Lesson 36: R ‘while’ Loops

We’ve seen R for loops in previous lessons, but there’s another kind of loop, while. It keeps iterating until some specified condition is met. We don’t know how many iterations will be needed, unlike the for case, with a fixed number of iterations.

As our example, consider AirPassengers, which consists of number of air travelers in thousands, in monthly data from January 1949. As usual, let’s glance at it first:

> str(airpass)
 Time-Series [1:144] from 1949 to 1961: 112 118 132 129 121 135 148 148 136 119 ...

Suppose we wish to know when the cumulative number of passengers first exceeded 10 million. A crude way would be to use R’s cumsum (“cumulative sums”) function:

> cumsum(airpass)
  [1]   112   230   362   491   612   747   895  1043  1179  1298  1402  1520
 [13]  1635  1761  1902  2037  2162  2311  2481  2651  2809  2942  3056  3196
 [25]  3341  3491  3669  3832  4004  4182  4381  4580  4764  4926  5072  5238
 [37]  5409  5589  5782  5963  6146  6364  6594  6836  7045  7236  7408  7602
 [49]  7798  7994  8230  8465  8694  8937  9201  9473  9710  9921 10101 10302
 [61] 10506 10694 10929 11156 11390 11654 11956 12249 12508 12737 12940 13169
 [73] 13411 13644 13911 14180 14450 14765 15129 15476 15788 16062 16299 16577
 [85] 16861 17138 17455 17768 18086 18460 18873 19278 19633 19939 20210 20516
 [97] 20831 21132 21488 21836 22191 22613 23078 23545 23949 24296 24601 24937
[109] 25277 25595 25957 26305 26668 27103 27594 28099 28503 28862 29172 29509
[121] 29869 30211 30617 31013 31433 31905 32453 33012 33475 33882 34244 34649
[133] 35066 35457 35876 36337 36809 37344 37966 38572 39080 39541 39931 40363

We see that that occurred in the 59th month. But though this approach would be convenient, it also would be wasteful: We are calculating all the cumulative sums, even though we don’t need them all. In a really long vector, this could be slow. Here is a less wasteful way:

 tot <- 0
> i <- 0
> while (i <= length(airpass) && tot < 10000) {
+    i <- i + 1
+    tot <- tot + airpass[i]
+ }
> i
[1] 59

So, the while loop keeps iterating until we get the desired cumulative total.

Key points here:

• The && operator stands for “and”.

• The condition within the while says that (a) we are not yet at the end of the airpass vector, AND (b) our total is still less than

• Note the need for the condition i <= length(airpass). It’s possible that tot will never exceed 10000 (not true here, but we wouldn’t know that a priori), so we need that condition so that the loop doesn’t iterate forever!

There’s more, though. The cumsum function is vectorized, so using it, though seemingly wasteful, may actually be faster than the loop