#R学习笔记
##Rstudio快捷键
ctrl+shift+c 对选中内容进行批量注释、解注释
Ctrl+Alt+R 运行当前文件
##R常用处理函数
round（）#四舍五入
round(target,digits=xx)
write.table(resource,target,append,quote)
length()#对象长度，R的index是从1开始的
is.na(a)
not variable
nan not a number
All the computing in R is element by element operation
names()
name of the component of one variable
c() v.s. list()
class(element_name[[]])
used to notificate the type of the object, embrace the element with a list
[],always get list
[[]], get the specific type of the element
always first row , second column in R(in setting)
to refer to certain position with []
LETTERS[] english characters
matrix() v.s. data.frame
as.data.frame() force convert to data frame
runif(number, intervalbegin,end)
which(condition)
cbind v.s. rbind (column and row)
order vs sort one for index, the other sequence
sapply(destination, operation)
head(destination, number of variable to release) see the head of the data.frame
tail
as.factor() find key to get access to the array
levels(object)
as.factor have a array and its levels ,levels for retrieval
as.numeric have the levels of each element in the array
detach("package:vegan", unload=TRUE)