TweetFollow Us on Twitter

Pascal/C II
Volume Number:10
Issue Number:1
Column Tag:Pascal/C workshop

The Pascal Programmer’s Guide
To Understanding ‘C’

Teach yourself to read another language - Part II

By Ken Gladstone, MacTech Magazine Technical Editor

Note: Source code files accompanying article are located on MacTech CD-ROM or source code disks.

This article is the second half of my Pascal Programmer’s Guide to Understanding “C”. If you haven’t already, I suggest you read the first half, which appeared in our December 1993 issue - otherwise you will probably be thoroughly confused by this half! That first half covered the following “C” concepts: comments, identifiers, operators, constants, program structure, and variable declarations and scope. So, let’s continue

PARAMETER PASSING

One key difference between C and Pascal is that C always passes parameters by value, never by reference. Therefore, you may be wondering how a C function can ever modify a passed in parameter. It can’t - but you can accomplish the same thing by passing a pointer to the value you wish to modify, and having the function modify the pointed to value. Here is an example:

/* 1 */
/************************ C Version *************************/

void doubleIt( int * pointerToIntParam )
{
  (*pointerToIntParam) *= 2;
}

int main()
{
  int myInt = 3;

  doubleIt( & myInt );

  return myInt;
}

/******************** End of C Version **********************/

(********************* Pascal Version ***********************)
program myProgram;
  var
    myInt: INTEGER;

  procedure doubleIt( var IntParam: INTEGER );
  BEGIN
    IntParam := IntParam * 2
  END;

BEGIN
  myInt := 3;

  doubleIt( myInt )
END.

(****************** End of Pascal Version *******************)

OLD VERSUS NEW FUNCTION DECLARATIONS

So far, I’ve been showing functions as follows:

int MyFunc( int a, char b, float c )
{
  /* Code goes here */
}

This way of writing functions is an ANSI extension that allows C to perform parameter type checking when calling a function. Things weren’t always so nice. In the original K&R C, functions were written as follows:

int MyFunc( a, b, c )
int a;
char b;
float c;
{
  /* Code goes here */
}

In original C compilers, when calling a function, there was no checking of parameter types, or often even of the number of parameters! In old C, you could write a call to a function before it had ever been defined, declared or mentioned in any way! Now, C compilers have much stronger type-checking. For example, Think C has a compiler option to require you to write a function prototype for every function.

FLOW CONTROL

So far, all of the examples that I’ve shown execute code sequentially - in fact, I’ve only shown declarations, assignment statements, function calls, and function return statements. Like Pascal, C has various loops and other constructs to control the flow of code. We’ll start with the while loop. The while loop in C is nearly identical to the one in Pascal, except that it needs parens around the test expression and it doesn’t have a DO keyword. Examples:

/* 2 */

while ( i < j ) i *= 2;   // First C example

while i < j DO i := i * 2;  {Pascal Equiv.}

while ( i < j )  // C example w/compound statement
{
  sysBeep( 1 );
  i *= 2;
}

while i < j DO   {Pascal Equiv.}
BEGIN
  sysBeep( 1 );
  i := i * 2
END

Next we have the C do statement. This is a loop with the test at the end of each iteration, like the Pascal REPEAT statement, but the sense of the while test at the end is the opposite of the Pascal UNTIL test. Unlike the Pascal version, the C version needs braces if the loop contains a compound statement. And again, the while condition needs parens. Example:

/* 3 */

do // C version
{
  sysBeep( x );
  ++ x;
}
while ( x != 10 );

REPEAT  {Pascal equiv.}
  sysBeep( x );
  x := x + 1
UNTIL x = 10;

Next we have the for loop.  The for loop in C is far more general than the one in Pascal. 
 It looks like this:

for ( expr1; expr2; expr3 )
  statement;

What it does is this: expr1 is an initialization that is performed before executing the loop for the first time. expr2 is a test that is performed before each iteration. As long as expr2 evaluates to non-zero, the looping continues. expr3 is a statement that is performed at the end of every iteration. C does not limit loops to simple count up and count down types. Any or all of the three expressions may be omitted, but the semicolons must remain. Any C for loop can be rewritten as follows:

