C++ All-In-One For Dummies®, 2nd Edition

Chapter 3. Reading with Input Streams

In This Chapter

• Reading with the extraction operators

• Dealing with the end of the file

• Reading various data types

• Reading data that is formatted with text

Well, isn't this nice. You have a file that you wrote to, but you need to read from it! After all, what good is a file if it's just sitting on your hard drive collecting dust?

In this chapter, we show you how you can read from a file. Reading a file is tricky because you can run into some formatting issues. For example, you may have a line of text in a file with a sequence of 50 digits. Do those 50 digits correspond to 50 one-digit numbers, or maybe 25 two-digit numbers, or some other combination? If you created the file, you probably know; but the fun part is getting your C++ program to properly read from them. The file might contain 25 two-digit numbers, in which case you make sure that the C++ code doesn't just try to read one enormous 50-digit number. In this chapter, we give you all the dirt on getting the dust off your hard drive and the file into memory. Have at it!

Extracting with Operators

When you read from a file, you can use the extraction operator, >>. This operator is very easy to use, provided you recognize that the phrase, "Look mom, no caveats!" just doesn't apply to the extraction operator.

Suppose you have a file called Numbers.txt with the following text on one line:

100 50 30 25

You can easily read in these numbers with the following code. First, make sure you #include <fstream> ...