The C Programing Langunge

Chapter 1 - A Tutorial Introduction 

Let us begin with a quick introduction in C. Our aim is to show the essential elements of the language in real programs, but without getting bogged down in details, rules, and exceptions. At this point, we are not trying to be complete or even precise (save that the examples are meant to be correct). We want to get you as quickly as possible to the point where you can write useful programs, and to do that we have to concentrate on the basics: variablesand constants,arithmetic, control flow, functions, and the rudiments of input and output. We are intentionally leaving out of this chapter features of C that are important for writing bigger programs. These include pointers,structures, most of C's rich set of operators, several control-
flow statements, and the standard library. 

This approach and its drawbacks. Most notable is that the complete story on any particular feature is not found here, and the tutorial, by being brief, may also be misleading. And because the examples do not use the full power of C, they are not as concise and elegant as they might be. We have tried to minimize these effects, but be warned. Another drawback is that later chapters will necessarily repeat some of this chapter. We hope that the repetition will help you more than it annoys. 

In any case, experienced programmers should be able to extrapolate from the material in this chapter to their own programming needs. Beginners should supplement it by writing small, similar programs of their own. Both groups can use it as a framework on which to hang the more detailed descriptions that begin in Chapter 2. 

1.1 Getting Started 
The only way to learn a new programming language is by writing programs in it. The first program to write is the same for all languages: 

Print the words 

hello, world 


This is a big hurdle; to leap over it you have to be able to create the program text somewhere, compile it successfully, load it, run it, and find out where your output went. With these mechanical details mastered, everything else is comparatively easy. 

In C, the program to print ``hello, world'' is 

#include


 main()
{ 
printf("hello, world\n");
} 


Just how to run this program depends on the system you are using. As a specific example, on the UNIX operating system you must create the program in a file whose name ends in ``.c'', such as hello.c, then compile it with the command 

cc hello.c 


If you haven't botched anything, such as omitting a character or misspelling something, the compilation will proceed silently, and make an executable file called a.out. If you run a.out by typing the command 

a.out 


it will print 


 hello, world


On other systems, the rules will be different; check with a local expert. 

Now, for some explanations about the program itself. A C program, whatever its size, consists of functions and variables. A function contains statements that specify the computing operations to be done, and variables store values used during the computation. C functions are like the subroutines and functions in Fortran or the procedures and functions of Pascal. Our example is a function named main. Normally you are at liberty to give functions whatever names you like, but ``main'' is special -your program begins executing at the beginning of 
main. This means that every program must have a main somewhere. 

main will usually call other functions to help perform its job, some that you wrote, and others from libraries that are provided for you. The first line of the program, 

#include  
tells the compiler to include information about the standard input/output library; the line appears at the beginning of many C source files. The standard library is described in Chapter 7 
and Appendix B. 

One method of communicating data between functions is for the calling function to provide a list of values, called arguments, to the function it calls. The parentheses after the function name surround the argument list. In this example, main is defined to be a function that expects no arguments, which is indicated by the empty list ( ). 

#include include information about standard 
library 
main() define a function called main 


that received no argument values 
{ statements of main are enclosed in braces 
printf("hello, world\n"); main calls library function printf 
to print this sequence of characters 
} \n represents the newline character 


The first C program 

The statements of a function are enclosed in braces {}. The function main contains only one 
statement, 

printf("hello, world\n");
A function is called by naming it, followed by a parenthesized list of arguments, so this calls 
the function printf with the argument "hello, world\n". printf is a library function that 
prints output, in this case the string of characters between the quotes. 

A sequence of characters in double quotes, like "hello, world\n", is called a character 
string or string constant. For the moment our only use of character strings will be as 
arguments for printf and other functions. 

The sequence \n in the string is C notation for the newline character, which when printed 
advances the output to the left margin on the next line. If you leave out the \n (a worthwhile 
experiment), you will find that there is no line advance after the output is printed. You must 
use \n to include a newline character in the printf argument; if you try something like 

printf("hello, world


"); 



the C compiler will produce an error message. 

printf never supplies a newline character automatically, so several calls may be used to build 
up an output line in stages. Our first program could just as well have been written 

#include


 main()


{


 printf("hello, ");


printf("world");


printf("\n");


}


to produce identical output. 

