爲什麼`as.factor`在通過R中的apply函數應用時不起作用？

Question

我不知道爲什麼as.factor功能在通過apply功能應用於R時不起作用？

> df.nrow <- 10 
> df <- data.frame(col1=sample(c("a","b","c"), df.nrow, TRUE), 
+     col2=sample(c("d","e","f"), df.nrow, TRUE), 
+     col3=sample(c("g","h","i"), df.nrow, TRUE)) 
> apply(df, 2, is.factor) 
col1 col2 col3 
FALSE FALSE FALSE 
> df <- apply(df, 2, as.factor) 
> apply(df, 2, is.factor) 
col1 col2 col3 
FALSE FALSE FALSE 

Answer 1

我想這是因爲apply如何簡化返回矩陣的結果。從?apply：

If ‘X’ is not an array but an object of a class with a non-null 
    ‘dim’ value (such as a data frame), ‘apply’ attempts to coerce it 
    to an array via ‘as.matrix’ if it is two-dimensional (e.g., a data 
    frame) or via ‘as.array’. 

事實上，您的原始數據框是如你所願。嘗試使用str(df)或sapply(df, is.factor)來驗證它。除非stringsAsFactors=FALSE，基本上字符向量總是被強迫因素。