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@realalien
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比如第四章习题7(g),答案使用的是(f)的代码。这道题有些搞脑子,花了我一两个小时。
毕竟大家是在学习工具;当工具稍微复杂了点,就照葫芦画瓢吧,节省大家时间。
#(g) Extract the <name> node for all presidents whose term started in or after the year 1960. library(stringr) choose <- function(x) { start <- xmlValue(xmlChildren(x)[["start"]]) name <- xmlValue(xmlChildren(x)[["name"]]) year_pat <- "([0-9]{4}$)" year <- as.numeric( str_extract(start, year_pat )) after1960 <- ifelse( year >= 1960 , name, NA ) return(after1960) } temp <- xpathSApply(potus, "//document/president", choose) presidents_after_1960 <- temp[!is.na(temp)]
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