r - R create strings in a loop

Question

How can I create variables and strings using the value passed by the loop?

---

Example

I have a set of countries = c('USA','Canada','Mexico').

I have a dataframe 'population'.

I want to query my database to get their population, and assign it to a column in a data frame. Don't worry about accessing the database, I am only concerned with dynamically creating the query string and the dataframe column name.

for (country in countries) {
   query = "SELECT population FROM population_database WHERE location='country';"
   population$country = mysql_query(query) 
}

Answer 1

您可能正在寻找该paste功能。

x <- "USA"
paste("location =", x)
#[1] "location = USA" 

Answer 2

现在，如果您想创建一个 for 循环然后创建新的字符串名称，您可以这样做

a=c()
for (i in 1:5) {
    d=paste("location = ", i)
    a=c(a,d)
}

因此，

Answer 3

你想要population[[country]]而不是population$country. 该$符号是[[“两点之间的最长距离是一条捷径”的捷径。

其他已经提到的pasteand past0，你也可以使用sprintforgsubfn包。