CMSI 2210: Welcome to Week 04

This Week's Agenda

For this week, here's the plan, Fran…

Character Encoding

In computer concepts, a character is a unit of information in textual data, which has a name associated with it. Some easy examples, of course, are the alphabet we are used to using, in which each of the symbols, or letters, is usually called a character. However, there are LOTS of other characters, which go by names such as:

A grapheme is a minimally distinctive unit of writing in some writing system. It is what a person usually thinks of as a character. However, it may take more than one character to make up a grapheme. For example, the grapheme:

is made up of two characters (1) LATIN CAPITAL LETTER R and (2) COMBINING RING ABOVE. The grapheme:

நி

is made up of two characters (1) TAMIL LETTER NA and (2) TAMIL VOWEL SIGN I. This grapheme:

🚴🏾

is made up of two characters (1) BICYCLIST and (2) EMOJI MODIFIER FITZPATRICK TYPE-5. This grapheme:

🏄🏻‍♀

is made up of four characters (1) SURFER, (2) EMOJI MODIFIER FITZPATRICK TYPE-1-2, (3) ZERO-WIDTH JOINER, (4) FEMALE SIGN. And this grapheme:

🇨🇻

requires two characters: (1) REGIONAL INDICATOR SYMBOL LETTER C and (2) REGIONAL INDICATOR SYMBOL LETTER V. It's the flag for Cape Verde (CV).

Finally, a glyph is a picture of a character [or grapheme]. Two or more characters can share the same glyph [e.g. LATIN CAPITAL LETTER A and GREEK CAPITAL LETTER ALPHA], and one character can have many glyphs [think fonts, e.g., A A A and so on…]

Investigation: Go find out from the Internet what you need to know to answer the question, What glyph can represent the three characters LATIN SMALL LETTER O WITH STROKE, DIAMETER SIGN, andEMPTY SET.? Then draw your own copy of it.

Investigation: Go find out from the Internet what you need to know to answer the question, What two characters would you use to make the Spanish flag?

Character Sets

A character set, then, is a set of these characters that are linked by some important unifying characteristic[s]. It has two parts: (1) a repertoire, which is the characters which make up the set, and (2) a code position mapping, which is a function mapping a non-negative integer to EACH INDIVIDUAL character in the repertoire.

As usual, the term mapping means a one-to-one correspondence between these two values. For example, if the value 65 is set up to correspond to the character Uppercase Latin A, that is the kind of mapping referred to here. Thus, another term for you to know is, when an integer i maps to a character c we say i is the code point of c.

A character set, then, can also be considered a set of code points.

BCD, EBCDIC, and ASCII

Three older examples of character sets are BCD [which stands for Binary-Coded Decimal], EBCDIC [which stands for Extended Binary Coded Decimal Interchange Code], and ASCII [which stands for American Standard Code for Information Interchange].

BCD is the oldest of the three. It is a way of representing decimal numbers in which each digit is encoded into a specific fixed number of bits. Usually, either four or eight bits are used, with four being the default since that is all that are required. Note that BCD is specifically used for numeric digit values, NOT for letter characters, and as implied by the name, it encodes decimal digits.

The main value of BCD is that it is more easily human readable than normal binary, because each four bits [or tetrade, another word for nybble] only handles one digit. Since the values only go from zero to nine, the bit patterns correspond to the binary tables we saw last week. BCD was a part of most operating systems and processor instruction sets up until modern times, but with the advent of modern processors such as the ARM and later versions of x86 it is no longer used. It is still useful to know, however, because there are hardware chips that use the incoding.

This link will give you more information, and this table shows the encoding scheme.

Digit
Value
BCD
Value
00000
10001
20010
30011
40100
50101
60110
70111
81000
91001
Zones
1111Unsigned
1100Positive
11D1Negative

Because in most computers the smallest unit of data storage is one byte, there is a possibility for a lot of wasted space with BCD. For example, the number 24310 could be stored in BCD as 0010 0100 0011. If that is extended to 8-bit format, but the same idea of one BCD value per 8-bits is kept, the binary string balloons to become 00000010 00000100 00000011. A more efficient way of encoding is using what is known as packed BCD in which two BCD values are stored within each byte.

