3. Simple Types, Variables, and Constants

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1 3. Simple Types, Variables, and Constants This section of the lectures will look at simple containers in which you can storing single values in the programming language C++. You might find it interesting to know that C++ is an enhancement of the C language, and it thus has a number of added and better features for this (in particular, boolean types, classes, and typed constants). Note that we will not cover aggregate types like arrays, structures, and classes in this introductory section. Readings: [Savitch99] Chapter 2. Chapters 3 and 4 will need to be read next week. 3. Simple Types, Variables, and Constants Setting the Stage Identifiers Simple Types Integer Types Floating Point Types Variables Copyright 1998 by R. Tront 3-1 Copyright 1998 by R. Tront 3-2

2 3.1 Setting the Stage Here is a simple C++ program. // //This program displays 2*pi on the //screen. // #include <iostream.h> int main() { //define and initialize a constant. const float PI = ; float twopi; //define a variable. twopi = 2. * PI; cout << twopi; return 0; } In many computer languages, your program has to be called main. It may be stored in a file with a different name, but as far as the compiler is concerned, it is a function called main(). The () indicates it is a function. Functions are discussed in Chapter 4 of [Savitch99]. To run the program, the operating system of the computer calls the main function. There can be many modules in a program, but only one can export a function called main. Not surprisingly, it is Copyright 1998 by R. Tront 3-3 called the main module. The body of the function is contained within the braces {}. PI is defined by the words const float to be a constant number which is not an integer. i.e. it has a decimal point. They are represented in RAM in what is called floating point representation. Constants usually are named with all upper case letters, This allows programmers to distinguish those containers that cannot be assigned new values. Contrast this with the variable named twopi. It starts off with a junk value (whatever was in RAM memory at that location before your program started up), and is assigned a new value during the execution of the program. The character * is used to indicate multiplication in almost all computer languages. Note that names that are composed of two words are often nice, but they cannot be written in a program as two separate words. I like to concatenate them and make the first letter of each word upper case. [Savitch99] alternatively uses underscore characters _. e.g. this_months_rainfall. Finally note that and 2. are called literal constants or literals. It is a good idea to add a decimal point to the 2 so that both operands of the multiply operator are floats. Copyright 1998 by R. Tront 3-4

3 cout is an output stream object that is connected to your computer stream. We will cover objects later in the course. << is the insertion operator. The statement containing cout tells the computer to insert the value of twopi into the stream of data going to the screen (i.e. write it to the screen). The << operator is very smart in that it knows that the variable on its right is in floating point representation and that it should convert that to ASCII for display on your screen (which only knows how to display ASCII and graphics). Where did cout come from? Well, I asked that one be included in my program with the #include directive at the top of the program. The return 0; statement indicates that the function is to end at that point and return control to the operating system. Returning zero indicates to the operating system that the program thinks it was successful. It is very important to document the purpose of each variable and also the goal of each part of a program. In early computer languages, names for types, variables, and constants could only be 6 characters long. Modern languages allow us to use quite long and therefore descriptive identifier names. Only if your names are so descriptive that anyone (including those not familiar with the program) could figure out what an identifier is used for, should you not Copyright 1998 by R. Tront 3-5 bother to add comments about identifiers. Generally you should comment every goal, and every messy conditional statement. Do not comment the obvious even though I have done this to some degree above. I didn t comment the assignment statement as it is obvious that the constant PI is being multiplied by 2. Assume a reader can read C++. But don t underestimate how difficult a program is to read if you were not the one to write it and your don t quite know what it does. There are two forms of comments. Any pair of forward slashes // delineates the start of a comment. A // style comment ends at the end of the line. Anything (including seemingly executable statements) between a // and the end of the line are ignored by the compiler. If you want a multi-line comment you must either put a // on every line, or use the alternative style shown in Section 2.5 of [Savitch99]. Finally note that almost every statement is terminated by a semi-colon. There are two main exceptions. Function bodies normally do not need a semicolon following the closing brace. And #include directives are not terminated by a semi-colon because they are so-called directives to the compiler, not statements. Copyright 1998 by R. Tront 3-6

4 3.2 Identifiers We use names to refer to variables, constants, and their types. The C++ language only allows us to use certain keyboard characters within such identifiers. An identifier must start with a letter of the alphabet (either upper or lower case) or an underscore _. Subsequent characters can include alphabetic letters, _, or digits. You cannot use any other characters such as %,,, -, or. as part of an identifier. Names can be any length, but sometimes only the first few are significant. For instance, the compiler may warn you that two different names that have the same first 31 characters may not be acceptable. And some old linkers only pay attention to the first 6 characters. Nowadays, you are pretty safe with 31 unique characters, but if you have problems exporting something from one module to other modules, and its name has the same first 6 characters as some other identifier exported by some other module, the linker may get them confused. Finally, you cannot use any C++ keywords as identifiers. Appendix 1 of [Savich99] has a complete list of C++ keywords. Examples are const, float, if, then, and else. 3.3 Simple Types To declare a variable or a constant, you must tell the computer what kind of container you will need for it. These are called types or classifications of variables. There are a number of basic types built into C++. And C++ allows you to create your own types (though ones you create are usually just aggregates of the basic types). In this section we will look at some (not all) of the basic built-in types. Many of the types are numerical in nature. The reason that there are so many types is that each type is optimized for some purpose. The choice of which to use is usually a trade-off between compactness versus numerical range. If we choose a short integer, it can only store numbers in the range of -32,768 and +32,767. If you add 1,000 to a short integer variable whose value is already 32,000, and print the result, you will not get 33,000. You will get garbage, probably negative garbage. You will learn why in CMPT 150. So the choice of what type you will use for a variable is critical to a program. Copyright 1998 by R. Tront 3-7 Copyright 1998 by R. Tront 3-8

