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Think Julia by Allen B. Downey, Ben Lauwens

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Chapter 14. Files

This chapter introduces the idea of persistent programs that keep data in permanent storage, and shows how to use different kinds of permanent storage, like files and databases.

Persistence

Most of the programs we have seen so far are transient, in the sense that they run for a short time and produce some output, but when they end, their data disappears. If you run the program again, it starts with a clean slate.

Other programs are persistent: they run for a long time (or all the time); they keep at least some of their data in permanent storage (a hard drive, for example); and if they shut down and restart, they pick up where they left off.

Examples of persistent programs are operating systems, which run pretty much whenever a computer is on, and web servers, which run all the time, waiting for requests to come in on the network.

One of the simplest ways for programs to maintain their data is by reading and writing text files. We have already seen programs that read text files; in this chapter we will see programs that write them.

An alternative is to store the state of the program in a database. In this chapter I will also present how to use a simple database.

Reading and Writing

A text file is a sequence of characters stored on a permanent medium like a hard drive or flash memory. We saw how to open and read a file in “Reading Word Lists”.

To write a file, you have to open it with mode "w" as a second parameter:

julia> fout = open("output.txt", "w")
IOStream(<file ...

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