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>

#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

/*
* 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:

Network Radar is an advanced network scanning and managing tool. Featuring an easy-to-use and streamlined design, the all-new Network Radar 2 has been engineered from the ground up as a modern Mac... Read more
Printopia 3.0.8 - Share Mac printers wit...
Run Printopia on your Mac to share its printers to any capable iPhone, iPad, or iPod Touch. Printopia will also add virtual printers, allowing you to save print-outs to your Mac and send to apps.... Read more
ForkLift 3.2.1 - Powerful file manager:...
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
BetterTouchTool 2.417 - Customize multi-...
BetterTouchTool adds many new, fully customizable gestures to the Magic Mouse, Multi-Touch MacBook trackpad, and Magic Trackpad. These gestures are customizable: Magic Mouse: Pinch in / out (zoom... Read more
Little Snitch gives you control over your private outgoing data. Track background activity As soon as your computer connects to the Internet, applications often have permission to send any... Read more
Google Chrome 65.0.3325.181 - Modern and...
Google Chrome is a Web browser by Google, created to be a modern platform for Web pages and applications. It utilizes very fast loading of Web pages and has a V8 engine, which is a custom built... Read more
TeamViewer 13.1.2991 - Establish remote...
TeamViewer gives you remote control of any computer or Mac over the Internet within seconds or can be used for online meetings. Find out why more than 200 million users trust TeamViewer! Free for non... Read more
Printopia 3.0.8 - Share Mac printers wit...
Run Printopia on your Mac to share its printers to any capable iPhone, iPad, or iPod Touch. Printopia will also add virtual printers, allowing you to save print-outs to your Mac and send to apps.... Read more
BetterTouchTool 2.417 - Customize multi-...
BetterTouchTool adds many new, fully customizable gestures to the Magic Mouse, Multi-Touch MacBook trackpad, and Magic Trackpad. These gestures are customizable: Magic Mouse: Pinch in / out (zoom... Read more
Mellel 4.1.0 - The word processor for sc...
Mellel is the leading word processor for OS X and has been widely considered the industry standard for long form documents since its inception. Mellel focuses on writers and scholars for technical... Read more

## Latest Forum Discussions

Destiny meets its mobile match - Everyth...
Shadowgun Legends is the latest game in the Shadowgun series, and it's taking the franchise in some interesting new directions. Which is good news. The even better news is that it's coming out tomorrow, so if you didn't make it into the beta you... | Read more »
How PUBG, Fortnite, and the battle royal...
The history of the battle royale genre isn't a long one. While the nascent parts of the experience have existed ever since players first started killing one another online, it's really only in the past six years that the genre has coalesced into... | Read more »
Around the Empire: What have you missed...
Oh hi nice reader, and thanks for popping in to check out our weekly round-up of all the stuff that you might have missed across the Steel Media network. Yeah, that's right, it's a big ol' network. Obviously 148Apps is the best, but there are some... | Read more »
All the best games on sale for iPhone an...
It might not have been the greatest week for new releases on the App Store, but don't let that get you down, because there are some truly incredible games on sale for iPhone and iPad right now. Seriously, you could buy anything on this list and I... | Read more »
Everything You Need to Know About The Fo...
In just over a week, Epic Games has made a flurry of announcements. First, they revealed that Fortnite—their ultra-popular PUBG competitor—is coming to mobile. This was followed by brief sign-up period for interested beta testers before sending out... | Read more »
The best games that came out for iPhone...
It's not been the best week for games on the App Store. There are a few decent ones here and there, but nothing that's really going to make you throw down what you're doing and run to the nearest WiFi hotspot in order to download it. That's not to... | Read more »
Death Coming (Games)
Death Coming 1.1.1.536 Device: iOS Universal Category: Games Price: \$1.99, Version: 1.1.1.536 (iTunes) Description: --- Background Story ---You Died. Pure and simple, but death was not the end. You have become an agent of Death: a... | Read more »
Hints, tips, and tricks for Empires and...
Empires and Puzzles is a slick match-stuff RPG that mixes in a bunch of city-building aspects to keep things fresh. And it's currently the Game of the Day over on the App Store. So, if you're picking it up for the first time today, we thought it'd... | Read more »
What You Need to Know About Sam Barlow’s...
Sam Barlow’s follow up to Her Story is #WarGames, an interactive video series that reimagines the 1983 film WarGames in a more present day context. It’s not exactly a game, but it’s definitely still interesting. Here are the top things you should... | Read more »
Pixel Plex Guide - How to Build Better T...
Pixel Plex is the latest city builder that has come to the App Store, and it takes a pretty different tact than the ones that came before it. Instead of being in charge of your own city by yourself, you have to work together with other players to... | Read more »