Packed BCD also allows for signed values, but the sign is put at the END instead of at the beginning, and consists of four bits, as shown in the Zones section of the table on the left. If our value 24310 is represented as a positive signed number it would become
0010 0100 0011 1100. Note that if there are even numbers of digits, there would still be four extra leading zeros required for the most significant byte.

EBCDIC is probably one of the oldest ways of encoding characters so that digital computers can haneld them. It started in 1963 – 1964, and was invented by IBM for use on its System/360 mainframe computers [which should give you a heads-up on something else all good computer scientists should know about, the book called The Mythical Man Month by Fred Brooks, who was the chief system architect of that system].

EBCDIC is an 8-bit encoding scheme, which was created to extend BCD and its follow-on, BCDIC. These two encodings were used to help insure the physical integrity of punched cards, which at the time were used to imput both code and data into the mainframe computers. All IBM mainframe computers used EBCDIC as the basic encoding scheme for many years. In fact, there is actually an EBCDIC-oriented Unicode Transformation Format called UTF-EBCDIC proposed by the Unicode Consortium, which is designed to allow EBCDIC software to handle Unicode!

talk about your hanging chads...

ASCII is, apart from the modern Unicode sets, the most well-known character encoding scheme. It was also developed primarily by IBM, and actually its development was in process during the same period that the EBCDIC set was in work.

well, there ya go then...

ASCII was originally a 7-bit coding, but over time, since the standardization of 8-bits for the byte and the need for more than 128 characters, it turned into an 8-bit code. The first 32 characters of ASCII are the so-called control characters, with code points from 0 – 31. Code point 32 is the space, and there is room for all the other characters on the standard QWERTY keyboard. One of the interesting offshoots of ASCII is that many of the characters are apart from each other by a factor of 32. For example, the lowercase a has code point 97, and the uppercase A has code point 65, which is 97 minus 32, with 32 being of course 16 × 2.

Another useful artifact of ASCII is that several programming languages [such as Java] treat individual characters as synonymous with their ASCII code points. In such cases, you can actually add and subtract numeric values from the characters, so they can be used as array indices and for other such operations.

Unicode

Another example of a character set is Unicode. Here is part of its code point mapping [note that code points are traditionally written in hex]:

      25  PERCENT SIGN — %
      2C  COMMA  — ,
      54  LATIN CAPITAL LETTER T  — T
      5D  RIGHT SQUARE BRACKET  — ]
      B0  DEGREE SIGN  — °
      C9  LATIN CAPITAL LETTER E WITH ACUTE  — É
     2AD  LATIN LETTER BIDENTAL PERCUSSIVE  — ʭ
     39B  GREEK CAPITAL LETTER LAMDA  — Λ
     446  CYRILLIC SMALL LETTER TSE  — ц
     543  ARMENIAN CAPITAL LETTER CHEH  — Ճ
     5E6  HEBREW LETTER TSADI  — צ
     635  ARABIC LETTER SAD  — ص
     BEF  TAMIL DIGIT NINE  — 
    13CB  CHEROKEE LETTER QUV  — 
    2023  TRIANGULAR BULLET  — 
    20A4  LIRA SIGN  — 
    2105  CARE OF  — 
    213A  ROTATED CAPITAL Q  — 
    21B7  CLOCKWISE TOP SEMICIRCLE ARROW  — 
    2226  NOT PARALLEL TO  — 
    2234  THEREFORE  — 
    2248  ALMOST EQUAL TO  — 
    265E  BLACK CHESS KNIGHT  — 
   1D122  MUSICAL SYMBOL F CLEF  — 𝄢
   1F08E  DOMINO TILE VERTICAL-06-01  — 🂎
   1F001  SQUID  — 🀁
   1F0CE  PLAYING CARD KING OF DIAMONDS  — 🃎
   1F382  BIRTHDAY CAKE  — 🎂
   1F353  STRAWBERRY  — 🍓
            

Because characters can have multiple glyphs, Unicode lets you represent characters unambiguously with U+ followed by four to six hex digits [e.g. U+00C9, U+1D122]. You can also represent them using HTML [as you can see from the code on this web page] by putting the prefix &#X followed by the hexadecimal code point, and ending with a semicolon. [You can also use the equivalent decimal value by converting the hex and dropping the X...]