/* 4 */

expr1;
while( expr2 )
{
  statement;
  expr3;
}
Example:

for ( i = 10; i != 0; --i ) // C example
  DoIt( i );

for ( i = 10; i; --i )    // An equivalent variation
  DoIt( i );

i = 10; // Another equivalent variation
while ( i )
{
  DoIt( i );// Could use pre or post decrement
  -- i;
}

i = 10; // Yet another equivalent variation
while ( i )
  DoIt( i-- );   // Must use post decrement

FOR i := 10 DOWNTO 0 DO   {Pascal equivalent}
 DoIt( i );

The C if statement is very similar to the Pascal version: The else clause is optional, and the statements can be either simple or compound. The only difference is that C needs parens around the expression, it doesn’t use the THEN keyword, and as always, every statement needs a semicolon. Example:

/* 5 */

if ( condition ) // C
  DoOneThing();
else
{
  DoAnother();
  AndAnother();
}

IF condition THEN {Pascal version}

/* 6 */

  DoOneThing
ELSE
BEGIN
  DoAnother;
  AndAnother
END

C has a case statement that is very similar to the Pascal version. An example should suffice:

/* 7 */

switch ( x )// C version
{
  case 1:
  case 2:
    DoTheOneOrTwoThing();
    break;  // Must explicitly leave each case
  case 3:
    DoTheThreeThing();
    AndTheOtherThreeThing();// Purposely fall through
  case 7:
    DoTheThreeAndSevenThing();
    break;
  default:
    DoTheDefaultThing();
}

CASE x OF { Pascal Version }

/* 8 */

  1, 2:  DoTheOneOrTwoThing;
  3:
    BEGIN
      DoTheThreeThing;
      AndTheOtherThreeThing;
      DoTheThreeAndSevenThing; { In Pascal, we need this twice }
    END;
  7:
      DoTheThreeAndSevenThing; {In Pascal, we need this twice}
  OTHERWISE DoTheDefaultThing
END

The previous example used the C break keyword. This keyword is like the Pascal Leave statement, and can be used to break out of the innermost while, do, for, or switch. C also has a continue keyword that like the Pascal Cycle statement. It skips to the next iteration of the innermost while, do or for loop.

Finally, C also has the dreaded goto statement (nothing seems to split programmers into warring factions as well as a goto statement does). Unlike in Pascal, you don’t declare labels in C, you just stick ‘em in the code, and they follow the same syntax as other identifiers. Example:

/* 9 */

{// C
  MyLabel:
    x := Function();
    if ( x == 10 ) goto MyLabel;
}

LABEL 333; { Pascal }
BEGIN   
  333: x = Function;
  IF x=10 THEN GOTO 333
END

LIBRARY FUNCTIONS

Standard C has oodles of library functions, such as malloc() and fread(), that you would use if you were programming on any computer unless you are programming on a Macintosh which you are. So for the most part, you will use calls like NewPtr() and FSRead() instead. You’ll need to look at your C compiler manual if you are interested in the standard C libraries.

STRINGS

Amazingly enough, standard C doesn’t really provide much built-in language support for strings. There are several standard C library functions that process strings, but no real string type or operators. C handles strings as simple arrays of the char type. In general, you would create a string in one of the following ways:

{
  char myString[100]; // 100 bytes of storage
  char Another[] = "LetTheCompilerCountTheSize";
  char * ptrToString;

  ptrToString = NewPtr( 100 );
}

C also has a different way of representing strings than the Pascal way. Instead of having a length byte followed by a number of characters, C starts immediately with characters, and the string is considered to continue until the occurrence of a zero byte. So this declaration:

char myString[] = "Foo";

creates four bytes of storage. It fills the first three with the word "Foo" and puts a zero byte in the fourth. This convention allows strings of arbitrary length.

This string representation doesn’t fit well with Pascal nor with the Mac toolbox, but don’t despair. C only uses this convention in two places: In string constants (like the "Foo" shown above) and in its library functions. The Mac solves the second problem by shunning the C library that is used by the rest of the world, in favor of its own toolbox. And the compilers on the Mac solve the first problem by introducing an ingenious extension, the \p escape sequence. Here is an example:

char pascalString[] = "\pFoo";

This causes the compiler to insert a Pascal-style length byte at the beginning of the string. It still generates a zero-byte at the end, however. So the above declaration would use five bytes: The first byte contains a 3 (for the Pascal length), the next three bytes contain the string, and the final byte contains the C-style zero byte. So pascalString can be used as a Pascal string, and (pascalString+1) or &pascalString[1] can be used as a C string.

