007 Stata循环：forvalues

Stata中的循环有三类：

• while循环
• foreach循环
• forvalues循环
  其中，foreach和forvalues都可以看作是while循环的变种。两者的区别在于foreach跟的对象可以是宏、变量名和文件名等，而forvalues跟的必须是数字。

最后我们介绍 forvalues 循环

        forvalues lname = range {
                Stata commands referring to `lname'
        }

    where range is

                                                    #1(#d)#2      meaning #1 to #2 in steps of #d     
                                                    #1/#2         meaning #1 to #2 in steps of 1      
                                                    #1 #t to #2   meaning #1 to #2 in steps of #t - #1
                                                    #1 #t :  #2   meaning #1 to #2 in steps of #t - #1

    The loop is executed as long as calculated values of `lname' are < #2, assuming that #d > 0.

    Braces must be specified with forvalues, and

        1.  the open brace must appear on the same line as forvalues;

        2.  nothing may follow the open brace except, of course, comments; the first command to be executed must appear on a new line;

        3.  the close brace must appear on a line by itself.

*示例1
clear
set obs 100
gen x1 = rnormal()
gen x2 = rnormal()
gen x3 = rnormal()
gen x4 = rnormal()
gen x5 = rnormal()

forvalues i = 1(1)5 {
    disp ""
    disp ""
    disp "T-test: the mean of variable x`i' is 0"
    ttest x`i' = 0
}

*示例2
clear
cap mkdir C:\Users\Van\Desktop\download\paper
cd C:\Users\Van\Desktop\download\paper

forvalues i = 26068(1)26072 {
    cap copy https://www.nber.org/papers/w`i'.pdf w`i'.pdf, replace
    while _rc != 0 {
        disp "w`i'下载失败，10秒后重新下载"
        sleep 10000
        cap copy http://www.nber.org/papers/w`i'.pdf w`i'.pdf, replace
    }
disp "w`i'下载完成"
}

此处解释下cap和_rc。cap有两个作用：一是不显示结果，类似于qui，二是即使所在行的命令有问题也可继续执行后续命令。_rc是stata运行cap命令时生成的一个系统标量，如果命令未出错，则_rc值为0，且该命令执行；若该命令出错，则_rc值为对应的错误代码，且该命令不执行。

参考资料：
官方帮助文档
【爬虫俱乐部】精通Stata之数据整理
stata 编程里 开头的capture 是什么意思？