//清除内存
clear
//设置5个样本
set obs 5
//增加变量v1,都是this is the day
gen v1 = "this is the my is day"
//增加新变量，v1_replace，其值是替换v1中的第一个is为X
gen v1_replace = subinstr(v1,"is","X",1)
// v1 v1_replace
// this is the my is day thX is the my is day
// this is the my is day thX is the my is day
// this is the my is day thX is the my is day
// this is the my is day thX is the my is day
// this is the my is day thX is the my is day
//增加新变量，v2_replace，其值是替换v1中的第一个 第二个is为X
gen v2_replace = subinstr(v1,"is","X",2)
order v2_replace ,after(v1)
// v1 v2_replace
// this is the my is day thX X the my is day
// this is the my is day thX X the my is day
// this is the my is day thX X the my is day
// this is the my is day thX X the my is day
// this is the my is day thX X the my is day
//增加新变量，v3_replace，其值是替换v1中的所有is为X
gen v3_replace = subinstr(v1,"is","X",.)
order v3_replace ,after(v1)
// v1 v3_replace
// this is the my is day thX X the my X day
// this is the my is day thX X the my X day
// this is the my is day thX X the my X day
// this is the my is day thX X the my X day
// this is the my is day thX X the my X day
