 从hopewell来的，是正确的。
[code:2q9qwhbk]
data sampling;
do sample=1 to 1000;
Y=0;
missiles=0;
do until(missiles >= 16);*number of missiles may be presumably more than 16,that should be Okay;
ran=ranuni(1);
missiles+ifn(ran<=0.7,1,ifn(ran<=0.9,2,3));*at last attack, u-boat may need more missiles than number of missiles left,but that still counts;
Y+1; *ships attacked and damaged;
/* output;*/
end;
output;
end;
run;[/code:2q9qwhbk]