Notice that \n represents only a single character. An escape sequence like \n provides a general and extensible mechanism for representing hard-to-type or invisible characters. Among the others that C provides are \t for tab, \b for backspace, \" for the double quote and \\ for the backslash itself. There is a complete list in Section 2.3. 
Exercise 1-1. Run the ``hello, world'' program on your system. Experiment with leaving out parts of the program, to see what error messages you get. 

Exercise 1-2. Experiment to find out what happens when prints's argument string contains \c, where c is some character not listed above. 

1.2 Variables and Arithmetic Expressions 
The next program uses the formula oC=(5/9)(oF-32) to print the following table of Fahrenheit temperatures and their centigrade or Celsius equivalents: 


 1 -17 
20 -6 
40 4 
60 15 
80 26 
100 37 
120 48 
140 60 
160 71 
180 82 
200 93 
220 104 
240 115 
260 126 
280 137 
300 148 


The program itself still consists of the definition of a single function named main. It is longer than the one that printed ``hello, world'', but not complicated. It introduces several new ideas, including comments, declarations, variables, arithmetic expressions, loops , and formatted output. 

#include


 /* print Fahrenheit-Celsius table


for fahr = 0, 20, ..., 300 */
main()
{


 int fahr, celsius;
int lower, upper, step;


 lower = 0; /* lower limit of temperature scale */
upper = 300; /* upper limit */
step = 20; /* step size */


 fahr = lower;


while (fahr <= upper) {
celsius = 5 * (fahr-32) / 9;
printf("%d\t%d\n", fahr, celsius);
fahr = fahr + step;


}
}


The two lines 

/* print Fahrenheit-Celsius table
for fahr = 0, 20, ..., 300 */ 


are a comment, which in this case explains briefly what the program does. Any characters between /* and */ are ignored by the compiler; they may be used freely to make a program easier to understand. Comments may appear anywhere where a blank, tab or newline can. 

In C, all variables must be declared before they are used, usually at the beginning of the function before any executable statements. A declaration announces the properties of variables; it consists of a name and a list of variables, such as 

int fahr, celsius;
int lower, upper, step;
The type int means that the variables listed are integers; by contrast with float, which means floating point, i.e., numbers that may have a fractional part. The range of both int and float depends on the machine you are using; 16-bits ints, which lie between -32768 and +32767, are common, as are 32-bit ints. A float number is typically a 32-bit quantity, with at least six significant digits and magnitude generally between about 10-38 and 1038. 

C provides several other data types besides int and float, including: 


char character - a single byte 
short short integer 
long long integer 
double double-precision floating point 

The size of these objects is also machine-dependent. There are also arrays, structures and 
unions of these basic types, pointers to them, and functions that return them, all of which we 
will meet in due course. 

Computation in the temperature conversion program begins with the assignment statements

 lower = 0;
upper = 300;
step = 20;


which set the variables to their initial values. Individual statements are terminated by 
semicolons. 

Each line of the table is computed the same way, so we use a loop that repeats once per output 
line; this is the purpose of the while loop 

while (fahr <= upper) { 
... 
}
The while loop operates as follows: The condition in parentheses is tested. If it is true (fahr 
is less than or equal to upper), the body of the loop (the three statements enclosed in braces) is 
executed. Then the condition is re-tested, and if true, the body is executed again. When the test 
becomes false (fahr exceeds upper) the loop ends, and execution continues at the statement 
that follows the loop. There are no further statements in this program, so it terminates. 

The body of a while can be one or more statements enclosed in braces, as in the temperature 
converter, or a single statement without braces, as in 

while (i < j)
i = 2 * i;
In either case, we will always indent the statements controlled by the while by one tab stop 
(which we have shown as four spaces) so you can see at a glance which statements are inside 
the loop. The indentation emphasizes the logical structure of the program. Although C 
compilers do not care about how a program looks, proper indentation and spacing are critical 
in making programs easy for people to read. We recommend writing only one statement per 
line, and using blanks around operators to clarify grouping. The position of braces is less 
important, although people hold passionate beliefs. We have chosen one of several popular 
styles. Pick a style that suits you, then use it consistently. 

Most of the work gets done in the body of the loop. The Celsius temperature is computed and 
assigned to the variable celsius by the statement 

celsius = 5 * (fahr-32) / 9;
The reason for multiplying by 5 and dividing by 9 instead of just multiplying by 5/9 is that in 
C, as in many other languages, integer division truncates: any fractional part is discarded. 
Since 5 and 9 are integers. 5/9 would be truncated to zero and so all the Celsius temperatures 
would be reported as zero. 

