按索引从data.frame中提取元素

Question

Suppose I have a calendar-like data frame in R:

df = data.frame(Sun = c("*","*","*","*","*","*","*","*","*","*"),
             Mon= c("*","s","*","*","*","*","*","*","*","*"),
             Tues = c("*","*","*","*","*","*","*","*","*","*"),
             Wedn = c("*","*","*","*","*","*","*","*","*","*"),
             Thur = c("*","*","*","*","*","*","*","*","*","*"),
             Fri = c("*","*","*","*","*","*","*","*","*","*"),
             Sat = c("*","*","*","*","e","*","*","*","*","*"))

> df
   Sun Mon Tues Wedn Thur Fri Sat
1    *   *    *    *    *   *   *
2    *   s    *    *    *   *   *
3    *   *    *    *    *   *   *
4    *   *    *    *    *   *   *
5    *   *    *    *    *   *   e
6    *   *    *    *    *   *   *
7    *   *    *    *    *   *   *
8    *   *    *    *    *   *   *
9    *   *    *    *    *   *   *
10   *   *    *    *    *   *   *

We may index this calendar as following:

df_index[1,1] = 1
df_index[1,2] = 2 
.
.
.
df_index[2,1] = 8
df_index[2,2] = 9

so on and so forth. That is, df[1,1] is the first day and df[2,1] is the 8th day. (And df_index[,] is non-existing, just to help understand better).

What I want to do is to subset this data frame by index. For example, I want to extract from 9th day to 35th day and generate a new data frame:

  Sun Mon Tues Wedn Thur Fri Sat
1  NA   s    *    *    *   *   *
2   *   *    *    *    *   *   *
3   *   *    *    *    *   *   *
4   *   *    *    *    *   *   e

Answer 1

A week is nothing but 7 days so to get the week number you just have to divide it by 7. Taking the ceiling value as index and extracting subset should work!

> df[ceiling(9/7):ceiling(35/7),]
  Sun Mon Tues Wedn Thur Fri Sat
2   *   s    *    *    *   *   *
3   *   *    *    *    *   *   *
4   *   *    *    *    *   *   *
5   *   *    *    *    *   *   e

Answer 2

You can vectorize the dataframe such that the first element of this vector is the first day, etc.

   > x = as.vector(t(df))
   > x
    [1] "*" "*" "*" "*" "*" "*" "*" "*" "s" "*" "*" "*" "*" "*" "*" "*" "*" "*" "*"
    [20] "*" "*" "*" "*" "*" "*" "*" "*" "*" "*" "*" "*" "*" "*" "*" "e" "*" "*" "*"
    [39] "*" "*" "*" "*" "*" "*" "*" "*" "*" "*" "*" "*" "*" "*" "*" "*" "*" "*" "*"
    [58] "*" "*" "*" "*" "*" "*" "*" "*" "*" "*" "*" "*" "*"

To get the 9 to 35 day you simply do

> x[9:35]
 [1] "s" "*" "*" "*" "*" "*" "*" "*" "*" "*" "*" "*" "*" "*" "*" "*" "*" "*" "*"
[20] "*" "*" "*" "*" "*" "*" "*" "e"

You can then put this in a matrix/dataframe again. However, you also need first a NA value as you miss the first day of the week. So probably should work with ceiling/floor and dividing by 7, as suggested by the other answer.