Finding a mean value of velocity by month (using R)

Question

I have data table here:

row      V1      velocity

1   2009-04-06  95.9230769230769

2   2009-04-11  95.0985074626866

3   2009-04-17  95.8064935064935

4   2009-04-22  94.6357142857143

5   2009-04-27  95.3626865671642

6   2009-05-03  95.9101265822785

7   2009-05-08  95.826582278481

8   2009-05-14  94.5126582278481

9   2009-05-20  95.8371428571429

10  2009-05-25  94.6981481481481

11  2009-05-30  96.397619047619

12  2009-06-05  94.8132530120482

13  2009-06-10  96.4558139534884

14  2009-06-16  94.9627906976744

15  2009-06-21  95.2666666666667

16  2009-06-26  95.2919540229885

17  2009-07-01  95.4333333333333

18  2009-07-07  95.3375

19  2009-07-12  95.0534246575343

20  2009-07-18  96.0277777777778

21  2009-07-24  95.6885057471264

22  2009-07-29  93.9375

23  2009-08-03  95.2776315789474

24  2009-08-08  94.9089285714286

25  2009-08-13  96.8906976744186

26  2009-08-19  95.4487804878049

27  2009-08-24  97.2444444444444

28  2009-08-30  95.1174418604651

I want to write a r code to find a mean value of velocity by month. (There are May, June, July, and August.

What could I do?

I am mesmerized by the speed of response. Thanks! – jamryu May 21 '15 at 17:07 — jamryu, May 21 '15 at 17:07

score 3 · Answer 1 · answered May 21 '15 at 17:05

3

Or jusr:

tapply(df$velocity, months(as.Date(df$V1)), mean)
   April   August     Juli     Juni      Mai 
95.36530 95.81465 95.24634 95.35810 95.53038

answered May 21 '15 at 17:05

DatamineR

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Or `aggregate(velocity ~ as.POSIXlt(V1)$mon, df, mean)` or `aggregate(velocity ~ months(as.Date(df$V1)), df, mean)` – David Arenburg May 21 '15 at 17:08

score 2 · Accepted Answer · answered May 21 '15 at 17:05

Here's how I would do it

Use lubridate to create a month variable to group by in dplyr and then get means.

library(lubridate)
library(dplyr)

df %>% group_by(month = month(df$V1)) %>% summarize(mean = mean(velocity))

         month     mean
1            4 95.36530
2            5 95.53038
3            6 95.35810
4            7 95.24634
5            8 95.81465

If you add label=T you get this:

df %>% group_by(month = month(df$V1,label=T)) %>% summarize(mean = mean(velocity))

  month     mean
1   Apr 95.36530
2   May 95.53038
3   Jun 95.35810
4   Jul 95.24634
5   Aug 95.81465

Why would you load `lubridate` if you have same functions in base R? — David Arenburg, May 21 '15 at 17:09

Finding a mean value of velocity by month (using R)

2 Answers2