For loops are a fundamental tool in R that allow you to perform repetitive tasks efficiently. Whenever you need to execute the same block of code multiple times—such as processing each element of a vector or generating a sequence of outputs—a for loop is often the best approach. The basic structure of a for loop in R involves specifying a loop variable and a sequence of values to iterate over, followed by the statements you want to execute for each value.

In the example above, the sequence 1:5 tells the loop to iterate over the numbers 1 through 5. The loop variable i takes on each value in the sequence, one at a time. On each iteration, the print(i) statement outputs the current value of i to the console. This simple structure can be adapted to many repetitive tasks in R.

When you need to accumulate results—such as summing all elements in a vector—it's important to initialize a variable before the loop starts. In the previous example, sum_total is set to 0 before the loop begins. The loop then adds each value of num to sum_total, updating the total on every iteration. Initializing variables outside the loop ensures your results are correct and not accidentally reset during the process.

A frequent mistake in for loops is accidentally shifting the range of values, leading to off-by-one errors. For example, writing 1:length(vec) + 1 actually creates a sequence that starts at 2 and ends at length(vec) + 1, which can cause errors or unexpected behavior. Always check your loop's range and ensure you are iterating over the intended indices or values.