How many characters are there?

How many characters are possible with Unicode? The powers that be have declared that the highest code point they will ever map is 10FFFF, so it seems like 1,114,112 code points are possible. However, they also declared they would never map characters to code points D800..DFFF, FDD0..FDEF, {0..F}FFFE, {0..F}FFFF, 10FFFE, 10FFFF [2114 code points]. So the maximum number of possible characters is 1,111,998.

How many characters have been assigned [so far]? See this cool table at Wikipedia to see how many characters have actually been mapped in each version of Unicode. [BTW, if you don't peek, Unicode Version 10.0 has 136,755 characters mapped, so there is a lot of room to grow.]

Where can I find all of them?

Please see the complete and up-to-date code charts, with example glyphs of every character. If you would like a much easier way to browse the characters [and why wouldn't you?], check out the beautiful charbase.com and codepoints.net. [Charbase and Codepoints are run by volunteers, so they may not be up to date.]

Blocks

The code points are not assigned haphazardly. They are allocated into blocks. There are around 300 blocks. Make sure to see the official blocks file at Unicode.org. If you need a little preview, here is a little slice of that file:

     0000 — 007F     Basic Latin
     0080 — 00FF     Latin-1 Supplement
     0250 — 02AF     IPA Extensions
     0400 — 04FF     Cyrillic
     0F00 — 0FFF     Tibetan
     2200 — 22FF     Mathematical Operators
     2700 — 27BF     Dingbats
     3100 — 312F     Bopomofo
    10860 — 1087F    Palmyrene
    12000 — 123FF    Cuneiform
    1D100 — 1D1FF    Musical Symbols
    1F000 — 1F02F    Mahjong Tiles
    F0000 — FFFFF    Supplementary Private Use Area-A
   100000 — 10FFFF   Supplementary Private Use Area-B
            

Categories

Every Unicode code point is assigned various properties, such as name and category. The complete list is beyond the scope of this discussion, but you can fine out more about them from the Internet.

Combining Characters, Modifiers, and Variation Selectors

It's common to use multiple characters to make up graphemes. This is often done to put marks on letters, modify skin tones, modify genders, and create more complex graphemes. Examples:

ş̌́ U+0073 LATIN SMALL LETTER S
U+0327 COMBINING CEDILLA
U+030C COMBINING CARON
U+0301 COMBINING ACUTE ACCENT
[Note that this looks different on different browsers…]
👶 U+1F476 BABY baby
👶🏻 U+1F476 BABY
U+1F3FB EMOJI MODIFIER FITZPATRICK TYPE-1-2
baby: light skin tone
👶🏾 U+1F476 BABY
U+1F3FE EMOJI MODIFIER FITZPATRICK TYPE-5
baby: medium-dark skin tone
👶🏿 U+1F476 BABY
U+1F3FE EMOJI MODIFIER FITZPATRICK TYPE-6
baby: dark skin tone
👩‍⚖️ U+1F469 WOMAN
U+200D ZERO WIDTH JOINER
U+2696 SCALES
U+FE0F VARIATION SELECTOR-16
woman judge
👩🏿‍🔬 U+1F469 WOMAN
U+1F3FF EMOJI MODIFIER FITZPATRICK TYPE-6
U+200D ZERO WIDTH JOINER
U+1F52C MICROSCOPE
woman scientist: dark skin-tone
🤽🏾‍♀️ U+1F93D WATER POLO
U+1F3FE EMOJI MODIFIER FITZPATRICK TYPE-5
U+200D ZERO WIDTH JOINER
U+2640 FEMALE SIGN
U+FE0F VARIATION SELECTOR-16
woman playing water polo: medium-dark
skin tone

See all the emojis at Unicode.org.

Investigation: Research and describe the function of VARIATION SELECTOR-16. How is it different from VARIATION SELECTOR-15?

Where can I get a list of all the fun stuff?

Awesome Codepoints is super cool.

