将JSON数据拉入R Dataframe

Question

我试图从USNO API中提取JSON月球周期数据。问题是我得到的是两个JSON数据数组。我没有看到一种方法来指定我从天文台得到的东西，所以我想我需要在R中清理它。这是我的代码：

library(sqldf);
library(jsonlite);

curr_date <- Sys.Date();
Q_date <- format.Date(curr_date, "%m/%d/%Y");
moon_call <- paste0("http://api.usno.navy.mil/moon/phase?date=",Q_date,"&nump=4");

moon_json <- fromJSON(moon_call, simplifyDataFrame =  TRUE);

moon_phases <- do.call("rbind.fill", lapply(moon_json$phasedata, as.data.frame));

我得到的数据如下所示：

"","error","apiversion","year","month","day","numphases","datechanged","phasedata.phase","phasedata.date","phasedata.time"
"1",FALSE,"2.1.0",2018,8,29,4,FALSE,"Last Quarter","2018 Sep 03","02:37"
"2",FALSE,"2.1.0",2018,8,29,4,FALSE,"New Moon","2018 Sep 09","18:01"
"3",FALSE,"2.1.0",2018,8,29,4,FALSE,"First Quarter","2018 Sep 16","23:15"
"4",FALSE,"2.1.0",2018,8,29,4,FALSE,"Full Moon","2018 Sep 25","02:52"

将它转换为数据框时，我得到：

"","X[[i]]"
"1","Last Quarter"
"2","New Moon"
"3","First Quarter"
"4","Full Moon"
"5","2018 Sep 03"
"6","2018 Sep 09"
"7","2018 Sep 16"
"8","2018 Sep 25"
"9","02:37"
"10","18:01"
"11","23:15"
"12","02:52"

但我想要的是选择了phasedata.phase/.date/.time列的数据框：

"","phase","date","time"
"1","Last Quarter","2018 Sep 03","02:37"
"2","New Moon","2018 Sep 09","18:01"
"3","First Quarter","2018 Sep 16","23:15"
"4","Full Moon","2018 Sep 25","02:52"

Answer 1

1. R允许您直接从数据框extract the three columns moon_json，如您所愿： moon_phases <- moon_json[, c('phasedata.phase', 'phasedata.date', 'phasedata.time')]

（do.call("rbind.fill", lapply(..., as.data.frame))不需要任何东西 - 这只是一种低效且折磨的切片方式，然后连接起来。）

2. 然后你想重命名你的df列以删除phasedata.前缀： names(moon_phases) <- c('phase', 'date', 'time')

或者：names(moon_phases) <- gsub('^phasedata\.', '', names(moon_phases))

3. 此外，您通常不希望在1,2,3...等数据框架上显式rownames moon_json，所以只需要做row.names(moon_json) <- NULL或data.frame(..., row.names=NULL)或as.data.frame(..., row.names=NULL)

（jsonlite（或其他R json软件包之一）应该有自动清理和重命名的选项，我不知道，我不会使用它们。检查出来并选择一个让刮擦更少痛苦的包装。）