THE PREPROCESSOR

C compilers include a preprocessor step that reads in the source file, expands macros, and then writes back a temporary file that is fed into the actual compiler. Keep in mind that preprocessor commands are purely compile-time, not run-time operations. They are similar to Pascal {$ } compiler directives. Instead of being embedded within comments, C preprocessor instructions begin with a number sign ‘#’. Here is a (somewhat contrived) code fragment that includes many of the common preprocessor instructions:

/* 10 */

#include <stdio.h>
#include "myheader.h"

#define DEBUG    // Delete this line before shipping program.
#define PI3.14159
#define square(a)( ( a ) * ( a ) )
#define cube(a)  ( ( a ) * square( a ) )
#define max(a,b) ( ( ( a ) > ( b ) ) ? ( a ) : ( b ) )

#pragma segment mySegment

double MaxSurfaceOrVolume( double radius )
/*
 * This is a strange function which will return either the 
 * surface area or the volume of a sphere, whichever is 
 * larger, for a given radius.
 */
{
#ifdef DEBUG
  printf( "Hey, we're in the MaxSurfaceOrVolume function" );
#else
  printf( "Hey, we're running the non-debug version" );
#endif

#if 0

I could have a bunch of lines of code in here, and they

wouldn't ever be executed, or even compiled.

/* 11 */

#endif

  return max( 4 * PI * square( radius ), 
    4.0/3.0 * PI * cube( radius ) );
}

The following table describes the preceding preprocessor statements:


Preprocessor Statement Meaning

#include <stdio.h> Similar to the Pascal {$I filename} directive. Paste the contents of the included file into here as if they had actually been typed into this file. The angle brackets generally tell the compiler to look for the include file in its list of “system” file folders. Include files are generally named with a .h at the end. They generally consist of things like typedefs, global variable definitions, function prototypes, preprocessor macros, etc. The Mac compilers provide header files that prototype all the toolbox functions so you don’t have to.

#include "myHeader.h" Same as above, but look in the list of “user” file folders instead of system file folders.

#define DEBUG Similar to the Pascal {$SETC DEBUG = 1} directive. Define the existence of a preprocessor variable. The existence of the variable can be checked later.

#define PI 3.14159 A simple text substitution. Replace all future occurences of PI with 3.14159.

#define square(a) ((a)*(a)) A substitution that takes parameters. Keep in mind that while a macro like this may look like a function call, it is purely text substitution, and therefore incurs none of the overhead of a function call.

#pragma segment mySegment The #pragma feature allows compiler specific instructions that are not actually part of the C language. Each compiler has its own pragmas. They are used for such things as turning optimizations on and off, disabling compiler warnings, or in this case, telling the compiler in what code segment to put this code. They perform many of the same functions as the miscellaneous Pascal {* } directives.

#ifdef DEBUG Similar to {$IFC } in Pascal. The subsequent statements will only be compiled if the variable is defined.

#else Similar to {$ELSEC}. The subsequent statements will only be compiled in the “else” case of the preceding #if.

#endif Similar to {$ENDC}. Ends a preprocessor #if or #ifdef construction.

#if 0 This is a quick way to disable a chunk of code. Change it to #if 1 to re-enable.

SUMMARY

You should now know enough C to be able to read C code listings. If you would like to get some more practice at seeing the differences between C and Pascal, you may wish to check out Dave Mark’s first few “Getting Started” articles. Dave wrote both a C and Pascal version for all of his programs in the 1992 columns. And while we didn’t print all of the listings in the magazine, we did include them in the source code disks and in our CD-ROM. Beyond that, you’ll probably have to break down a buy a couple of C books.

Million dollar (no, we won’t pay you, even if you have a good answer!) bonus question: K&R say that the term define is used when actually creating storage for a variable, and that the term declare is used when describing the characteristics of a variable (and only possibly creating storage). So why is it that type declarations, which allocate no storage, are spelled typedef (short for type definition) instead of being spelled typedecl? Perhaps this has been discussed somewhere before, but not that I’ve seen. Personally, I like the C keyword, and think that K&R have the define and declare terms backwards throughout their book!