Other Character Sets

Unicode is really the only character set you should be working with. However, other character sets exist, and you should probably know something about them.

ISO8859-1

ISO8859-1 is a character set that is exactly equivalent to the first 256 mappings of Unicode. Obviously it doesn't have enough characters, but it also maps nicely to ASCII for backward compatibility.

ISO8859-2 through ISO8859-16

These 15 charsets also have 256-character repertoires. They all share the same characters in the first 128 positions, but differ in the next 128. Again ASCII and backward compatibility.

Details at http://www.unicode.org/Public/MAPPINGS/ISO8859/.

Windows-1252

This character set, with a repertoire of 256 characters, also known as CP1252, can be found at http://www.unicode.org/Public/MAPPINGS/VENDORS/MICSFT/WINDOWS/CP1252.TXT. It is very close to ISO8859-1. Be careful with this set! Users of Windows systems often unknowingly produce documents with this character set, then forget to specify it when making these documents available on the web or transporting them via other protocols with tend to default to Unicode. Then the end result is annoying. It's best to avoid this.

ASCII

The last thing about ASCII is that is exactly equivalent to the first 128 mappings of Unicode. Obviously it doesn't have enough characters for EVERYTHING. However it is commonly used, because many Internet protocols require it! It is a common subset of many character sets and something most people can agree on. Here is a link to an ASCII table.

Ways to Encode Characters

A character encoding specifies how a character [or character string] is encoded in a bit string. Character sets with 256 characters or less have only a single encoding: they encode each character in a single byte [so in some sense the character set and the character encode are about the same]. But there are, naturally, many encodings of Unicode. The most important are UTF-32, UTF-16 and UTF-8.

UTF-32

This is the simplest. Just encode each character in 32 bits. The encoding of a character is simply its code point! Couldn't be more straightforward. Of course, you try to convince people to actually USE four bytes per character.

There are actually two kinds: UTF-32BE [Big Endian] and UTF-32LE [Little Endian]. Examples:

Unicode CharacterUTF-32BEUTF-32LE
RIGHT SQUARE BRACKET [U+005D] 00 00 00 5D5D 00 00 00
CHEROKEE LETTER QUV [U+13CB] 00 00 13 CBCB 13 00 00
MUSICAL SYMBOL F CLEF [U+1D122]00 01 D1 2222 D1 01 00

Note that every character sequence can be encoded into a byte sequence, but not every byte sequence can be decoded into a character sequence. For example, the byte sequence CC CC CC CC does not decode to any character because there is not code point CCCCCCCC in Unicode.

UTF-32
The Good: It's fixed width! Constant-time to find the nth character in a string [provided you care].
The Bad: It's pretty bloated.

UTF-16

In UTF-16 some characters are encoded in 16 bits and some in 32 bits.

Character RangeBit Encoding
U+0000 ... U+FFFF xxxxxxxx xxxxxxxx
U+10000 ... U+10FFFFlet y = X-1000016 in
110110yy yyyyyyyy 110111yy yyyyyyyy

Note that all characters requiring 32-bits have their first 16 bits in the range D800..D8FF. Pretty slick, right? Those code points are never assigned to any character in Unicode.... How perfect, dontcha think?

There are actually two kinds: UTF-16BE and UTF-16LE. Examples:

Unicode CharacterUTF-16BEUTF-16LE
RIGHT SQUARE BRACKET [U+005D] 00 5D 5D 00
CHEROKEE LETTER QUV [U+13CB] 13 CB CB 13
MUSICAL SYMBOL F CLEF [U+1D122]D8 34 DD 2222 DD 34 D8

UTF-16
The Good: Nothing is good about this encoding.
The Bad: Variable width, almost always uses more space than UTF-8, even for East Asian scripts, people.
The Ugly: It's what JavaScript thinks characters are. Facepalm.

UTF-8

Here's another variable length encoding.

