抑制函数结果中attr()的输出 [英] Suppress output from attr() in function result

问题描述

在data.frame的源代码中，最后三行代码设置属性并返回结果.

In the source code for data.frame, the last three lines of code set the attributes and return the result.

    ...
    attr(value, "row.names") <- row.names
    attr(value, "class") <- "data.frame"
    value
}

在我编写的函数中，结果是由lapply创建的命名列表.在函数主体中设置任何属性之前，结果如下.

In a function I wrote, the result is a named list created by lapply. Before I set any attributes in the function body, result is as follows.

> x <- data.frame(a = 1:5, b = letters[1:5])    
> (g <- grep.dataframe("a|c", x))
# ...    
# $b
#   value row
# 1     a   1
# 2     c   3
> attributes(g)  # I want "list" in here...
# $names
# [1] "a" "b"

我希望"class"包含在属性列表中，所以我在res之前添加attr(res, "class") <- "list"(res是最终结果).现在，类别"显示在属性列表中.但是，它也可以打印出我不想要的功能的结果.我尝试用invisible包装它，但是没有用.

I'd like "class" to be included in the attributes list, so I add attr(res, "class") <- "list" (res is the final result) just before res. "class" now shows up in the attributes list. However,it also prints out with the result of the function, which I don't want. I tried wrapping it with invisible, but that didn't work.

为什么手动分配的属性随函数结果一起打印，但在我创建的新数据框中却被抑制?

Why do the manually assigned attributes print with the function result, but are suppressed in a new data frame I create?

> (h <- grep.dataframe("a|c", x))
# ...    
# $b
#   value row
# 1     a   1
# 2     c   3

# attr(,"class")  # ...This prints with the result. I don't want that.
# [1] "list"
> attributes(h)   # ...But I want these attributes
# $names
# [1] "a" "b"

# $class
# [1] "list"

推荐答案

?class文档提供了一些指针:

The ?class documentation offers some pointers:

许多R对象都有一个class属性，一个字符向量给出了该对象继承自的类的名称.如果对象不具有类属性，则其具有隐式类矩阵"，数组".或mode(x)的结果(除了整数向量具有隐式类整数"). (函数oldClass和oldClass<-获取并设置属性，也可以直接完成.)

Many R objects have a class attribute, a character vector giving the names of the classes from which the object inherits. If the object does not have a class attribute, it has an implicit class, "matrix", "array" or the result of mode(x) (except that integer vectors have implicit class "integer"). (Functions oldClass and oldClass<- get and set the attribute, which can also be done directly.)

当将通用函数fun应用于具有类属性c("first"，"second")的对象时，系统搜索名为fun.first的函数，如果找到该函数，则将其应用于目的.如果找不到这样的函数，则尝试一个名为fun.second的函数.如果没有任何类名会产生合适的函数，则使用fun.default函数(如果存在).如果没有class属性，则尝试使用隐式类，然后使用默认方法.

When a generic function fun is applied to an object with class attribute c("first", "second"), the system searches for a function called fun.first and, if it finds it, applies it to the object. If no such function is found, a function called fun.second is tried. If no class name produces a suitable function, the function fun.default is used (if it exists). If there is no class attribute, the implicit class is tried, then the default method.

从那开始并运行一些简单的测试，我得到了这一点:

From that and running a few simple tests, I gather that:

• 列表是这些隐式类之一:请参见attributes(list(1))，typeof(list(1))
• 在列表上调用print时，它使用的是print.default
• print.default打印对象的属性

• a list is one of these implicit classes: see attributes(list(1)), typeof(list(1))
• when print is called on a list, it is using print.default
• print.default prints the attributes of an object

因此您可以定义一个print.list来处理您的特殊情况:

So you could define a print.list that will handle your special case:

 print.list <- function(x, ...) {
    if (is.list(x)) attr(x, "class") <- NULL
    print.default(x, ...)
 }

res <- list(1)
attr(res, "class") <- "list"
res
# [[1]]
# [1] 1

attributes(res)
# $class
# [1] "list"

这篇关于抑制函数结果中attr()的输出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！