The R Book

Addresses within Vectors

There are two important functions for finding addresses within arrays. The function which is very easy to understand. The vector y (see above) looks like this:

y

[1] 8 3 5 7 6 6 8 9 2 3 9 4 10 4 11

Suppose we wanted to know which elements of y contained values bigger than 5. We type

which(y>5)

[1] 1 4 5 6 7 8 11 13 15

Notice that the answer to this enquiry is a set of subscripts. We don't use subscripts inside the which function itself. The function is applied to the whole array. To see the values of y that are larger than 5, we just type

y[y>5]

[1] 8 7 6 6 8 9 9 10 11

Note that this is a shorter vector than y itself, because values of 5 or less have been left out:

length(y)

[1] 15

length(y[y>5])

[1] 9

To extract every nth element from a long vector we can use seq as an index. In this case I want every 25th value in a 1000-long vector of normal random numbers with mean value 100 and standard deviation 10:

xv<-rnorm(1000,100,10)
xv[seq(25,length(xv),25)]

 [1]     100.98176     91.69614     116.69185     97.89538     108.48568     100.32891     94.46233
 [8]     118.05943     92.41213     100.01887    112.41775     106.14260      93.79951    105.74173
[15]     102.84938     88.56408     114.52787     87.64789     112.71475     106.89868    109.80862
[22]      93.20438     96.31240      85.96460    105.77331      97.54514      92.01761     97.78516
[29]      87.90883     96.72253      94.86647     90.87149      80.01337      97.98327     92.77398
[36]     121.47810     92.40182      87.65205    115.80945      87.60231