R 和 Python 中的双重 for 循环

Question

我正在尝试在 R 中转换 Python 中的双 for 循环。当我在粘贴函数中使用它时，它可以正常工作。我试图弄清楚为什么没有它就无法访问这些。

在 Python 中

l1 = ['appetizer','main course']
l2 = ['italian','mexican','french']

for i in l1:
    for j in l2:
        print(i,j)

在R中

l1 = list('appetizer','main course')
l2 = list('italian','mexican','french')

这会引发错误

for (i in l1) {
  for (j in l2) {
    print(i,j)
  }
}

错误

Error in print.default(i, j) : invalid 'digits' argument
In addition: Warning message:
In print.default(i, j) : NAs introduced by coercion
> for (i in l1) {

这不会引发错误

for (i in l1) {
  for (j in l2) {
    print(paste(i,j,sep=","))
  }
}

输出

[1] "appetizer,italian"
[1] "appetizer,mexican"
[1] "appetizer,french"
[1] "main course,italian"
[1] "main course,mexican"
[1] "main course,french"

Answer 1

索引之间存在差异。 Python 将向量（数组）作为索引传递，而 R 传递值。请注意，索引以 1 开头，即 1, 2, ..., n。您还需要concatenate 要打印的对象，将它们作为单个参数传递。因此，您可以执行以下操作：

l1 <- list('appetizer', 'main course')
l2 <- list('italian', 'mexican', 'french')

for (i in seq_along(l1)) {
  for (j in seq_along(l2)) {
    print(c(l1[[i]], l2[[j]]))
  }
}
# [1] "appetizer" "italian"  
# [1] "appetizer" "mexican"  
# [1] "appetizer" "french"   
# [1] "main course" "italian"    
# [1] "main course" "mexican"    
# [1] "main course" "french"  

Answer 2

请参阅下面对print 方法的说明。

print(x, digits = NULL, quote = TRUE,
  na.print = NULL, print.gap = NULL, right = FALSE,
  max = NULL, useSource = TRUE, …)

x       the object to be printed.

digits  a non-null value for digits specifies the minimum number of 
        significant digits to be printed in values. The default, NULL, uses 
        getOption("digits"). (For the interpretation for complex numbers see 
        signif.) Non-integer values will be rounded down, and only values 
        greater than or equal to 1 and no greater than 22 are accepted.
...

显然第二个变量不被接受为有效参数。

虽然这有点奇怪，但更重要的一点是 print(a,b) 不是打印多个值的语法正确方法。

所以你可以继续使用paste()函数

或者像print(c(l1[[i]], l2[[j]]))这样的索引