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PRELIMINARIES: simple c++ program, compiling, variables, simple I/O, comments, memory, integer number, real numbers, characters, names, strings & an exercise C++ PROGRAMMING LANGUAGE COURSE 1 - PRELIMINARIES This chapter introduces the basic elements of a C++ program. We will use simple examples to show the structure of C++ programs and the way they are compiled. Elementary concepts such as constants, variables, and their storage in memory will also be discussed. The following is a cursory description of the concept of programming for the benefit of those who are new to the subject.
Programming
A digital computer is a useful tool for solving a great variety of problems. A solution to a problem is called an algorithm ; it describes the sequence of steps to be performed for the problem to be solved. A simple example of a problem and an algorithm for it would be: Problem : Sort a list of names in ascending lexicographic order. Algorithm : Call the given list list1 ; create an empty list, list2 , to hold the sorted list. Repeatedly find the ‘smallest’ name in list1 , remove it from list1 , and make it the next entry of list2 , until list1 is empty. An algorithm is expressed in abstract terms. To be intelligible to a computer, it needs to be expressed in a language understood by it. The only language really understood by a computer is its own machine language. Programs expressed in the machine language are said to be executable. A program written in any other language needs to be first translated to the machine language before it can be executed. A machine language is far too cryptic to be suitable for the direct use of programmers. A further abstraction of this language is the assembly language which provides mnemonic names for the instructions and a more intelligible notation for the data. An assembly language program is translated to machine language by a translator called an assembler. Even assembly languages are difficult to work with. High-level languages such as C++ provide a much more convenient notation for implementing algorithms. They liberate programmers from having to think in very low-level terms, and help them to focus on the algorithm instead. A program written in a high-level language is translated to assembly language by a translator called a compiler. The assembly code produced by the compiler is then assembled to produce an executable program. A simple c++ program Listing 1.1 shows our first C++ program, which when run, simply outputs the message Hello World. Listing
- #include <iostream.h>
- int main (void)
- {
- cout << "Hello World\n"; Annotation
PRELIMINARIES: simple c++ program, compiling, variables, simple I/O, comments, memory, integer number, real numbers, characters, names, strings & an exercise 1 This line uses the preprocessor directive #include to include the contents of the header file iostream.h in the program. Iostream.h is a standard C++ header file and contains definitions for input and output. 2 This line defines a function called main. A function may have zero or more parameters; these always appear after the function name, between a pair of brackets. The word void appearing between the brackets indicates that main has no parameters. A function may also have a return type; this always appears before the function name. The return type for main is int (i.e., an integer number). All C++ programs must have exactly one main function. Program execution always begins from main. 3 This brace marks the beginning of the body of main. 4 This line is a statement. A statement is a computation step which may produce a value. The end of a statement is always marked with a semicolon (;). This statement causes the string "Hello World\n" to be sent to the cout output stream. A string is any sequence of characters enclosed in double-quotes. The last character in this string (\n) is a newline character which is similar to a carriage return on a type writer. A stream is an object which performs input or output. Cout is the standard output stream in C++ (standard output usually means your computer monitor screen). The symbol << is an output operator which takes an output stream as its left operand and an expression as its right operand, and causes the value of the latter to be sent to the former. In this case, the effect is that the string "Hello World\n" is sent to cout, causing it to be printed on the computer monitor screen. 5 This brace marks the end of the body of main. • Compiling a simple C++ program Dialog 1.1 shows how the program in Listing 1.1 is compiled and run in a typical UNIX environment. User input appears in bold and system response in plain. The UNIX command line prompt appears as a dollar symbol ($). Dialog 1. Annotation 1 The command for invoking the AT&T C++ translator in a UNIX environment is CC. The argument to this command (hello.cc) is the name of the file which contains the program. As a convention, the file name should end in .c, .C, or .cc. (This ending may be different in other systems.) 2 The result of compilation is an executable file which is by default named a.out. To run the program, we just use a.out as a command. 3 This is the output produced by the program. 4 The return of the system prompt indicates that the program has completed its execution. The CC command accepts a variety of useful options. An option appears as -name, where name is the name of the option (usually a single letter). Some options take arguments. For example, the output option (-o) allows you to specify a name for the executable file produced by the compiler instead of a.out. Dialog 1.Error! Bookmark not defined. illustrates the use of this option by specifying hello as the name of the executable file. Dialog 1. 1 $ CC hello.cc $ a.out Hello World $ $ CC hello.cc -o hello $ hello Hello World
