R data.table fread using named colClasses without header (e.g. no col.names?)

问题

update (June 2016)

col.names was added on data.table 1.9.6 so issue is over and everyone super happy :) I think I can now convert all my read.csv calls to fread calls without worries of destruction

original question

using data.table 1.9.4

I'm importing read.csv calls to fread due to HUGE performance improvements we've noticed. Most issues I can handle but I've reached a point where I'm clueless and wonder if anyone has an elegent solution.

My problem is that I have named colClasses but the input has no header (it's a grep function), here's a silly example to make sense:

males.students <- read.csv(pipe("grep Male students.csv"), 
                           col.names=c("id", "name", "gender"), 
                           colClasses=(id="numeric"))

now in fread I still want the named colClasses but I have no col names so just using

males.students <- fread("grep Male students.csv"), 
                        colClasses=(id="numeric"))

fails with

Column name 'id' in colClasses[[1]] not found

How can I fix that? are there plans to add col.names?

回答1:

Add the names in the command line:

fread('echo "id,name,gender"; grep Male students.csv', colClasses = c(id='numeric'))

回答2:

Answering the original question, if the problem is that grep removes the header, you could use awk instead, to print the first line and any lines containing "Male":

fread("awk 'NR==1 || /Male/' students.csv"), colClasses=(id="numeric"))

This might help people that still use the old version of data.table.

来源：https://stackoverflow.com/questions/28602337/r-data-table-fread-using-named-colclasses-without-header-e-g-no-col-names