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Question

使用data.table柱更新內參數I具有以下數據表和函數，其提取的參數，並增加了它作爲數據表中的列：

library(stringr) 
library(data.table) 

pq <- c("cm_mmc=MSN-_-Exact-_-BrandExact-_-CheddarCheese" 
    ,"cm_mmc=Google-_-cheeseTypes-_-cheddar-_-cheesedelivered&gclid=CMXyy2D81MsCFcsW0w0dMVoPGw" 
    ,"cm_mmc=MSN-_-worldcitiesuslocations-_-cheese-_-cheeseshops" 
    ,"cm_mmc=MSN-_-worldcitiesuslocations-_-cheese-_-cheeseshops") 


rq <- c("q=cheese&src=IE-SearchBox&FORM=IESR02", 
        "sa=L", 
        "q=london+cheese+shop&src=IE-TopResult&FORM=IETR02&pc=WCUG", 
        "q=london+cheese+shop&src=IE-TopResult&FORM=IETR02&pc=WCUG") 

DT = data.table(page_urlquery = pq, refr_urlquery = rq) 

# Extracts a paramater from the relevant query and adds it to the table as a column 
extract_param <- function(dt, source = c("page_urlquery", "refr_urlquery"), param_name){ 
    source <- match.arg(source) 
    regexp <- paste("(?i)", param_name, "=([^&]+)", sep="") 
    col_name <- switch(source 
        ,"page_urlquery" = paste("url_", param_name, sep = "") 
        ,"refr_urlquery" = paste("ref_", param_name, sep = "") 
) 

    dt[,(col_name):= str_match((source), regexp)[,2]] 
} 

然而，當我調用函數作爲如下：

extract_param(DT, "page_urlquery", "cm_mmc") 

它創建列，但內容是空白的。我認爲它在數據表（源）參數中的語法有問題。我錯過了什麼？

Answer 1

改變內部函數的代碼從

dt[,(col_name):= str_match((source), regexp)[,2]] 

到

dt[,(col_name):= str_match(get(source), regexp)[,2]]