PRELIMINARIES: simple c++ program, compiling, variables, simple I/O, comments, memory, integer number, real numbers, characters, names, strings & an exercise
Variables
A variable is a symbolic name for a memory location in which data can be stored and subsequently recalled. Variables are used for holding data values so that they can be utilized in various computations in a program. All variables have two important attributes: A type which is established when the variable is defined (e.g., integer, real, character). Once defined, the type of a C++ variable cannot be changed. A value which can be changed by assigning a new value to the variable. The kind of values a variable can assume depends on its type. For example, an integer variable can only take integer values (e.g., 2, 100, - 12). Listing 1.2 illustrates the uses of some simple variable Listing 1. 1 2 3 4 5 6 7 8 9 10 11 12 13 Annotation 4 This line defines an int (integer) variable called workDays, which will represent the number of working days in a week. As a general rule, a variable is defined by specifying its type first, followed by the variable name, followed by a semicolon. 5 This line defines three float (real) variables which, respectively, represent the work hours per day, the hourly pay rate, and the weekly pay. As illustrated by this line, multiple variables of the same type can be defined at once by separating them with commas. 6 This line is an assignment statement. It assigns the value 5 to the variable workDays. Therefore, after this statement is executed, workDays denotes the value
7 This line assigns the value 7.5 to the variable workHours. 8 This line assigns the value 38.55 to the variable payRate. 9 This line calculates the weekly pay as the product of workDays, workHours, and payRate (* is the multiplication operator). The resulting value is stored in weeklyPay. 10-12 These lines output three items in sequence: the string "Weekly Pay = ", the value of the variable weeklyPay, and a newline character. #include <iostream.h> int main (void) { int workDays; float workHours, payRate, weeklyPay; workDays = 5; workHours = 7.5; payRate = 38.55; weeklyPay = workDays * workHours * payRate; cout << "Weekly Pay = "; cout << weeklyPay; cout << '\n'; }
PRELIMINARIES: simple c++ program, compiling, variables, simple I/O, comments, memory, integer number, real numbers, characters, names, strings & an exercise When run, the program will produce the following output: Weekly Pay = 1445. When a variable is defined, its value is undefined until it is actually assigned one. For example, weeklyPay has an undefined value (i.e., whatever happens to be in the memory location which the variable denotes at the time) until line 9 is executed. The assigning of a value to a variable for the first time is called initialization. It is important to ensure that a variable is initialized before it is used in any computation. It is possible to define a variable and initialize it at the same time. This is considered a good programming practice, because it pre-empts the possibility of using the variable prior to it being initialized. Listing 1.3 is a revised version of Listing 1.2 which uses this technique. For all intents and purposes, the two programs are equivalent. Listing 1. 1 2 3 4 5 6 7 8 9 10 11
Simple Input/Output
The most common way in which a program communicates with the outside world is through simple, character-oriented Input/Output (IO) operations. C+
- provides two useful operators for this purpose: >> for input and << for output. We have already seen examples of output using <<. Listing 1.4 also illustrates the use of >> for input. Listing 1. 1 2 3 4 5 6 7 8 9 10 11 12 13 Annotation #include <iostream.h> int main (void) { int workDays = 5; float workHours = 7.5; float payRate = 38.55; float weeklyPay = workDays * workHours * payRate; cout << "Weekly Pay = "; cout << weeklyPay; cout << '\n'; } #include <iostream.h> int main (void) { int workDays = 5; float workHours = 7.5; float payRate, weeklyPay; cout << "What is the hourly pay rate? "; cin >> payRate; weeklyPay = workDays * workHours * payRate; cout << "Weekly Pay = "; cout << weeklyPay; cout << '\n'; }
PRELIMINARIES: simple c++ program, compiling, variables, simple I/O, comments, memory, integer number, real numbers, characters, names, strings & an exercise What are the work hours and the hourly pay rate? 7.5 33. Weekly Pay = 1258.