This example also shows a bit more of how printf works. printf is a general-purpose 
output formatting function, which we will describe in detail in Chapter 7. Its first argument is a 
string of characters to be printed, with each % indicating where one of the other (second, third, 


...) arguments is to be substituted, and in what form it is to be printed. For instance, %d 
specifies an integer argument, so the statement 

printf("%d\t%d\n", fahr, celsius);
causes the values of the two integers fahr and celsius to be printed, with a tab (\t) between 
them. 

Each % construction in the first argument of printf is paired with the corresponding second 
argument, third argument, etc.; they must match up properly by number and type, or you will 
get wrong answers. 

By the way, printf is not part of the C language; there is no input or output defined in C 
itself. printf is just a useful function from the standard library of functions that are normally 
accessible to C programs. The behaviour of printf is defined in the ANSI standard, however, 
so its properties should be the same with any compiler and library that conforms to the 
standard. 

In order to concentrate on C itself, we don't talk much about input and output until chapter 7. 
In particular, we will defer formatted input until then. If you have to input numbers, read the 
discussion of the function scanf in Section 7.4. scanf is like printf, except that it reads 
input instead of writing output. 

There are a couple of problems with the temperature conversion program. The simpler one is 
that the output isn't very pretty because the numbers are not right-justified. That's easy to fix; if 
we augment each %d in the printf statement with a width, the numbers printed will be right-
justified in their fields. For instance, we might say 

printf("%3d %6d\n", fahr, celsius);


to print the first number of each line in a field three digits wide, and the second in a field six 
digits wide, like this: 

0 -17 
20 -6 
40 4 
60 15 
80 26
100 37 
... 


The more serious problem is that because we have used integer arithmetic, the Celsius temperatures are not very accurate; for instance, 0oF is actually about -17.8oC, not -17. To get 
more accurate answers, we should use floating-point arithmetic instead of integer. This requires some changes in the program. Here is the second version: 

#include


 /* print Fahrenheit-Celsius table


for fahr = 0, 20, ..., 300; floating-point version */
main()
{


 float fahr, celsius;
float lower, upper, step;


 lower = 0; /* lower limit of temperatuire scale */
upper = 300; /* upper limit */
step = 20; /* step size */


 fahr = lower;


while (fahr <= upper) {
celsius = (5.0/9.0) * (fahr-32.0);
printf("%3.0f %6.1f\n", fahr, celsius); 



 fahr = fahr + step;
}
}
This is much the same as before, except that fahr and celsius are declared to be float and the formula for conversion is written in a more natural way. We were unable to use 5/9 in the previous version because integer division would truncate it to zero. A decimal point in a constant indicates that it is floating point, however, so 5.0/9.0 is not truncated because it is 
the ratio of two floating-point values. 

If an arithmetic operator has integer operands, an integer operation is performed. If an arithmetic operator has one floating-point operand and one integer operand, however, the 
integer will be converted to floating point before the operation is done. If we had written (fahr-32), the 32 would be utomatically converted to floating point. Nevertheless, writing 
floating-point constants with explicit decimal points even when they have integral values emphasizes their floating-point nature for human readers. 

The detailed rules for when integers are converted to floating point are in Chapter 2. For now, notice that the assignment 

fahr = lower;


and the test 

while (fahr <= upper)
also work in the natural way - the int is converted to float before the operation is done. 

The printf conversion specification %3.0f says that a floating-point number (here fahr) is to be printed at least three characters wide, with no decimal point and no fraction digits. %6.1f describes another number (celsius) that is to be printed at least six characters wide, with 1 digit after the decimal point. The output looks like this: 

0 -17.8 
20 -6.7
40 4.4 
... 
Width and precision may be omitted from a specification: %6f says that the number is to be at least six characters wide; %.2f specifies two characters after the decimal point, but the width is not constrained; and %f merely says to print the number as floating point. 

%d print as decimal integer 
%6d print as decimal integer, at least 6 characters wide 
%f print as floating point 
%6f print as floating point, at least 6 characters wide 
%.2f print as floating point, 2 characters after decimal point 
%6.2f print as floating point, at least 6 wide and 2 after decimal point 

Among others, printf also recognizes %o for octal, %x for hexadecimal, %c for character, %s for character string and %% for itself. Exercise 1-3. Modify the temperature conversion program to print a heading above the table. Exercise 1-4. Write a program to print the corresponding Celsius to Fahrenheit table.