Entering an End-of-File from the Keyboard

When we enter input to a program from the keyboard, different operating systems use different conventions to allow us to indicate end-of-file. On Windows systems we enter an end-of-file by typing a control-z—hold down the Ctrl key and press z—followed by hitting either the Enter or Return key. On UNIX systems, including on Mac OS X machines, end-of-file is usually control-d.

Compilation Revisited

Part of the compiler’s job is to look for errors in the program text. A compiler cannot detect whether a program does what its author intends, but it can detect errors in the form of the program. The following are the most common kinds of errors a compiler will detect.

Syntax errors: The programmer has made a grammatical error in the C++ language. The following program illustrates common syntax errors; each comment describes the error on the following line:

// error: missing ) in parameter list for main
int main ( {
    // error: used colon, not a semicolon, after endl
    std::cout << "Read each file." << std::endl:
    // error: missing quotes around string literal
    std::cout << Update master. << std::endl;
    // error: second output operator is missing
    std::cout << "Write new master." std::endl;
    // error: missing ; on return statement
    return 0
}

Type errors: Each item of data in C++ has an associated type. The value 10, for example, has a type of int (or, more colloquially, “is an int”). The word "hello", including the double quotation marks, is a string literal. One example of a type error is passing a string literal to a function that expects an int argument.

Declaration errors: Every name used in a C++ program must be declared before it is used. Failure to declare a name usually results in an error message. The two most common declaration errors are forgetting to use std:: for a name from the library and misspelling the name of an identifier:

#include <iostream>
int main()
{
    int v1 = 0, v2 = 0;
    std::cin >> v >> v2; // error: uses "v" not "v1"
    // error: cout not defined; should be std::cout
    cout << v1 + v2 << std::endl;
    return 0;
}

Error messages usually contain a line number and a brief description of what the compiler believes we have done wrong. It is a good practice to correct errors in the sequence they are reported. Often a single error can have a cascading effect and cause a compiler to report more errors than actually are present. It is also a good idea to recompile the code after each fix—or after making at most a small number of obvious fixes. This cycle is known as edit-compile-debug.