A computer program is a sequence of instructions that tell the computer what to do.
Statements and expressions
The most common type of instruction in a program is the statement. A statement in C++ is the smallest independent unit in the language. In human language, it is analogous to a sentence. We write sentences in order to convey an idea. In C++, we write statements in order to convey to the compiler that we want to perform a task. Statements in C++ are terminated by a semicolon.
There are many different kinds of statements in C++. The following are some of the most common types of simple statements:
x = 5;
std::cout << x;
int x is a declaration statement. It tells the compiler that x is a variable. You may remember variables from math in school. They serve the same purpose here: to represent an arbitrary value. All variables in a program must be declared before they are used. We will talk more about variables shortly.
x = 5 is an assignment statement. It assigns a value (5) to a variable (x).
std::cout << x; is an output statement. It outputs the value of x (which we set to 5 in the previous statement) to the screen.
The compiler is also capable of resolving expressions. An expression is an mathematical entity that evaluates to a value. For example, in math, the expression 2+3 evaluates to the value 5. Expressions can involve values (such as 2), variables (such as x), operators (such as +) and functions (which return an output value based on some input value). They can be singular (such as 2, or x), or compound (such as 2+3, 2+x, x+y, or (2+x)*(y-3)).
For example, the statement
x = 2 + 3; is a valid assignment statement. The expression 2+3 evaluates to the value of 5. This value of 5 is then assigned to x.
In C++, statements are typically grouped into units called functions. A function is a collection of statements that executes sequentially. Every C++ program must contain a special function called main. When the C++ program is run, execution starts with the first statement inside of main. Functions are typically written to do a very specific job. For example, a function named “max” might contain statements that figures out which of two numbers is larger. A function named “calculateGrade” might calculate a student’s grade. We will talk more about functions later.
Libraries and the C++ Standard Library
A library is a collection of precompiled code (e.g. functions) that has been “packaged up” for reuse in many different programs. Libraries provide a common way to extend what your programs can do. For example, if you were writing a game, you’d probably want to include a sound library and a graphics library.
The C++ core language is actually very small and minimalistic (and you’ll learn most of it in these tutorials). However, C++ also comes with a library called the C++ standard library that provides additional functionality for your use. One of the most commonly used parts of the C++ standard library is the iostream library, which contains functionality for writing to the screen and getting input from a console user.
Taking a look at a sample program
Now that you have a brief understanding of what statements, functions, and libraries are, let’s look at a simple “hello world” program:
std::cout << "Hello world!";
Line 1 is a special type of statement called a preprocessor directive. Preprocessor directives tell the compiler to perform a special task. In this case, we are telling the compiler that we would like to add the contents of the iostream header to our program. The iostream header allows us to access functionality in the iostream library, which will allow us to write to the screen.
Line 2 has nothing on it, and is ignored by the compiler.
Line 3 declares the main() function, which as you learned above, is mandatory. Every program must have a main() function.
Lines 4 and 7 tell the compiler which lines are part of the main function. Everything between the opening curly brace on line 4 and the closing curly brace on line 7 is considered part of the main() function.
Line 5 is our first statement (you can tell it’s a statement because it ends with a semicolon), and it is an output statement. std::cout is a special object that represents the console/screen. The << symbol is an operator (much like + is an operator) called the output operator. std::cout understands that anything sent to it via the output operator should be printed on the screen. In this case, we’re sending it the text “Hello world!”.
Line 6 is a new type of statement, called a return statement. When an executable program finishes running, the main() function sends a value back to the operating system that indicates whether it was run successfully or not.
This particular return statement returns the value of 0 to the operating system, which means “everything went okay!”. Non-zero numbers are typically used to indicate that something went wrong, and the program had to abort. We will discuss return statements in more detail when we discuss functions.
All of the programs we write will follow this template, or a variation on it. We will discuss each of the lines above in more detail in the upcoming sections.
Syntax and syntax errors
In English, sentences are constructed according to specific grammatical rules that you probably learned in English class in school. For example, normal sentences end in a period. The rules that govern how sentences are constructed in a language is called syntax. If you forget the period and run two sentences together, this is a violation of the English language syntax.
C++ has a syntax too: rules about how your programs must be constructed in order to be considered valid. When you compile your program, the compiler is responsible for making sure your program follows the basic syntax of the C++ language. If you violate a rule, the compiler will complain when you try to compile your program, and issue you a syntax error.
For example, you learned above that statements must end in a semicolon.
Let’s see what happens if we omit the semicolon in the following program:
std::cout << "Hello world!"
Visual studio produces the following error:
c:\users\apomeranz\documents\visual studio 2013\projects\test1\test1\test1.cpp(6): error C2143: syntax error : missing ';' before 'return'
This is telling you that you have an syntax error on line 6: You’ve forgotten a semicolon before the return. In this case, the error is actually at the end of line 5. Often, the compiler will pinpoint the exact line where the syntax error occurs for you. However, sometimes it doesn’t notice until the next line.
Syntax errors are common when writing a program. Fortunately, they’re often easily fixable. The program can only be fully compiled (and executed) once all syntax errors are resolved.
The following quiz is meant to reinforce your understanding of the material presented above.
1) What is the difference between a statement and an expression?
2) What is the difference between a function and a library?
3) What symbol do statements in C++ end with?
4) What is a syntax error?
To see these answers, select the area below with your mouse.
|1.2 -- Comments|
|0.7 -- A few common C++ problems|