C++ Notes: Idiom - cin loop

Idiom: Read from cin in a while loop

Example -- Adding numbers in the input

int sum = 0;
int x;

while (cin >> x) {
   sum = sum + x;
}

cout << sum;

Testing the value from an input operation

• Produces a value. The input operation with cin not only reads values into variables, but it also produces a value. That's because >> is an operator that produces a value. This value can be tested in loops and ifs.
• true. The value of cin >> x is true if a value was read into x.
• false. The value of cin >> x is false if it was unable to read. There are several possible causes for a reading failure.
  • EOF. When there is no more data, the EOF (End-Of-File) condition occurs. Every stream of input has an end so this is a normal event. It happens when reading a disk file and the end is reached. It's also possible to signal an EOF from the console by entering a special key combination, which depends on which operating system and C++ library you are using. Generally, you can use Control-Z followed by Enter to send an EOF from the keyboard. See End-Of-File (EOF).
  • Bad data. The read may fail if the data isn't formatted correctly. For example, when trying to read a floating-point temperature and the user types "zero", the read will fail.

Use this pattern when reading. Another style, which is not generally used is Anti-idiom - Using cin in three places, instead of one.