Character RangeBit EncodingNumber of Bits
U+0000 ... U+007F 0xxxxxxx7
U+0080 ... U+07FF 110xxxxx 10xxxxxx11
U+0800 ... U+FFFF 1110xxxx 10xxxxxx 10xxxxxx16
U+10000 ... U+1FFFFF 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx21
U+200000 ... U+3FFFFFF 111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx26
U+4000000 ... U+7FFFFFFF1111110x 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx31

Examples:

Unicode CharacterUTF-8 Encoding
RIGHT SQUARE BRACKET [U+005D] 5D
LATIN CAPITAL LETTER E WITH ACUTE [U+00C9]C3 89
CHEROKEE LETTER QUV [U+13CB] E1 8F 8B
MUSICAL SYMBOL F CLEF [U+1D122] F0 9D 84 A2

And yeah, you never have to care about big-endian or little-endian with UTF-8.

UTF-8 absolutely rocks. The number of advantages it has is stunning. For example:

Here's Tom Scott's overview of UTF-8:


You should also read the UTF-8 Everywhere Manifesto.

NOrmalization

Here are two character sequences:

In some sense they should represent the same text, right? In fact, they only differ in that the second sequence has a pre-composed character. Since the character sequences are different, we can make get them to compare equal only if we normalize them. Unicode defines four NORMALIZATION FORMS: NFD, NFC, NFKD, and NFKC. These forms are used to interpret unicode and decompose the character sequences so that they can be handled properly for things like searching and comparing. For example, normalizing the two strings given above [given the proper normalization form] will compress the first one into the second one. This saves space and also allows searching/comparing for things like the tilde-n sequence.

About Strings

How hard can this be? Well, do you know the answers to these two simple questions?

  1. What is the length of a string? Oy. Maybe it is the number of graphemes? Or the number of characters? Or the number of bytes when encoded as UTF-8? Or the number of code units in a UTF-16 encoding? Does the BOM count at all? Oh and wait: do we count pre-composed characters as one character or multiple characters? or…… AAAAAAAARRRRRRRRGGGGGGGGGGHHHHHHHHHHHHH!!!
  2. When are two strings equal? Yes there is the whole nastiness of whether we have reference equality (same string object) vs. value equality (same value), but what the heck does it mean to compare strings by value? Should we normalize first? If so, which normalization format should be use?

Well, it's hard. But if you've read this far, you at least are now better able to debug problems.

But wait, think about this a bit. Maybe the fact that these questions are hard means these are the wrong questions. Really. Consider:

  1. Why do you care how long a string is? You should really only care about (1) how many bytes of storage are needed for it, or (2) how many pixels or characters wide your the rendered string is going to take up. The number of characters is quite often irrelevant!
  2. Why are you comparing strings for equality anyway? Are you checking passwords? YOU BETTER BE HASHING THOSE, so then you will be comparing byte sequences or integer numbers! Are you looking up dictionary keys, or doing a search for something? In these cases, printable, normalized characters will be just fine. (Use normalization when building the search index and when parsing the query string.)

Aren't these problems with strings widely known? Well, Edaqa Motoray has written about this. He says programming languages dont't need a string type at all. And in fact, if your language does have one, it is probably badly broken. It helps, if you want to get a handle on all this, that strings and text are not the same.

Further Reading

Unicode is a huge topic. The standard is over 1,000 pages long. There is a ridiculous amount to learn. These notes have barely scratched the surface. Maybe you would like to go deeper. Maybe you would like to know everything there is to know.

Start with these articles, and follow links within them as well:

Got weeks of free time? You can read the entire one-thousand page Unicode standard.

And Now For Something Completely Different… More about C

We have seen the basics of a C program from the previous exercises. This week we'll go a step further, and get into some details that you will need so that you can understand the code you'll be seeing and writing during the semester when you do your homework sets.

Because Java was originally based on C, the syntax is very similar. The if statement, the for loop, the while loop, the switch statement, as well as things like break, continue, and other keywords are all [pretty much] the same in C as you see them in Java. Further, the languages are both statically typed, which is another similarity. Also, variable declarations are as you would expect, such as int i = 23; or double d; or long l = 32547619845;. All statements end with a semi-colon, as you would expect, and curly braces, square brackets, and parentheses all work the way you think. So do all the operators.

