For-loop can be used to iterate a range of numbers - or actually, any type of data structure that contains values after each other. (More of this in Chapter 5.) Thus, the for-loop is not only used to repeat commands or explore the content of files. The value of the iterator changes every time a new loop is started. We can consider the iterator as a stepper. The values of the iterator change one step at a time, or one round of a loop at a time. They go through all numbers in the range in a systematic manner, succession values after each other. Therefore, the iterator changes values until it has been each of the values in the range of numbers, or the data container.