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Together 3.6.1 - Store and organize all...
Together helps you organize your Mac, giving you the ability to store, edit and preview your files in a single clean, uncluttered interface. Features Smart storage. With simple drag-and-drop... Read more
Cloud 4.1.1 - File sharing from your men...
Cloud is simple file sharing for the Mac. Drag a file from your Mac to the CloudApp icon in the menubar and we take care of the rest. A link to the file will automatically be copied to your clipboard... Read more
OmniFocus 2.7.1 - GTD task manager with...
OmniFocus helps you manage your tasks the way that you want, freeing you to focus your attention on the things that matter to you most. Capturing tasks and ideas is always a keyboard shortcut away in... Read more
CleanApp 5.1.1 - Application deinstaller...
CleanApp is an application deinstaller and archiver.... Your hard drive gets fuller day by day, but do you know why? CleanApp 5 provides you with insights how to reclaim disk space. There are... Read more
ForkLift 3.0 Beta 2 - Powerful file mana...
ForkLift is a powerful file manager and ferociously fast FTP client clothed in a clean and versatile UI that offers the combination of absolute simplicity and raw power expected from a well-executed... Read more
Sublime Text 3126 - Sophisticated text e...
Sublime Text is a sophisticated text editor for code, markup, and prose. You'll love the slick user interface, extraordinary features, and amazing performance. Features Goto Anything. Use Goto... Read more
1Password 6.3.3 - Powerful password mana...
1Password is a password manager that uniquely brings you both security and convenience. It is the only program that provides anti-phishing protection and goes beyond password management by adding Web... Read more
WhatsApp 0.2.1880 - Desktop client for W...
WhatsApp is the desktop client for WhatsApp Messenger, a cross-platform mobile messaging app which allows you to exchange messages without having to pay for SMS. WhatsApp Messenger is available for... Read more
NeoFinder 6.9.3 - Catalog your external...
NeoFinder (formerly CDFinder) rapidly organizes your data, either on external or internal disks, or any other volumes. It catalogs all your data, so you stay in control of your data archive or disk... Read more
Amadeus Pro 2.3.1 - Multitrack sound rec...
Amadeus Pro lets you use your Mac computer for any audio-related task, such as live audio recording, digitizing tapes and records, converting between a variety of sound formats, etc. Thanks to its... Read more

Zip—Zap (Games)
Zip—Zap 1.01 Device: iOS Universal Category: Games Price: $1.99, Version: 1.01 (iTunes) Description: Touch to contract.Release to let go.Bring the clumsy mechanical beings home. · · · over 100 levelsno adsno in-app-purchases Zip—... | Read more »
Paperback: The Game (Games)
Paperback: The Game 1.0 Device: iOS Universal Category: Games Price: $3.99, Version: 1.0 (iTunes) Description: You are an author trying to finish kitschy paperback novels. Complete Westerns, Science Fiction, Romance or even a Crime... | Read more »
How to Rule With a Firm Hand in My Majes...
My Majesty is a kingdom management sim not unlike August’s magisterial hit, Reigns. It’s essentially a reskin of developer Tigrido’s previous management sim, Dictator. As supreme ruler of the land, you must consult with a number of subjects to... | Read more »
Our 5 Favorite iMessage Sticker Packs
At long last, iMessage joins the ranks of messaging apps the likes of LINE and Whatsapp, adding an impressive collection of stickers. They’re a great way to add a little something extra to your daily conversations. [Read more] | Read more »
How to get past Vulture Island's tr...
Vulture Island is a colorful and quirky mish-mash of platforming and puzzles. It’s creative and fresh, but sometimes the game can throw a curveball at you, leaving you stuck as to how you should progress. These tips will help you explore smoothly... | Read more »
The new Clash of Kings is just for Weste...
If you’ve played the original Clash of Kings, you’ll probably recognise the city building, alliance forging and strategic battles in Clash of Kings: The West. What sets this version apart is that it’s tailor made for a Western audience and the... | Read more »
Frost - Survival card game (Games)
Frost - Survival card game 1.12.1 Device: iOS Universal Category: Games Price: $3.99, Version: 1.12.1 (iTunes) Description: *Warning: the game will work on iPhone 5C and above and iPad Pro / 4. Other devices are not supported* | Read more »
How to build and care for your team in D...
Before you hit the trail and become a dog sledding legend, there’s actually a fair bit of prep work to be done. In Dog Sled Saga, you’re not only racing, you’re also building and caring for a team of furry friends. There’s a lot to consider—... | Read more »
How to win every race in Dog Sled Saga
If I had to guess, I’d say Dog Sled Saga is the most adorable racing game on the App Store right now. It’s a dog sled racing sim full of adorable, loyal puppies. Just look at those fluffy little tails wagging. Behind that cute, pixelated facade is... | Read more »
Let the war games commence in Gunship Ba...
Buzz Lightyear famously said, “This isn’t flying, this is falling – with style!” In the case of Gunship Battle: Second War, though, this really is flying - with style! The flight simulator app from Joycity puts you in control of 20 faithfully... | Read more »

