对数据表进行行操作 [英] Row wise operation on data.table

问题描述

假设我想在一行一行的基础上计算几个列的范围的幅度。

Let's say I'd like to calculate the magnitude of the range over a few columns, on a row-by-row basis.

set.seed(1)
dat <- data.frame(x=sample(1:1000,1000),
                  y=sample(1:1000,1000),
                  z=sample(1:1000,1000))

使用data.frame（），我会这样做：

Using data.frame(), I would do something like this:

dat$diff_range <- apply(dat,1,function(x) diff(range(x)))

为了更简单，我在寻找这个操作，在每一行：

To put it more simply, I'm looking for this operation, over each row:

diff(range(dat[1,]) # for i 1:nrow(dat)

如果我为整个表，它将是：

If I were doing this for the entire table, it would be something like:

setDT(dat)[,diff_range := apply(dat,1,function(x) diff(range(x)))]

）rows？

推荐答案

pmax 和 以向量化方式查找列之间的最小值和最大值，这比单独分割和处理每一行要好得多。它也很简洁：

pmax and pmin find the min and max across columns in a vectorized way, which is much better than splitting and working with each row separately. It's also pretty concise:

dat[, r := do.call(pmax,.SD) - do.call(pmin,.SD)]


        x   y   z   r
   1: 266 531 872 606
   2: 372 685 967 595
   3: 572 383 866 483
   4: 906 953 437 516
   5: 201 118 192  83
  ---                
 996: 768 945 292 653
 997:  61 231 965 904
 998: 771 145  18 753
 999: 841 148 839 693
1000: 857 252 218 639

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