The C Programming Language

C was originally designed for and implemented on the UNIX operating system on the DEC PDP-11, by Dennis Ritchie. The operating system, the C compiler, and essentially all UNIX applications programs (including all of the software used to prepare this book) are written in C. Production compilers also exist for several other machines, including the IBM System/370, the Honeywell 6000, and the Interdata 8/32. C is not tied to any particular hardware or system, however, and it is easy to write programs that will run without change on any machine that supports C.

Functions 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.

Comment 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.

Declaration A declaration announces the properties of variables; it consists of a name and a list of variables C provides several other data types besides int and float, including char, short, long and double. Arrays, structures, unions, pointers, functions

Assignment statements 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.

While loop 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. 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. 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. 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. Body of loop 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.