Price Scanner via MacPrices.net

15-inch 2.2GHz Retina MacBook Pro on sale for...
B&H Photo has the 2015 15″ 2.2GHz Retina MacBook Pro (MJLQ2LL/A) on sale for $1799, including free shipping plus NY sales tax only. Amazon also has the 2015 15″ 2.2GHz Retina MacBook Pro (... Read more
Toughbook Celebrates 20 Years of Ruggedized M...
Panasonic System Communications Company of North America, Division of Panasonic Corporation of North America (Panasonic) today celebrates the 20th anniversary of its industry-leading Toughbook mobile... Read more
12-inch 1.1GHz Gray Retina MacBook on sale fo...
B&H Photo has the 2016 12″ 1.1GHz Gray Retina MacBook on sale for $1199.99 including free shipping plus NY sales tax only. Their price is $100 off MSRP. Read more
13-inch 2.5GHz MacBook Pro (Apple refurbished...
Apple has Certified Refurbished 13″ 2.5GHz MacBook Pros available for $829, or $270 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free: - 13″ 2.5GHz MacBook Pros... Read more
Save 30% on Camera Guard’s Secure Protection...
To celebrate the release of macOS Sierra, Miami-based security solutions company, ProtectStar has announced a special 30% discount on Camera Guard Professional for Mac 2016. This innovative security... Read more
DVDFab Special Deal – Get a 1-Year Free Licen...
Beijing, China based specialist in the field of DVD, Blu-ray and video backup solutions, Fengtao Software has launched its Autumn Special Deals 2016, giving a 1-year free license of a randomly picked... Read more
21-inch iMacs on sale for up to $120 off MSRP
B&H Photo has 21″ iMacs on sale for up to $120 off MSRP including free shipping plus NY sales tax only: - 21″ 3.1GHz iMac 4K: $1379 $120 off MSRP - 21″ 2.8GHz iMac: $1199.99 $100 off MSRP - 21″ 1... Read more
13-inch 2.7GHz/256GB Retina MacBook Pro on sa...
Amazon.com has the 13″ 2.7GHz/256GB Retina Apple MacBook Pro on sale for $151 off MSRP including free shipping: - 13″ 2.7GHz/256GB Retina MacBook Pro (sku MF840LL/A): $1348 $151 off MSRP Read more
Apple TVs on sale for up to $50 off MSRP
Best Buy has 32GB and 64GB Apple TVs on sale for $40-$50 off MSRP on their online store. Choose free shipping or free local store pickup (if available). Sale prices for online orders only, in-store... Read more
Apple refurbished 13-inch Retina MacBook Pros...
Apple has Certified Refurbished 13″ Retina MacBook Pros available for up to $270 off the cost of new models. An Apple one-year warranty is included with each model, and shipping is free: - 13″ 2.7GHz... Read more

Jobs Board

*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
Restaurant Manager (Neighborhood Captain) - A...
…in every aspect of daily operation. WHY YOU'LL LIKE IT: You'll be the Big Apple . You'll solve problems. You'll get to show your ability to handle the stress and Read more
Sr. *Apple* Mac Engineer - Net2Source Inc....
…staffing, training and technology. We have following position open with our client. Sr. Apple Mac Engineer6+ Months CTH Start date : 19th Sept Travelling Job If Read more
*Apple* Retail - Multiple Positions-Norfolk,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Restaurant Manager (Neighborhood Captain) - A...
…in every aspect of daily operation. WHY YOU'LL LIKE IT: You'll be the Big Apple . You'll solve problems. You'll get to show your ability to handle the stress and Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.