带索引的R lapply语句 [英] R lapply statement with index

问题描述

有没有一种方法可以使lapply语句也显示索引?更具体地说，请考虑以下示例:

Is there a way I can make an lapply statement also show the index? More specifically, consider the following example:

mylist <- list(c(5,4),c(3,2), c(1,3))

myfunction<- function(values){
  print("Adding values: ")
  return(values[1] + values[2])
}

lapply(mylist, myfunction)

有没有办法让我以某种方式打印"Adding Values:1"(添加值:1)"Adding Values:2"(添加值:2)等.列表中每个元素的一个元素?

Is there a way I can somehow make it print "Adding Values: 1" "Adding Values: 2" etc.. one element of each in the list?

谢谢！

推荐答案

函数mapply的作用类似于lapply，但允许您提供多个向量或列表.

The function mapply works like lapply but allows you to supply multiple vectors or lists.

在您的情况下，您可以创建第二个向量，该向量与列表的长度相同，只需向上计数即可:

In your case you can create a second vector, the same length of the list, just counting up:

mapply(myfunction, mylist, seq_along(mylist))

让我们尝试一下:

myfunction<- function(values, index){ 
  cat("Adding values (", index, "): ", values[1], "...", values[2], " = ", sum(values), "\n" )
  invisible(values[1] + values[2])
}

mylist <- list(c(5, 4), c(3, 2), c(1, 3))

mapply(myfunction, mylist, seq_along(mylist))

结果:

Adding values ( 1 ):  5 ... 4  =  9 
Adding values ( 2 ):  3 ... 2  =  5 
Adding values ( 3 ):  1 ... 3  =  4 

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高级用户乐趣

仅出于娱乐目的，仔细阅读?lapply手册页后，发现以下内容同样适用:

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Advanced user fun

Just for fun, a careful reading of the ?lapply manual page reveals that the following also works:

myfunction<- function(values){
    print(sprintf("Adding values: %i",substitute(values)[[3]]))
    return(values[1] + values[2])
}

lapply(mylist, myfunction)

这建议可以创建一个通用函数适配器来为原始函数(或任何其他函数)提供索引，并对其进行修改以期望有第二个索引参数:

Which suggests a general function adaptor could be created to supply index to your original function (or any other), modified to expect a second index argument:

myfunction<- function(values,index){
    print(sprintf("Adding values: %i",index))
    return(values[1] + values[2])
}

现在是适配器

lapply_index_adaptor=function(f)function(x,...)f(x,substitute(x)[[3]],...)

，现在是使用适配器的lapply调用:

and now the lapply call, with the adaptor:

lapply(mylist, lapply_index_adaptor(myfunction))

这篇关于带索引的R lapply语句的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！