From the Publisher

For beginning programmers, this updated edition answers all C programming questions. This bestseller talks to readers at their level, explaining every aspect of how to get started and learn the C language quickly. Readers also find out where to learn more about C. This book includes tear-out reference card of C functions and statements, a hierarchy chart, and other valuable information. It uses special icons, notes, clues, warnings, and rewards to make understanding easier. And the clear and friendly style presumes no programming knowledge.

Product Details

• Table of Contents
• Read a Sample Chapter

Table of Contents

Read a Sample Chapter

[Figures are not included in this sample chapter]

ABG to C, 2E



- 3 -

How Do I Know What's Happening?

Through Comments

Your computer must be able to understand your programs. Because the computer is
a dumb machine, you must be careful to spell C commands exactly right and type them
in the same order you want them executed. However, people also read your programs.
You will change your programs often, and if you write programs for a company, the
company's needs will change over time. You must ensure that your programs are understandable
to people as well as to computers. Therefore, you should document your programs
by explaining what they do.

Commenting on Your Code

Throughout a C program, you should add comments. Comments are messages
scattered throughout your programs that explain what's going on. If you write a program
to calculate payroll, the program's comments explain the gross pay calculations,
state tax calculations, federal tax calculations, social security calculations, and
all the other calculations that are going on.

---

NOTE If you write the program and only you will use it, you don't really need
comments, right? Well, not exactly. C is a cryptic programming language. Even if
you write the program, you aren't always able to follow it later.

---

---

Clue: Add comments as you write your programs. Get in the habit now, because
programmers rarely go back and add comments later. When they must make a change later,
programmers often lament about their program's lack of comments.

There is another advantage to commenting as you write the program instead of waiting
until after you finish. While writing programs, you often refer back to statements
you wrote earlier in the process. Instead of reinterpreting C code you've already
written, you can scan through your comments, finding sections of code that you need
faster. If you didn't comment, you would have to decipher your C code every time
you looked through a piece of it.

---

Program maintenance is the process of changing a program, over time, to
fix hidden bugs and to adapt the program to a changing environment. If you write
a payroll, program for a company, that company could eventually change the way it
does payroll, and you (or another programmer) will have to modify the payroll, program
to conform to the company's new payroll procedures. Commenting speeds program maintenance.
With comments, you or another programmer can quickly scan through a program listing
finding the areas that need changing.

Comments are not C commands. C ignores every comment in your program. Comments
are for people, and the programming statements residing between the comments are
for the computer. (See Figure 3.1.)

FIGURE 3.1. Comments are for people, and C programming statements are
for the computer.

Consider the following C statement:

return ((s1 < s2) ? s1 : s2); 

You don't know C yet, but even if you did, this statement takes some study to
figure out. Isn't this better:

return ((s1 < s2) ? s1 : s2); /* Gets the smaller of 2 values */ 

The next section explains the syntax of comments, but for now, you can see that
the message between the /* and the */ is a comment. The closer
a comment is to spoken language and the further a comment is from C code, the better
the comment is. Don't write a comment just for the sake of commenting. The following
statement's comment is useless:

printf("Payroll");  /* Prints the word "Payroll" */ 

---

WARNING You don't know C yet, and you still don't need the preceding
line's comment! Redundant comments are a waste of your time, and they don't add anything
to programs. Add comments to explain what is going on to people (including yourself)
who might need to read your program.

---

Specifying Comments

C comments begin with /* and end with */. Comments can span
several lines in a program, and they can go just about anywhere in a program. All
of the following lines contain C comments:

/* This is a comment that happens to span two lines 
before coming to an end */ 
/* This is a single-line comment */ 
for (i = 0; i < 25; i++)  /* Counts from 0 to 25 */ 

---

NOTE Notice that comments can go on lines by themselves or before or after
programming statements. The choice of placement depends on the length of the comment
and the amount of code the comment describes.

---

The Blackjack program in Appendix B contains all kinds of comments. By reading
through the comments in that program, you can get an idea of what the program does
without ever looking at the C code itself.

Don't comment every line. Usually only every few lines need comments. Many programmers
like to place a multiline comment before a section of code and then insert a few
smaller comments on lines that need them. Here is a complete program with different
kinds of comments:

/* Written by: Perilous Perry, finished on April 9, 1492 */ 
/* Filename: AVG.C */ 
/* Computes the average of three class grades */ 
#include <stdio.h> 
main() 
{ 
  float gr1, gr2, gr3;  /* Variables to hold grades */ 
  float avg;            /* Variable to hold average */ 
  /* Asks for each student's grade */ 
  printf("What grade did the first student get? "); 
  scanf(" %f", &gr1); 
  printf("What grade did the second student get? "); 
  scanf(" %f", &gr2); 
  printf("What grade did the third student get? "); 
  scanf(" %f", &gr3); 
  avg = (gr1 + gr2 + gr3) / 3.0;  /* Computes average */ 
  printf("\nThe student average is %.2f", avg); 
  return 0;  /* Goes back to DOS */ 
} 

Many companies require that their programmers embed their own names in comments
at the top of programs they write. If changes need to be made to the program later,
the original programmer can be found to help out. It's also a good idea to include
the filename that you use to save the program on disk at the beginning of a program
so that you can find a program on disk when you run across a printed listing.

