blocks|key|1601487|text|生成和计算字符串|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|1601488|Q1+=+1,+Q2+=+2,+Q3+=+3,+Q4+=+4不像"Q1+=+1,+Q2+=+2,+Q3+=+3,+Q4+=+4"是字符串那样是字符串。有一些R函数将接受字符串对象并将其计算为代码。例如：|CODE|1601489|>+eval(parse(text="print('hello+world')"))

#>+[1]+"hello+world"|code-block|syntax|javascript|1601490|但是，在dbplyr翻译中，这可能不太好。如果你设法让这种方法奏效，最好能把它作为一个答案发布出来。|1601491|使用循环|1601492|与其将其作为单个字符串执行，另一种方法是使用循环：|1601493|db+<-+tbl(con,+"mtcars")+%25>%25
++++select(carb)+%25>%25
++++distinct(carb)+%25>%25
++++arrange(carb)

for(i+in+1:n){
++++var+=+paste0("Q",i)
++++db+<-+db+%25>%25
++++++++mutate(!!sym(var)+:=+i)
}

db+<-+collect(db)|1601494|!!sym()必须告诉dplyr，您希望将文本参数作为变量处理。懒惰的评估可以给你奇怪的结果，没有它。需要进行:=分配，因为需要对LHS进行评估。|1601495|这种方法大致相当于每个变量的一个可变语句(例如下面的例子)，但是dbplyr转换可能不像在一个变体语句中那样优雅。|1601496|db+<-+tbl(con,+"mtcars")+%25>%25
++++select(carb)+%25>%25
++++distinct(carb)+%25>%25
++++arrange(carb)+%25>%25
++++mutate(Q1+=+1)+%25>%25
++++mutate(Q2+=+2)+%25>%25
++++...
++++mutate(Qn+=+n)+%25>%25
++++collect()|1601497|entityMap^0|0|8|0|0|U|W|W|0|0|4|6|0|0|0|0|0|7|B|5|1J|2|0|W|6|0|0^^$0|@$1|2|3|4|5|6|7|13|8|@$9|14|A|15|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|16|8|@$9|17|A|18|B|H]|$9|19|A|1A|B|H]]|D|@]|E|$]]|$1|I|3|J|5|K|7|1B|8|@]|D|@]|E|$L|M]]|$1|N|3|O|5|6|7|1C|8|@$9|1D|A|1E|B|H]]|D|@]|E|$]]|$1|P|3|Q|5|6|7|1F|8|@]|D|@]|E|$]]|$1|R|3|S|5|6|7|1G|8|@]|D|@]|E|$]]|$1|T|3|U|5|K|7|1H|8|@]|D|@]|E|$L|M]]|$1|V|3|W|5|6|7|1I|8|@$9|1J|A|1K|B|H]|$9|1L|A|1M|B|H]|$9|1N|A|1O|B|H]]|D|@]|E|$]]|$1|X|3|Y|5|6|7|1P|8|@$9|1Q|A|1R|B|H]]|D|@]|E|$]]|$1|Z|3|10|5|K|7|1S|8|@]|D|@]|E|$L|M]]|$1|11|3|-4|5|6|7|1T|8|@]|D|@]|E|$]]]|12|$]]

Generating and evaluating the string

<code>Q1 = 1, Q2 = 2, Q3 = 3, Q4 = 4</code> is not a string in the same way that <code>"Q1 = 1, Q2 = 2, Q3 = 3, Q4 = 4"</code> is a string. There are some R functions that will take a string object and evaluate it as code. For example:

<pre class="lang-r prettyprint-override"><code>&gt; eval(parse(text="print('hello world')"))

#&gt; [1] "hello world"
</code></pre>

However, this may not play nicely inside <code>dbplyr</code> translation. If you manage to get something like this approach working it would be good to see it posted as an answer.

Using a loop

Instead of doing it as a single string, an alternative is to use a loop:

<pre class="lang-r prettyprint-override"><code>db &lt;- tbl(con, "mtcars") %&gt;%
 select(carb) %&gt;%
 distinct(carb) %&gt;%
 arrange(carb)

for(i in 1:n){
 var = paste0("Q",i)
 db &lt;- db %&gt;%
 mutate(!!sym(var) := i)
}

db &lt;- collect(db)
</code></pre>

The <code>!!sym()</code> is required to tell <code>dplyr</code> that you want the text argument treated as a variable. Lazy evaluation can give you odd results without it. The <code>:=</code> assignment is required because the LHS needs to be evaluated.

This approach is roughly equivalent to one mutate statement for each variable (example below), but the <code>dbplyr</code> translation might not look as elegant as doing it all within a single mutate statement.

