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Loops

A loop can be used to perform the same function repeatedly. For example:

for(i in 1:3){x12[i] <- x12[i] + 1}

1. for(i in 1:3){} sets up the loop to run over three values of i, 1, 2, and 3 for whatever commands are entered in the curly brackets {}.
2. x12[i] indicates the ith entry in x12. When i = 1, x12[i] is the same as x12[1]. When i = 2, x12[i] is the same as x12[2].
3. x12[i] <- x12[i] + 1

   Here the value in x12[i] is replaced by the value in x12[i] plus 1.

In this case the same result is obtained more simply by:

x12 <- x12 + 1

A generalization if the function is to be applied to each entry of x12 would be the following.

for(i in 1:length(x12)){x12[i] <- x12[i] + 1}

Here i runs from one to the length of the x12 vector.

The use of a temporary variable named i is arbitrary, another name could be used.

for(Joe in 1:length(x12)){x12[Joe] <- x12[Joe] + 1}

You might choose to apply a function only to entries for which a certain criterion is true.

for(i in 1:length(x12)){if(x12[i] < 4){x12[i] <- x12[i] + 1}}

Here the if() statement controls which entries in the vector x12 are modified.

In this case, only those entries that are less than 4 have 1 added.

Another way to accomplish the same thing would be as follows.

index12 <- x12 < 4x12[index12] <- x12[index12] + 1

Minimizing the use of loops can make R programs more efficient. Often the use of a loop can be avoided if operations are performed on whole objects at a time, as in:

x12 <- x12 + 1