Lesson 5 – Arrays, pointers and strings

1) Arrays
2) Pointers
3) Strings

        This part of c language is very important because you cannot make a program without using an array. Indeed the arrays are able to deal great data. Your program is more readable and also more possible. The strings are more specific and you can use a pseudo-constant to display text instead of using a string variable. Yet the strings are useful when you want to make a game. For instance, when you want to display a score, the number of lives, … you had to use a string variable. The “DrawStr” function uses only a character input and you cannot display directly a number unlike of “printf” function. On the other hand, the pointers are specific to c language. The pointers are very powerful and are able to decrease the memory and increase the performance of your program. I deal with these three notions because they are equivalent; the writing changes only.


1) Arrays

        An array gathers several variables with the same type. I will explain how an array works later with the pointers. The arrays are very useful to deal with an amount of data because each line of your array is a variable and each variable has a number instead of a name. What means you can use another variable so as to use the variable of the line of your array. Thus, you can easily use the array within a loop and deal with a great amount of data. Now we can see how we can use the arrays.
        The array works like a variable, you must declare and assign an array to be able to use. The syntax is:
Declaration:
    Type name [the number of lines]
Assignment:
    Name [the number of the line] = value
        In fact, in c language, the arrays always begin from 0. Of course the last number of row is (the number of lines – 1). Now to illustrate this new notion a brief example:

  // C Source File
  // Created 01/01/2003; 17:48:11

  #define USE_TI89              // Produce .89z File
  #define USE_TI92PLUS          // Produce .9xz File

  #define OPTIMIZE_ROM_CALLS    // Use ROM Call Optimization

  #define SAVE_SCREEN           // Save/Restore LCD Contents

  #include <tigcclib.h>         // Include All Header Files

  // Main Function
  void _main(void)
  {
    int array1[4];
    short array2[17];
    short a = 22;

    array1[0] = 21;
    array1[1] = a;
    array1[2] = array1[0] + 2;
    a = 3;
    array1[a] = 24;


    clrscr();

    for (a = 0; a < 4; a++)
      printf ("%d ", array1[a]);

    for (a = 0; a < 17; a++)
      array2[a] = a;

    ngetchx();
  }

        There is another type of arrays: the matrix; in fact you have rows and colons. The syntax is :
Declaration:
    Type name[the number of rows] [the number of colons]
Assignment:
    Name[the number of the row] [the number of the colon] = value
        The matrix gathers several arrays. I have done a small graphics to see, it is easier to imagine and understand the working of a matrix.

                Type matrix [n][m]
               
                matrix [i][j] = black box











        As I discussed previously, the arrays are especially powerful to deal with an amount of data. Let us see an example to sort an array!

  // C Source File
  // Created 01/01/2003; 19:00:30

  #define USE_TI89              // Produce .89z File
  #define USE_TI92PLUS          // Produce .9xz File

  #define OPTIMIZE_ROM_CALLS    // Use ROM Call Optimization

  #define SAVE_SCREEN           // Save/Restore LCD Contents

  #include <tigcclib.h>         // Include All Header Files

  // Main Function
  void _main(void)
  {
    short array[13];
    short a,z;
    short aux;

    randomize ();
    clrscr();

    printf ("An random array:\n");

    for (a = 0; a < 13; a++) {
      array[a] = random (10); // 0 <= short <= 9
      printf ("%d ", array[a]);
    }

    for (a = 0; a < 12; a++) {
      for (z = a + 1; z < 13; z++) {
        if (array[a] > array[z]) {
          aux = array[z];
          array[z] = array[a];
          array[a] = aux;
        }
      }
    }

    printf("\n\nThe sorted array:\n");
    for (a = 0; a < 13; a++)
      printf ("%d ", array[a]);

    ngetchx();
  }

        Do not worry with the new functions: randomize() and random(). I will explain you later in details. For this moment, you should know only that “random(number)” function gives you a number between 0 and number; exactly: 0 <= random(number) <= number –1.
On the other hand, the two loops that are able to sort the array are more complex to understand. There are several ways to sort an array but this way is for me the best on the speed and the size of memory. So as to understand how the sort works, you can see the bellow diagram:

2) Pointers

        It is a specificity of c language. The pointers are very powerful and now I am going to explain only the basis. In the next lessons, I explain more interesting uses: procedures, functions, files, …
        A pointer is a variable which the main aim is to remember the address of an area. Unlike a variable which remembers a value. This address is a hexadecimal value which points on an area of memory. In fact, you can get the value of this area and after you can use a pointer like a variable. Yet, I’ve never told you but you can also get the address of a variable, even if the main aim of a variable is its value. The declaration and the assignment of a pointer is not harder than with a variable. You should remember only that you work on the address. Here is an example.
 

  // C Source File
  // Created 28/01/2003; 18:40:15

  #define USE_TI89              // Compile for TI-89
  #define USE_TI92PLUS          // Compile for TI-92 Plus

  #define OPTIMIZE_ROM_CALLS    // Use ROM Call Optimization

  #define SAVE_SCREEN           // Save/Restore LCD Contents

  #include <tigcclib.h>         // Include All Header Files

  // Main Function
  void _main(void)
  {
    int *pointer1;
    int *pointer2;
    int *pointer3;

    int a = 13;
    int z;

    pointer1 = &a;
    pointer2 = pointer1;
    *pointer2 += 4;
    pointer3 = &z;
    z = 17;

    clrscr();
    printf ("pointer1: %d\npointer2: %d\npointer3: %d\n", *pointer1, *pointer2, *pointer3);
    ngetchx();
  }

        To declare a pointer, you must define the type of your pointer and its name like a variable but you add an ‘*’ so as to inform the compiler. It is not harder! ;-) After you do not initialize a pointer with a value but with an address of a memory address. This address is one of a variable or one of another pointer. When you do not use a ‘*’, it means you work on the address of pointer or else you work on the value of the address of your pointer. Remember that with pointers you work on the address and you can get the value of the pointed area.
 

3) Strings

        We are going to see several functions to work with the strings. Indeed, the strings are a specific type and you cannot use the usual assignment acronym, it creates errors which are hard to understand and it is very easy to make mistakes!
        The declaration of your string depends on the using of your string: variable or constant. If you want to use only a constant string, you can declare without giving the size of your string. If you want to use a string like a variable, you have to tell compiler the size of your string and especially you mustn’t use the ‘=’ acronym! The syntax is:
    char name[] = “data”;
    char name[value];
with:
    ‘name’: the name of your string
    ‘data’: your string of characters
    ‘value’: the size of your string
        After you want to initialize the string variable, you must use a specific function and not the ‘=’ acronym! This function is ‘strcpy’ (copy the string) and here the syntax:
    strcpy (string1, string2);
with:
     ‘string1’: the name of the string that you want to initialize
    ‘string2’: the name of the string which you assign at the string: ‘string1’ or you can replace ‘string2’ by “data” (a constant string of characters)
        If you want to concatenate two strings, in other words, to add two strings, you should use the ‘strcat’ function whose the syntax is:
    strcat (string1, string2);
        In fact, ‘string2’ can be a constant string of characters. The result is equivalent to: ‘string1 = string1 + string2’ but you cannot use the ‘=’ acronym!