<pre class="lang-r prettyprint-override"><code>db &lt;- tbl(con, "mtcars") %&gt;%
 select(carb) %&gt;%
 distinct(carb) %&gt;%
 arrange(carb) %&gt;%
 mutate(Q1 = 1) %&gt;%
 mutate(Q2 = 2) %&gt;%
 ...
 mutate(Qn = n) %&gt;%
 collect()
</code></pre>

blocks|key|1601508|text|我最近读到了更多关于这个主题的文章，我发现下面的代码工作得很好，因此dbplyr编写了一个更干净的SQL代码。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1601509|#+Libraries

library(RSQLite)
library(dplyr)
library(dbplyr)
library(DBI)

#+Example+database

con+<-+DBI::dbConnect(RSQLite::SQLite(),+path+=+":memory:")

copy_to(con,+mtcars,+"mtcars",+temporary+=+FALSE)

#+Parameter+for+number+of+variables+to+be+created

n+<-+4+

#+Variable+list

var+<-+list()+
for(i+in+1:n){
++++j+<-+paste0("Q",+i)+
++++var[[j]]+<-+i
}+

#+Query/computation

db+<-+tbl(con,+"mtcars")+%25>%25+
++++select(carb)+%25>%25+
++++distinct(carb)+%25>%25+
++++arrange(carb)+%25>%25+
++++mutate(!!!+var)+%25>%25+
++++show_query()+%25>%25+
++++collect()|code-block|syntax|javascript|1601510|诀窍是构建一个具有正确名称的列表，并使用!!!将其放入!!!函数中。此外，我读到应该避免解析和计算字符串，所以我切换到列表。|offset|length|style|CODE|1601511|entityMap^0|0|0|K|3|R|3|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@$I|R|J|S|K|L]|$I|T|J|U|K|L]]|9|@]|A|$]]|$1|M|3|-4|5|6|7|V|8|@]|9|@]|A|$]]]|N|$]]

I recently read more about the topic and I found that the following code works quite nicely, causing dbplyr to write a cleaner SQL code.

<pre><code># Libraries

library(RSQLite)
library(dplyr)
library(dbplyr)
library(DBI)

# Example database

con &lt;- DBI::dbConnect(RSQLite::SQLite(), path = ":memory:")

copy_to(con, mtcars, "mtcars", temporary = FALSE)

# Parameter for number of variables to be created

n &lt;- 4 

# Variable list

var &lt;- list() 
for(i in 1:n){
 j &lt;- paste0("Q", i) 
 var[[j]] &lt;- i
} 

# Query/computation

db &lt;- tbl(con, "mtcars") %&gt;% 
 select(carb) %&gt;% 
 distinct(carb) %&gt;% 
 arrange(carb) %&gt;% 
 mutate(!!! var) %&gt;% 
 show_query() %&gt;% 
 collect()
</code></pre>

The trick was to build a list with proper names and to put it into the <code>mutate()</code> function using <code>!!!</code>. Furthermore, I read that parsing and evaluating strings should be avoided, so I switched to lists.

I asked a similar question before (<a href="https://stackoverflow.com/questions/59440107/how-can-i-dynamically-create-new-variables-columns-on-databases-in-r-using-dplyr">Link</a>). The given answer works fine. However, it turns out, that it does not fully apply to my use case.

Please consider the following minimal working example:

<pre><code>library(RSQLite)
library(dplyr)
library(dbplyr)
library(DBI)
library(stringr)

con &lt;- DBI::dbConnect(RSQLite::SQLite(), path = ":memory:")

copy_to(con, mtcars, "mtcars", temporary = FALSE)

db &lt;- tbl(con, "mtcars") %&gt;%
 select(carb) %&gt;%
 distinct(carb) %&gt;%
 arrange(carb) %&gt;% 
 mutate(Q1=1, Q2=2, Q3=3, Q4=4) %&gt;% 
 collect()
</code></pre>

I am interested in dynamically building the string <code>Q1=1, Q2=2, Q3=3, Q4=4</code> such that it could be <code>Q1=1, Q2=2, ..., Qn = n</code>.

One idea I had is to build the string like that:

<pre><code>n_par &lt;- 4
str_c('Q', 1:n_par, ' = ', 1:n_par, collapse = ', ')
</code></pre>

such that n_par could be any positive number. However, due to dplyr's non-standard evaluation, I cannot make it work like that. However, this is exactly what I need.

Can somebody help?

How can I dynamically build a string and pass it to dplyr's mutate() function in R?

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我以前问过一个类似的问题()。给出的答案很好。然而，事实证明，它并不完全适用于我的用例。请考虑以下最低限度的工作实例：library(RSQLite)library(dplyr)library(dbplyr)library(DBI)library(stringr)con <- DBI::dbConnect(RSQLit...

问如何动态构建字符串并将其传递给R中dplyr的mutate()函数？
EN

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问如何动态构建字符串并将其传递给R中dplyr的mutate()函数？EN