As pointed out in chapter 2, a file of data is normally required in a sorted form. An author index in a library would be of little use if the cards were filed randomly. Similarly, a telephone directory would be valueless if all the entries were in telephone number order. Numerical ordering is necessary, of course, in a great many activities — imagine the problems that would be created if householders were allowed to choose their own house numbers! In some cases, however, ordering will be required on data that is a mixture of letters and numbers; lists of Premium Bond winning numbers provide a good example of this.
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