用cat()和paste()连接字符串的区别是什么?

What are the differences between concatenating strings with cat and paste?

用cat和paste连接字符串的区别是什么?

In particular, I have the following questions.

特别是，我有以下几个问题。

1. Why does R not use the double quote (") when it prints the results of calling cat (but it uses quotes when using paste)?

   为什么R在打印调用cat的结果时不使用双引号(")(但在使用粘贴时使用引号)?

   > cat("test")test> paste("test")[1] "test"

2. Why do the functions length and mode, which are functions available for almost all objects in R, not "work" on cat?

   为什么函数长度和模式(在R中几乎对所有对象都可用)不能在cat上“工作”?

   > length(cat("test"))test[1] 0> mode(cat("test"))test[1] "NULL"

3. Why do C-style escape sequences work with cat, but not with paste?

   为什么C-style转义序列可以用cat，而不能用paste?

   > cat("1)Line1\n 2)Line2\n 3)Line3")1)Line1 2)Line2 3)Line3> paste("1)Line1\n 2)Line2\n 3)Line3")[1] "1)Line1\n 2)Line2\n 3)Line3"

4. Why doesn't R's recycling rule work with cat?

   为什么R的回收规则不能适用于猫?

   > cat("Grade", c(2, 3, 4, 5))Grade 2 3 4 5> paste("Grade", c(2, 3, 4, 5))[1] "Grade 2" "Grade 3" "Grade 4" "Grade 5"

1 个解决方案

#1

cat and paste are to be used in very different situations.

cat和paste是在非常不同的情况下使用的。

---

paste is not print

When you paste something and don't assign it to anything, it becomes a character variable that is print-ed using print.default, the default method for character, hence the quotes, etc. You can look at the help for print.default for understanding how to modify what the output looks like.

当您粘贴某个东西而不将其赋值给任何东西时，它就会成为一个字符变量，使用print.default、字符的默认方法、引号等进行打印。

• print.default will not evaluate escape characters such as \n within a character string.
• 默认情况下，不会在字符串中计算转义字符，例如\n。

Look at the answers to this question for how to capture the output from cat.

查看这个问题的答案，了解如何捕获cat的输出。

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Quoting from the easy to read help for cat (?cat)

引用易读的帮助猫(?猫)

Concatenate and Print

Description

Outputs the objects, concatenating the representations. cat performs much less conversion than print.

输出对象，连接表示。cat执行的转换比打印要少得多。

...

…

Details

cat is useful for producing output in user-defined functions. It converts its arguments to character vectors, concatenates them to a single character vector, appends the given sep= string(s) to each element and then outputs them.

cat用于在用户定义函数中生成输出。它将参数转换为字符向量，将它们连接到单个字符向量，将给定的sep= string(s)附加到每个元素，然后输出它们。

Value

None (invisible NULL).

(看不见的NULL)。

cat will not return anything, it will just output to the console or another connection.

cat不会返回任何内容，它只会输出到控制台或其他连接。

Thus, if you try to run length(cat('x')) or mode(cat('x')), you are running mode(NULL) or length(NULL), which will return NULL.

因此，如果您尝试运行length(cat('x'))或mode(cat('x'))，那么您正在运行mode(NULL)或length(NULL)，它将返回NULL。

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The help for paste is equally helpful and descriptive

粘贴的帮助同样有帮助和描述性

Concatenate Strings

Description

Concatenate vectors after converting to character.

转换为字符后连接向量。

....

....

Value

A character vector of the concatenated values. This will be of length zero if all the objects are, unless collapse is non-NULL in which case it is a single empty string.

连接值的字符向量。如果所有对象都是，那么它的长度将为0，除非折叠是非空的，在这种情况下，它是一个空字符串。