loops

Basic programming concepts in R 2.1 Loops A loop is typically a sequence of statements that is executed a specific number of times. How often the code ‘inside’ the loop is executed depends on a clearly defined control statement. If we know in advance how often the code inside of the loop has to be executed, we typically write a so-called ‘for-loop’. If the number of iterations is not clearly known before executing the code, we typically write a so-called ‘while-loop’. The following subsections illustrate both of these concepts in R. 2.1.1 For-loops In simple terms, a for-loop tells the computer to execute a sequence of commands ‘for each case in a set of n cases’. The flow-chart in Figure ?? illustrates the concept. For example, a for-loop could be used to sum up each of the elements in a numeric vector of fix length (thus the number of iterations is clearly defined). In plain English, the for-loop would state something like: “Start with 0 as the current total value, for each of the elements in the vector add the value of this element to the current total value.” Note how this logically implies that the loop will ‘stop’ once the value of the last element in the vector is added to the total. Let’s illustrate this in R. Take the numeric vector c(1,2,3,4,5). A for loop to sum up all elements can be implemented as follows:

2.1.2 Nested for-loops In some situations a simple for-loop might not be sufficient. Within one sequence of commands there might be another sequence of commands that also has to be executed for a number of times each time the first sequence of commands is executed. In such a case we speak of a ‘nested for-loop’. We can illustrate this easily by extending the example of the numeric vector above to a matrix for which we want to sum up the values in each column. Building on the loop implemented above, we would say ‘for each column j of a given numeric matrix, execute the for-loop defined above’.

2.1.3 While-loop In a situation where a program has to repeatedly run a sequence of commands but we don’t know in advance how many iterations we need in order to reach the intended goal, a while-loop can help. In simple terms, a while loop keeps executing a sequence of commands as long as a certain logical statement is true. The flow chart in Figure 2 illustrates this point. For example, a while-loop in plain English could state something like “start with 0 as the total, add 1.12 to the total until the total is larger than 20.” We can implement this in R as follows.

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