PRELIMINARIES: simple c++ program, compiling, variables, simple I/O, comments, memory, integer number, real numbers, characters, names, strings & an exercise COMMENTS A comment is a piece of descriptive text which explains some aspect of a program. Program comments are totally ignored by the compiler and are only intended for human readers. C++ provides two types of comment delimiters: Anything after // (until the end of the line on which it appears) is considered a comment. Anything enclosed by the pair /* and / is considered a comment. Listing 1.6 illustrates the use of both forms. Listing 1. 1 2 3 4 5 6 7 8 9 10 11 12 13 Comments should be used to enhance (not to hinder) the readability of a program. The following two points, in particular, should be noted: A comment should be easier to read and understand than the code which it tries to explain. A confusing or unnecessarily-complex comment is worse than no comment at all. Over-use of comments can lead to even less readability. A program which contains so much comment that you can hardly see the code can by no means be considered readable. Use of descriptive names for variables and other entities in a program, and proper indentation of the code can reduce the need for using comments. The best guideline for how to use comments is to simply apply common sense. MEMMORY A computer provides a Random Access Memory (RAM) for storing executable program code as well as the data the program manipulates. This memory can be thought of as a contiguous sequence of bits , each of which is capable of storing a binary digit (0 or 1). Typically, the memory is also divided into groups of 8 consecutive bits (called bytes ). The bytes are sequentially addressed. Therefore each byte can be uniquely identified by its address (see Figure 1.2). #include <iostream.h> / This program calculates the weekly gross pay for a worker, based on the total number of hours worked and the hourly pay rate. */ int main (void) { int workDays = 5; // Number of work days per week float workHours = 7.5; // Number of work hours per day float payRate = 33.50; // Hourly pay rate float weeklyPay; // Gross weekly pay weeklyPay = workDays * workHours * payRate; cout << "Weekly Pay = " << weeklyPay << '\n'; }
PRELIMINARIES: simple c++ program, compiling, variables, simple I/O, comments, memory, integer number, real numbers, characters, names, strings & an exercise While the exact binary representation of a data item is rarely of interest to a programmer, the general organization of memory and use of addresses for referring to data items (as we will see later) is very important.
Integer Numbers
An integer variable may be defined to be of type short, int, or long. The only difference is that an int uses more or at least the same number of bytes as a short, and a long uses more or at least the same number of bytes as an int. For example, on the author’s PC, a short uses 2 bytes, an int also 2 bytes, and a long 4 bytes. short age = 20; int salary = 65000; longprice = 4500000; By default, an integer variable is assumed to be signed (i.e., have a signed representation so that it can assume positive as well as negative values). However, an integer can be defined to be unsigned by using the keyword unsigned in its definition. The keyword signed is also allowed but is redundant. unsigned short age = 20; unsigned int salary = 65000; unsigned long price = 4500000; A literal integer (e.g., 1984 ) is always assumed to be of type int, unless it has an L or l suffix, in which case it is treated as a long. Also, a literal integer can be specified to be unsigned using the suffix U or u. For example: 1984L 1984l 1984U 1984u 1984LU 1984ul Literal integers can be expressed in decimal, octal, and hexadecimal notations. The decimal notation is the one we have been using so far. An integer is taken to be octal if it is preceded by a zero ( 0 ), and hexadecimal if it is preceded by a 0x or 0X. For example: 92 // decimal 0134// equivalent octal 0x5C// equivalent hexadecimal Octal numbers use the base 8, and can therefore only use the digits 0-7. Hexadecimal numbers use the base 16, and therefore use the letter A-F (or a- f) to represent, respectively, 10-15. Octal and hexadecimal numbers are calculated as follows: 0134 = 1 × 82 + 3 × 8^1 + 4 × 80 = 64 + 24 + 4 = 92 0x5C = 5 × 161 + 12 × 160 = 80 + 12 = 92
PRELIMINARIES: simple c++ program, compiling, variables, simple I/O, comments, memory, integer number, real numbers, characters, names, strings & an exercise
Real Numbers
A real variable may be defined to be of type float or double. The latter uses more bytes and therefore offers a greater range and accuracy for representing real numbers. For example, on the author’s PC, a float uses 4 and a double uses 8 bytes. float interestRate = 0.06; double pi = 3.141592654; A literal real (e.g., 0.06) is always assumed to be of type double, unless it has an F or f suffix, in which case it is treated as a float, or an L or l suffix, in which case it is treated as a long double. The latter uses more bytes than a double for better accuracy (e.g., 10 bytes on the author’s PC). For example: 0.06F 0.06f 3.141592654L 3.141592654l In addition to the decimal notation used so far, literal reals may also be expressed in scientific notation. For example, 0.002164 may be written in the scientific notation as: 2.164E-3 or 2.164e- The letter E (or e) stands for exponent. The scientific notation is interpreted as follows: 2.164E-3 = 2.164 × 10 -
