Chapter 5. Controlling Program Flow

In This Chapter

  • Controlling the flow through the program

  • Executing a group of statements repetitively

  • Avoiding infinite loops

The simple programs that appear in Chapters 1 through 4 process a fixed number of inputs, output the result of that calculation, and quit. However, these programs lack any form of flow control. They cannot make tests of any sort. Computer programs are all about making decisions. If the user presses a key, the computer responds to the command.

For example, if the user presses Ctrl+C, the computer copies the currently selected area to the Clipboard. If the user moves the mouse, the pointer moves on the screen. If the user clicks the right mouse button with the Windows key depressed, the computer crashes. The list goes on and on. Programs that don't make decisions are necessarily pretty boring.

Flow-control commands allow the program to decide what action to take based on the results of the C++ logical operations performed (see Chapter 4). There are basically three types of flow-control statements: the branch, the loop, and the switch.

Controlling Program Flow with the Branch Commands

The simplest form of flow control is the branch statement. This instruction allows the program to decide which of two paths to take through C++ instructions, based on the results of a logical expression (see Chapter 4 for a description of logical expressions).

In C++, the branch statement is implemented using the if statement:

if (m > n) { // Path 1 // ...

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