Lesson 4: The loops

1) The “for” loop
2) The “while” loop
3) The “until” loop
4) Some keywords used in the loops: “break” and “continue”

        The loops are a part of all languages, which is very interesting. Indeed, the aim is to deal with a great number of data and write less code than without loops. Often writing a code without loops is impossible. There are two or three kinds of loops. I say two or three because two loops are very similar. I will call these three loops: the “for” loop, the “while” loop and the “until” loop. The “until” loop do not really exist in C language but I will explain it later.


1) The “for” loop

        I begin with this loop because it is the first loop I have learned. Indeed, before starting dealing with much data, the most interesting is, for example, to repeat several times the same text on the screen. Of course, if you want, you can make several times “copy & paste” but your code will become very huge and unreadable. I start with a brief example and after I will explain the syntax and the aims of this loop.

  // C Source File
  // Created 01/11/2002; 21:58:08

  #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 i;

    clrscr();

    for (i = 1; i < 10; i++ ) {
      printf ("You have to type 9 times anywhich key to exit the program\n");
      ngetchx();
    }
  }

        If you would not use a loop, you should write this code:

  // C Source File
  // Created 01/11/2002; 21:58:08

  #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)
  {
    clrscr();

    printf ("You have to type 9 times anywhich key to exit the program\n");
    ngetchx();
    printf ("You have to type 9 times anywhich key to exit the program\n");
    ngetchx();
    printf ("You have to type 9 times anywhich key to exit the program\n");
    ngetchx();
    printf ("You have to type 9 times anywhich key to exit the program\n");
    ngetchx();
    printf ("You have to type 9 times anywhich key to exit the program\n");
    ngetchx();
    printf ("You have to type 9 times anywhich key to exit the program\n");
    ngetchx();
    printf ("You have to type 9 times anywhich key to exit the program\n");
    ngetchx();
    printf ("You have to type 9 times anywhich key to exit the program\n");
    ngetchx();
    printf ("You have to type 9 times anywhich key to exit the program\n");
    ngetchx();

  }

        You can see this second code has 5 lines in more. However, if you do not know how many times you want to display the same text, you had to use a loop or else you stop programming. :-) The syntax is:
    For (var = initial value; condition; step) {
    Statement
    }

        In fact, firstly you assign your variable called var. This assignment is the initial value of your variable. Afterwards, I will often call this variable “a” and “z” because I use a French keyboard and it is easier. It is a good or a bad habit? I do not know; it is my way to program. Secondly, you must tell your computer when it has to stop the loop and goes on the following code. That is why the syntax is like a condition: “var < ended value + 1”. We can translate with the following sentence: “the computer goes on the loop until the value of ‘var’ is equal to the ended value”. Thirdly, you should give the step of your loop i.e. you choose the values which your variable will take. In fact, it is like an assignment. The assignment: “var++” is generally used but you can also use another assignment, for example, var += 2 if you want to get the odd or even values. Here I am showing the greatest part of loop. Indeed, remember that var is a variable and the variable is very interesting and can be used with the statement of your loop so as to deal with data. To simplify, we can see a brief example. If you want to display the numbers from 1 to 10. You can write:

    clrscr();
    printf ("1\n2\n3\n3\n4\n5\n6\n7\n8\n9\n10");
    ngetchx();

        However, that is not the aim of this part. That is why we are going to use a for loop to simplify and especially to understand the aim and the power of loops.

    int a;
    clrscr();
    for (a = 1; a < 11 ; a++)
    printf ("%u\n", a);
    ngetchx();

        For 10 numbers, it is not really faster but if you want to display 100 numbers, it is a compulsory or else you are stubborn and stupid. ;:) You can view the code:

  // C Source File
  // Created 23/12/2002; 18:01:29

  #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)
  {
    clrscr();
    printf ("1\n2\n3\n3\n4\n5\n6\n7\n8\n9\n10");
    ngetchx();

    int a;
    clrscr();
    for (a = 1; a < 11 ; a++)
      printf ("%u\n", a);
    ngetchx();
  }

  // C Source File
  // Created 23/12/2002; 18:34:10

  #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 a = 1;

    clrscr();

    while (a < 11) {
      printf ("%u\n", a);
      a++;
    }
    ngetchx();
  }

        As you can see, you can use a for or a while loop but you should choose the loop which has the simplest for your algorithm. In fact, one is more readable but it is not true for each algorithm, you should choose your way to program. Now, we can see the syntax:
    While (condition) {
    statement
    }
        On the other hand, you should be careful. You mustn’t forget the first assignment “a = 1” or else the loop makes anything and especially the loop goes on and never stops!!! This loop is more dangerous because you can easily forget this assignment. Indeed, this assignment does not belong to the syntax of the loop unlike the for loop. Nevertheless, you should not avoid this loop because of this little inconvenient and you can also make mistakes with a complex for loop. Remember that you can make several tries before finding the good algorithm. If the loop never stops, you stop easily the emulator and you search the mistake, it is simple, isn’t it? That is why I advise you to use an emulator rather your calculator to make your try; to stop an emulator is easier than to take out the batteries when your program crashes. In addition, you can use the function of the emulator: “revert to saved state”; you do not need to start again the emulator!
        The applications of this loop are the same ones with any loops. The big differences between the for and while loop is that you control the counter more easily but all loops can make the same work you should think more, it is all! So as to improve, you should play around this two loops and other knowledges. It is the best way to improve efficiently.
 

  // C Source File
  // Created 23/12/2002; 19:15:20

  #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 a = 1;

    clrscr();

    do {
      printf ("%u\n", a);
      a++;
    } while (a < 11);

    ngetchx();
  }

        You can note that the test to check up the end of the loop is at the end and not at the beginning like the “while” loop. In the others languages like “Pascal”, when the test is at the end, the loop is called “until” loop and the keyword: “until” is written and means “until” and not “while”. The syntax of this loop is:
    do {
    Statement
    } while (condition)
 

1:      // C Source File
2:      // Created 23/12/2002; 19:38:02
3:    
4:      #define USE_TI89              // Produce .89z File
5:      #define USE_TI92PLUS          // Produce .9xz File
6:    
7:      #define OPTIMIZE_ROM_CALLS    // Use ROM Call Optimization
8:    
9:      #define SAVE_SCREEN           // Save/Restore LCD Contents
10:   
11:     #include <tigcclib.h>         // Include All Header Files
12:   
13:     // Main Function
14:     void _main(void)
15:     {
16:       int go_on = 1; // this variable is a boolean: TRUE=1 & FALSE=0
17:       int a = 0;
18:   
19:       clrscr();
20:   
21:       do {
22:         a++;
23:         if (a == 13) {
24:           continue;
25:           break;
26:         }
27:         if (a == 17) {
28:           break;
29:           continue;
30:         }
31:         printf ("%d\n", a);
32:       } while (go_on);
33:   
34:       ngetchx();
35:     }