5 3.3.1 Integer Types There are 8 basic integral types: char short int long unsigned char unsigned short unsigned int unsigned long Short integers are declared using the keyword short or optionally short int. e.g. short outsidetemperature; short int insidetemperature; Short integers have a range of -32,768 to 32,767 and only take up 2 bytes of space. Long integers are declared with the keyword long or optionally long int. They have a range of approximately -2 billion to +2 billion and take up 4 bytes. See Display 2.2 in [Savitch99] for details on all the types. The type int is troublesome, because when compiling for so called 16 bit operating systems like MS-DOS and Windows 3.1, they are two bytes long. And when Copyright 1998 by R. Tront 3-9 compiling for 32 bit systems like Win32 (i.e. Windows 98 or Windows NT) or Unix, they are 4 bytes long. If you move a program from a 32 system to a 16 bit system and it crashes or just produces slightly wrong results, this may be the problem. While int is widely used, to be safe, when you need a 4 byte container for an integer, specify a long. The unsigned integer types have a range of zero to basically twice what the upper limit of the corresponding signed type is. Often we know values will never be negative, and for those we can use the unsigned variants. Each type normally has a set of operators that go with it. For integer variables, they are +, -, *, /, = (assignment), == (test for equality), and several others described in your text. Operators are a critical aspect of a type, and the type is useless unless you know what operators are available and not available. Note that raising an integer to a power like X 3, is not directly available in C++ (though there is a function you can call to get this done for you). Though rarely used, a one byte integer is called a char. Often we store characters in them, but they can be used for very small integers in the range -128 to You can add and subtract chars, but if they contain things like A +3 you will get strange results like D! Copyright 1998 by R. Tront 3-10

6 3.3.2 Floating Point Types Computers don t handle numbers with decimal points very well, because they basically store binary bits. There are two types of variables with decimal points used in computers. One is called decimal or more correctly Binary Coded Decimal. It is used mainly by financial programs for which numbers do not get very large (< 1 billion), and which must be accurate to the penny. C++ does not directly support BCD numbers. Computers are also used to compute very small and very large numbers. Examples are the weight of an electron, the distance to the edge of the universe, or the number atoms in 1 kilogram of hydrogen (6.02 * ). In this case we need extreme range, but not necessarily extreme precision. Basically, the representation used for floating point numbers uses the same mechanism as the scientific notation used above for Avogadro s number. The decimal point is not necessarily between the units digit and the 1/10s digit, but floats as specified by the exponent. There are 3 floating point types supported by C++: float - which is stored in 4 bytes, has a precision of only 7 places, but still has a pretty good range: to But because of the limited precision, even a simple float calculation like might result in an answer of zero instead of ! double - which is stored in 8 bytes, has a precision of 15 digits and a very good range of to In addition, large libraries of useful functions like sine and cosine exist for type double (though you can use these libraries for floats if you are careful). long double - which is stored in 10 bytes and rarely used. Copyright 1998 by R. Tront 3-11 Copyright 1998 by R. Tront 3-12

7 3.4 Variables The actual containers we use in our recipes are called variables. We may need more than one 1 liter measuring cup. These are called multiple instances of the same type. Or using object-oriented terminology, multiple instances of the same class of container (classification of container). Before we start executing our recipe, we must specify in our program the number and class of every container we will need to execute the recipe. This is called defining our variables. (Note: In the computer language Pascal this is often called declaring our variables. As you will see later in the course though, the term declaring has a different meaning in C++ than in Pascal.) There is really nothing to defining our variables. We just name them preceded by type we want them to have. Note that this is opposite to the Pascal ordering! int temperature; //C++ style. //temperature: INTEGER; Pascal style. Though you should generally declare each variable in its own statement and line, it is possible to declare more than one in a single statement, if they are all of a common type: int outsidetemp, insidetemp; Note that you should assume that variables initially have garbage values. Some, but not all compilers will warn you when you use a variable before assigning it a proper value. If you want to initialize a variable in the same statement that it is defined in (this is sometimes nice), that is permissible. e.g. int temperature = 100; But, unlike a constant, the variable temperature will not always be 100 because the program is allowed to assign different values to it during the course of execution. Copyright 1998 by R. Tront 3-13 Copyright 1998 by R. Tront 3-14

8 3.5 Constants Constant are widely used in computer programming. Though they cannot change value during execution, often when enhancing a program you might want to change a constant. A good example is a constant specifying the maximum length of a university student s family name. Perhaps this was initially set to 20 characters. If it is discovered that some students have names longer than this, we would like to change this and recompile/link the program. If the literal constant 20 was used throughout the many modules of the program, the maintenance programmer must try to find all the places 20 was specified in the program and change this to 25. Generally he misses one, or changes one too many (perhaps a literal 20 was also used for the maximum number of credits allowed to be registered for in a semester, and the maintenance programmer mistakenly changed that literal to 25 too)! Instead, it is better to define a named constant, use it everywhere in the module it is needed, and if necessary export it for use by the other modules that need it. So, the main reasons for constants are: easing maintenance like described above. preventing other programs from changing a constant s value because of some other programmer s mistake. giving them names meaningful to other programmers. Copyright 1998 by R. Tront 3-15 Constants are considerably improved in C++ compared to the C language, which in fact didn t have them. To declare a simple constant, you add the keyword const in front of the definition. In contrast to the optional initialization of a variable, the initialization of a constant MUST be provided by you in the SAME statement it is defined (because the compiler forbids you from later initializing it since it is supposed to be constant!). If you try to assign a new value to a constant, the compiler will choke when translating the program and present you with an error message telling you to correct this mistake! Copyright 1998 by R. Tront 3-16

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