## Price Scanner via MacPrices.net

Back in stock! Apple’s full line of Certified...
Save \$300-\$300 on the purchase of a 2017 13″ MacBook Pro today with Certified Refurbished models at Apple. Apple’s refurbished prices are the lowest available for each model from any reseller. A... Read more
Back in stock: 13-inch 2.5GHz MacBook Pro (Ce...
Apple has Certified Refurbished 13″ 2.5GHz MacBook Pros (MD101LL/A) available for \$829, or \$270 off original MSRP. Apple’s one-year warranty is standard, and shipping is free: – 13″ 2.5GHz MacBook... Read more
Apple restocks Certified Refurbished 2017 13″...
Apple has Certified Refurbished 2017 13″ MacBook Airs available starting at \$849. An Apple one-year warranty is included with each MacBook, and shipping is free: – 13″ 1.8GHz/8GB/128GB MacBook Air (... Read more
8-Core iMac Pro on sale for \$4699, save \$300
Amazon has the 8-core iMac Pro on sale for \$4699 including free shipping. Their price is \$300 off MSRP, and it’s the currently lowest price available for an iMac Pro. For the latest up-to-date prices... Read more
10″ 512GB WiFi iPad Pros on sale for \$849, sa...
B&H Photo has Space Gray and Rose Gold 10.5″ 512GB WiFi iPad Pros on sale for \$849. Their price is \$150 off MSRP, and it’s the lowest price available for these models, new, from any Apple... Read more
MacBook Pro sale! B&H drops prices on new...
B&H Photo has dropped prices on new 2017 13″ MacBook Pros, with models now on sale for up to \$200 off MSRP. Shipping is free, and B&H charges sales tax for NY & NJ residents only. Their... Read more
13″ MacBook Airs on sale for \$100-\$150 off MS...
B&H Photo has 13″ MacBook Airs on sale for \$100-\$150 off MSRP. Shipping is free, and B&H charges sales tax for NY & NJ residents only: – 13″ 1.8GHz/128GB MacBook Air (MQD32LL/A): \$899, \$... Read more
Huge iMac sale! Apple reseller now offering 2...
B&H Photo has new 2017 21″ & 27″ iMacs on sale today for up to \$300 off MSRP. Shipping is free, and B&H charges sales tax for NY & NJ residents only: – 27″ 3.8GHz iMac (MNED2LL/A): \$... Read more
Sale! 1.4GHz Mac mini for \$399, \$100 off MSRP
B&H Photo has the 1.4GHz Mac mini on sale for \$399 for a limited time. Their price is \$100 off MSRP, and it’s the lowest price available for a mini from any Apple reseller: – 1.4GHz Mac mini (... Read more
Sale of the year continues as Apple resellers...
Adorama has new 2017 15″ MacBook Pros on sale for \$250-\$300 off MSRP. Shipping is free, and Adorama charges sales tax in NJ and NY only: – 15″ 2.8GHz Touch Bar MacBook Pro Space Gray (MPTR2LL/A): \$... Read more

## Jobs Board

*Apple* Genius - Technical Customer Service...
Job Description:Job SummaryAs a Genius at the Apple Store, you maintain customers' trust in Apple as the skilled technical customer service expert, Read more