The differences arise when you begin to look at things like Strings and Arrays. What we usually treat in Java as objects can't be handled the same in C. Further, Java handles the objects internally by treating them as entities that the JVM will manage for you, but in C the programmer has to take on the task of that management herself. As an example, here are a couple of code snippets:

  // in Java...
   public static final int MY_SYMBOLIC_CONSTANT = 23;

   String s = new String( "This is a test string" );
   int[] myRA1 = new int[10];
   int[] myRA2 = { 1, 2, 3, 4, 5 };

   System.out.println( "String s contains: " + s );
   for( int i = 0; i < myRA1.length; i++ ) {
      myRA1[i] = i * 2;
   }
            
  // in C
   #define MY_SYMBOLIC_CONSTANT 23

   char s[] = "This is a test string";
   int myRA1[10];
   int myRA2[5];

   int i = 0;
   for( i = 0; i < 5; i++ ) {
      myRA2[i] = i + 1;
   }
   printf( "String s contains: %s", s );
   for( i = 0; i < 10; i++ ) {
      myRA1[i] = i * 2;
   }
            

Note several things here. Pick this code apart.

There are quite a few differences shown here, which also shows you why programmers that have cut their teeth on Java and other nice languages are loath to return to those thrilling days of yesteryear when we had to handle a lot of things on our own. Nevertheless, THIS IS WHERE MUCH OF THE POWER OF THE C LANGUAGE COMES FROM, because the programmer has the flexibility to do a LOT of things that Java won't let her do.

Another Small Example

Here is some code from the K&R book that shows something we did in CMSI 281. We implemented a program to copy a file. Here is something similar, that copies one character at a time from its input to its output:

   #include <stdio.h>

   int main( int argc, char * argv[] ) {

      int c;

      c = getchar();
      while( c != EOF ) {
         putchar( c );
         c = getchar();
      }
   }

  // Because C has expressions like Java, we can write this as:
   #include <stdio.h>

   int main( int argc, char * argv[] ) {

      int c;

      ;
      while( (c = getchar()) != EOF ) {
         putchar( c );
      }
   }

            

Note several things here. Pick this code apart.

A Bit More Sophistication…

In Java, sub-program units are called methods. In C, as in JavaScript, they are called functions. A function provides a convenient way to perform some computation, and once the function is tested and shown to operate properly, it can be used over and over again without worrying about its internals. All that is needed at that point is for the person using the function to know its calling signature. As we've seen, there are a large number of functions that are defined for us, which are part of the C standard library. Here is a function that provides one of the methods of Java's Math.pow() method, along with the test code for it:

   #include <stdio.h>

   int power( int m, int n );

   int main( int argc, char * argv[] ) {

      int i;

      for( i = 0; i < 10; i++ ) {
         printf( "%d %d %d\n", i, power( 2, i ), power( -3, i ) );
      }
      return 0;

   }

   int power( int base, int n ) {

      int i, p;

      p = 1;
      for( i = 1; i <= n; ++i ) {
         p = p * base;
      }
      return p;

   }

            

Note several things here. Pick this code apart.

But Hey, Mr. K&R… What About Strings!?

The string is arguably one of the most used arrays in the c language. Yes, strings are treated as arrays, in this language, specifically an an array of characters. Also, they are a one-dimensional array, linear, as you'd expect. However, what you DON'T expect is that you have to do the management of the string yourself. The special character \0 is used to mark the end of the string, so you need to have the string dimensioned one character longer than the maximum length of the string to account for the space required. Here is one last program for this week, which does a string manipulation. We'll write several functions for this effort, and put them all into the same source file. We'll pick it apart to mine the gold. See if you can figure out just what this code does.