---

NOTE This book might comment too much in some places, especially in the beginning
chapters. You are so unfamiliar with C that every little bit of explanation helps.

---

---

SKIP THIS, IT'S TECHNICAL

For testing purposes, you might find it useful to comment out a section of
code by putting a /* and */ around it. By doing this, you cause
C to ignore that section of code, and you can concentrate on the piece of code you're
working on. Do not, however, comment out a section of code that already contains
comments because you cannot embed one comment within another. The first */
that C runs across triggers the end of the comment you started. When C finds the
next */ without a beginning /*, you get an error.

---

White Space

White space is the collection of spaces and blank lines you find in many
programs. In a way, white space is just as important in making your programs more
readable than comments are. People need white space when looking through C programs
instead of a program that runs together too much. Consider the following program:

#include <stdio.h> 
main(){float s,t;printf("How much do you make? ");scanf(" %f", 
&s);t=.33*s;printf("You owe %.2f in taxes.",t);return 0;} 

This program is a perfectly good C program--to a C compiler, but not to a person
looking at the program. Although the code is simple and it doesn't take a lot of
effort to figure out what is going on, the following program, even though it has
no comments, is much easier to decipher:

#include <stdio.h> 
main() 
  { 
    float s, t; 
    printf("How much do you make? "); 
    scanf(" %f", &s); 
    t = .33 * s; 
    printf("You owe %.2f in taxes.", t); 
    return 0; 
} 

This program listing is identical to the previous program except that this one
includes comments whereas the previous one did not. The physical length of a program
does not determine readability; the amount of whitespace does. (Of course, a few
comments would improve this program too, but the purpose of this exercise is to show
you the difference between no white space and good white space.)

---

NOTE You might be wondering why the first line of the squeezed program, the
one with the #include, did not contain code after the closing angle brace.
After all, it would seem that the point of unreadable code would be made even more
strongly if the #include contained trailing code. The author (that's me)
tried to do just that! Many, if not all, C compilers refuse to allow code after a
#include (or any other statement that begins with a pound sign (#)).
Some C compilers even refuse to allow comments at the end of such lines, although
many of today's C compilers do let you put comments there.

---

The Future of Comments

Many of today's C compilers support another kind of comment that was originally
developed for C++ programs. This new kind of comment is not approved for use by ANSI
C, but might be someday soon because it's so popular. The new style of comment begins
with two slashes (//) and ends only at the end of the line. Here is an example
of the new style of comment:

// Short program! 
#include <stdio.h> 
main() 
{ 
  printf("Looking good!");  // A message 
  return 0; 
} 

Because the new style of comment isn't sanctioned by the ANSI C committee, this
book doesn't use it again. However, you should become familiar with this style because
it's easier to use than /* and */, and many C programmers are beginning
to use it.

Rewards

• The three rules of programming are comment, comment, comment. Use comments abundantly.
  

  
  

• When you want to comment, begin with /*. End the comment with */.
  
  
  
  
• If you want to use the new style of comment, begin the comment with //.
  This kind of comment, however, isn't yet approved by ANSI C.
  

Pitfalls

• Don't use redundant comments. Worthless comments aren't helpful, and they waste
  valuable programming time.
  
  
  
  
• Don't nest one comment inside another. If you want to comment out a section of
  your program, you must make sure that the section doesn't contain other comments.
  
  
  
  
• Don't write programs that have little white space. Put as much indention and
  as many extra lines throughout a program as needed to group lines that go together.
  As you learn more about the C language, you'll learn where white space adds to a
  program's readability.
  

In Review

You must add comments to your programs, not for computers, but for people. Although
C programs can be cryptic, comments eliminate lots of confusion. A comment is just
a message that describes what's going on in the C code. Anything between the /*
and */ is a C comment. C ignores all comments because it knows that comments
are for people. In addition to comments, add lots of white space to your programs
to make your programs more readable. If a program is crunched together without blank
lines and helpful indention, you'll feel as if you're reading an entire book with
one long paragraph when you go back and study and modify the code later. Easing program
maintenance through comments and ample white space saves you time and energy if you
change the program later.



Code Example

Here are two lines without comments:

scanf(" %d", &a); 
yrs = (a >= 21) ? 0 : 21 - a; 

Here are the same two lines with comments:

scanf(" %d", &a);  /* Gets the user's age */ 
yrs = (a >= 21) ? 0 : 21 - a;  /* Calculates the number of */ 
                               /* years until adulthood */ 

Code Analysis As you can see from these lines, it's not always obvious
what goes on in C programs. Comments explain in plain, spoken language exactly what's
going on with the code. Not every line in every C program needs a comment, but many
do to clarify what's happening.