Characters
A character variable is defined to be of type char. A character variable occupies a single byte which contains the code for the character. This code is a numeric value and depends on the character coding system being used (i.e., is machine-dependent). The most common system is ASCII (American Standard Code for Information Interchange). For example, the character A has the ASCII code 65, and the character a has the ASCII code 97. charch = 'A'; Like integers, a character variable may be specified to be signed or unsigned. By the default (on most systems) char means signed char. However, on some systems it may mean unsigned char. A signed character variable can hold numeric values in the range -128 through 127. An unsigned character variable can hold numeric values in the range 0 through 255. As a result, both are often used to represent small integers in programs (and can be assigned numeric values like integers): signed char offset = -88; unsigned char row = 2, column = 26;
PRELIMINARIES: simple c++ program, compiling, variables, simple I/O, comments, memory, integer number, real numbers, characters, names, strings & an exercise next line. The above string is equivalent to the single line string: "Example to show the use of backslash for writing a long string" A common programming error results from confusing a single-character string (e.g., "A") with a single character (e.g., 'A'). These two are not equivalent. The former consists of two bytes (the character 'A' followed by the character '
0'), whereas the latter consists of a single byte. The shortest possible string is the null string ("") which simply consists of the null character.
Names
Programming languages use names to refer to the various entities that make up a program. We have already seen examples of an important category of such names (i.e., variable names). Other categories include: function names, type names, and macro names, which will be described later in this book. Names are a programming convenience, which allow the programmer to organize what would otherwise be quantities of plain data into a meaningful and human-readable collection. As a result, no trace of a name is left in the final executable code generated by a compiler. For example, a temperature variable eventually becomes a few bytes of memory which is referred to by the executable code by its address, not its name. C++ imposes the following rules for creating valid names (also called identifiers ). A name should consist of one or more characters, each of which may be a letter (i.e., 'A'-'Z' and 'a'-'z'), a digit (i.e., '0'-'9'), or an underscore character ('_'), except that the first character may not be a digit. Upper and lower case letters are distinct. For example: salary // valid identifier salary2 // valid identifier 2salary // invalid identifier (begins with a digit) _salary // valid identifier Salary // valid but distinct from salary C++ imposes no limit on the number of characters in an identifier. However, most implementation do. But the limit is usually so large that it should not cause a concern (e.g., 255 characters). Certain words are reserved by C++ for specific purposes and may not be used as identifiers. These are called reserved words or keywords and are summarized in Table 1.1: Table 1.1 C++ keywords. asm continue float new signed try auto default for operator sizeof typedef break delete friend private static union case do goto protected struct unsigned
PRELIMINARIES: simple c++ program, compiling, variables, simple I/O, comments, memory, integer number, real numbers, characters, names, strings & an exercise catch double if public switch virtual char else inline register template void class enum int return this volatile const extern long short throw while
Exercises
1.1 Write a program which inputs a temperature reading expressed in Fahrenheit and outputs its equivalent in Celsius, using the formula: C
( F 32)
Compile and run the program. Its behavior should resemble this: Temperature in Fahrenheit: 41 41 degrees Fahrenheit = 5 degrees Celsius 1.2 Which of the following represent valid variable definitions? int n = -100; unsigned int i = -100; signed int = 2.9; long m = 2, p = 4; int 2k; double x = 2 * m; float y = y * 2; unsigned double z = 0.0; double d = 0.67F; float f = 0.52L; signed char = -1786; char c = '$' + 2; sign char h = '\111'; char *name = "Peter Pan"; unsigned char *num = "276811"; 1.3 Which of the following represent valid identifiers? identifier seven_ unique gross-income gross$income 2by default average_weight_of_a_large_pizza variable object.oriented 1.4 Define variables to represent the following entities: Age of a person. Income of an employee. Number of words in a dictionary.