   #include <stdio.h>
   #define  MAXLINE  100

   int getline( char line[], int maxline );
   void copy( char to[], char from[] );

  /* main program - what does this actually *DO*?? */
   int main( int argc, char * argv[] ) {

      int len;             /* current line length */
      int max;             /* maximum length seen so far */
      char line[MAXLINE];
      char longest[MAXLINE];

      max = 0;

      while( (len = getline( line, MAXLINE )) > 0 ) {
         if( len > max ) {
            max = len;
            copy( longest, line );
         }
      }
      if( max > 0 ) {
         printf( "%s", longest );
      }
      return 0;

   }

  /* read a line into string s, and return the length */
   int getline( char s[], int lim ) {

      int c, i;

      for( i = 0; ((i < lim - 1) && ((c = getchar()) !- EOF) && (c != '\n')); ++i ) {
         s[i] = c;
      }
      if( c == '\n' ) {
         s[i] = c;
         ++i;
      }
      s[i] = '\0';
      return i;

   }

  /* copy values in "from[]" into "to[]"; assume that "to[]" is big enough! */
   void copy( char to[], car from[] ) {

      int i = 0;

      while( (to[i] = from[i]) != '\0' ) {
         ++i;
      }

   }
            

Note several things here. Pick this code apart.

In-class Assignment #4

Learning outcomes: The student will learn more about 1) the concept of addressing; 2) how values are stored in memory; 3) how values can be accessed by their memory addresses; 4) how the idea of a pointer and associated pointer math can be used to access memory in a type-dependent fashion.

One of the strongest parts of the C language is the ability to access the address of a variable, not just its value. In Java we don't have this ability, because we only either have primitive types which are allocated on the stack, or we have REFERENCE variables which are allocated on the heap. However, in C we can at any time specifically gain access to the address in memory of a variable. There are two operators which facilitate this, the star or asterisk, and the and or ampersand operator. Notice that these two things are overloaded. Star is also used for multiplication, and ampersand is also used for logical ANDing operations. The compiler has to use the context of the code to determine which you mean, but the compiler is very good at this, and will also tell you if it doesn't understand or if it thinks you've screwed up.

This facility involves what is commonly known as pointers. If V is a variable, let's say an int, then to get the address of where V is in memory we use the unary operator & and write &V. This is read the address of V; since the unary operator is right to left associative, the compiler will determine that you mean the address of V. Addresses can be declared in programs, as well. The corresponding thing, the pointer, is declared in the program and can then be used to obtain an address as a value. The declaration int *p; declared p to be a variable of the type pointer to int. In a certain sense, the star operator is the inverse of the ampersand operator in this context.

Because of the way arrays are stored in memory, the name of an array is treated as the pointer to the first location in memory of that array. Further, since strings in C are treated as an array of characters, the name of a string is treated as the pointer to the first location of the string. This can be a bit conf using when it is first encountered. The in-class exercise for this week will give you practice with pointers, addresses, strings, and arrays.

If p is a pointer, then the direct value of p is a memory location and the vlaue of *p is the value at the memory location which is stored in p. Suppose we declare:

   double x, y, *p;
            

…then the two statements:

   p = &x;
   y = *p;
            

…are equivalent to the statement:

   y = *&x;
            

…which in turn is equivalent to the statement:

   y = x;
            

The following short program should help to illustrate the distinction between a pointer value and its dereferenced value:

   int main( int argc, char * argv[] ) {

      int i = 17;
      int *p;

      p = &i;
      printf( "\n  The value of i is %d\n", *p );
      printf( "\n  The location [address] of i is %d\n\n", p );

   }
            

For the exercise, write a program called goFish.c that sums and averages a list of numbers, then creates a string and produces a count. You must use EITHER an array, or a set of pointers as the data storage. The program's specifications are as follows:

  1. The numbers must be prompted for in a loop
  2. The special value -9999 is used to indicate that the entries are complete
  3. Store the values in a structure of some kind that is initially declared to be size 25 int elements
  4. Loop through the structure and sum the elements, then output the sum to the console
  5. Count the number of elements in the structure, and output their average to the console
  6. Loop through the structure again, and contatenate all the values into a long string
  7. Output the string to the console
  8. Loop through the string, and count all the 7 [seven] characters; this is like in the game Go Fish when you ask another player, GIMME ALL YOUR SEVENS.
  9. Output the count to the console

Homework Assignment #3

Homework number three is due Wednesday/Thursday this week. Links to all the homework assignments are available on the syllabus page, but this reminder is just to make sure you know…

Week Four Wrap-up

That's probably enough for the this week. Be sure to check out the class links page to see what's in the pipeline for next week.

🌒