How to setup the tsp attribute of a dataset

The first two components of the tsp vector are in time units as
mentioned in ?tsp . Thus you would want:

x <- 1:100
tsp(x) <- c(1, 25.75, 4)

but normally you don't have to set it explicitly in the first place.
Just use ts:

x <- 1:100
x.ts <- ts(x, start = 1, frequency = 4)
tsp(x.ts)

[1]  1.00 25.75  4.00

x.ts

Qtr1 Qtr2 Qtr3 Qtr4
1     1    2    3    4
2     5    6    7    8
3     9   10   11   12
4    13   14   15   16
5    17   18   19   20
6    21   22   23   24
7    25   26   27   28
8    29   30   31   32
9    33   34   35   36
10   37   38   39   40
11   41   42   43   44
12   45   46   47   48
13   49   50   51   52
14   53   54   55   56
15   57   58   59   60
16   61   62   63   64
17   65   66   67   68
18   69   70   71   72
19   73   74   75   76
20   77   78   79   80
21   81   82   83   84
22   85   86   87   88
23   89   90   91   92
24   93   94   95   96
25   97   98   99  100

Hello,
I am wondering how I should set up the tsp attribute (available through
attr(x, "tsp")) of a dataset x? Let's assume that x has 100 points, and I
want to set the frequency to 4.
I tried:

attr(x,"tsp")<-c(1,100,4)

Error in attr(x, "tsp") <- c(1, 100, 4) :
?invalid time series parameters specified
Is there any other way to set the frequency of the data?
Thanks.
-Samik

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