T223 'C' Tutorial - Outputting text to the screen

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Contents

• Required Code
  • main( ) function
  • { }
• cprintf( )
• Differences between cprintf()and printf()
• clrscr ( )
• ;
• White space
• Times Table Project

• Students on OU course T223 - Warning re Reference Manual
• First Program
  • Premature closing of Dos Window.
  • Backslash format control codes / Escape sequence
• \n
• \r
• Error Exercises
• Display of line numbers
• Solutions to Exercises
• Comments

Required Code

main()
      {
	      
      }

Exercise 1. Code explanation main() function

• Is a special function in C.
• It is the first function that will be executed.

• Indicate the start and end of the code for the proceeding function.
• They are used as a matching pair.
• They are normally indented to the function.
  • It is easier to check that pairs match if they are vertically aligned.

• You are restricted to 8 characters for the name.
• A single . period
• 3 characters for the extension after the period.
  • c is the extension used for 'C' files
  • cpp is the extension for 'C++' files

• tXX is the Tutorial number.
• exYY is the exercise number.
• Z is a spare character that is usually a letter a-z and usually used for alternative program files, temporary files etc.

main()
      {
	      
      }

A window should appear that states that the Source file has been successfully compiled, along with other information

It is possible that the window may close itself automatically, on modern computers this can be so fast that you may see a quick flicker or nothing at all. If this is the case you will be told how to correct this after the next exercise.

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Students on OU course T223

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First Program

Exercise 2. cprintf(), formatting
T223 Students will find that the course team use cprintf( ), rather than printf( ). Differences between cprintf()and printf()

Most programs place text onto the screen to inform the user about something. The next exercise will show you how to do this.

printf( ) writes formatted output to the stdout, standard output device such as a screen, a printer a stream, or a string. The screen is the default output.
cprintf( ) writes formatted output to the screen.

In this lesson, I will use cprintf(), in later lessons I will use printf().

main()
      {
	      cprintf("Hello World.\nThis is my\nfirst ' C ' program");
       }

Hello World.
            This is my
                       first'C' program

Note: the layout of the lines, this is produced by the backslash formatting code \n

Premature closing of Dos Window.

Important Note: It is possible that the DOS window that displays the above result, may close after the program finishes. This depends on the setup of your computer. This can be cured by entering the 2 additional line as follows.

Main()
      {
	      cprintf("Hello World.\nThis is my\nfirst ' C ' program");
        cprintf("\n\r\n\r\n\rPress the Space bar to Exit.");
        getch();
       }

Code explanation

• Is a function that displays a formatted string on to the screen.
• Our your code is calling the function cprintf() and passing all of the contents within the parenthesis ( ) as a parameter.
• Is a pre written function that is held in a library of functions.
• The string in the code is

  "Hello World.\nThis is my\nfirst'C' program" 

• The pair of double quotes enclose the entire string, but are not outputted onto the output screen.
• The string contains ...
  • The actual text that will be displayed
  • The backslash formatting control code \n
    • This code, \n is used twice.
    • More backslash control codes are shown below in the table.

• Is used to mark the end of a line of code;
• Most lines end in a ;
• There are a few exceptions to this which will be pointed out.
  • Note there is no ; on the end of the

    main()

    line

The line

cprintf("\n\r\n\r\n\rPress the Space bar to Exit.");

\n\r\n\r\n\r
Places 3 blank lines onto the screen.
Further information on \n\r is below.

Press the Space bar to Exit. Is the text that will be displayed. It is an informative message to the user, to inform the user what to do. This is a very important step to include when coding. Please not that the pressing of almost any key will end the program

Note the position of the cursor.

The getch() code is expecting a key to be pressed before continuing. Almost any key, but not them all, will work.
Hence the prompt to use the space bar.
Further information on getch() is given in tutorial 7.

White space
The above code could be written as ...

main(){cprintf("Hello World.\nThis is my\nfirst'C' program");cprintf("\n\r\n\r\n\rPress the Space bar to Exit.");getch();}

• I think that you will agree that the original is far easier to read.
• This is more noticeable as the code becomes more complicated.
• White space is ignored by the compiler.
• As you read different peoples code you will notice how the use of white space varies.
• If you are studying at a college or university they may have a "white space style" that they require you to follow.

Backslash Format Control Code / Escape sequences.

These are used in 'C' strings and character variables

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The characters " ' \ ? can sometimes create problems if used by themselves, hence the use of the preceding \ backslash.
Note: I have scattered a few ' in the code examples and solutions to test you

If you require the combined codes \n\r then use them in the order shown

There are many more code that start with the % percent character. These are later in the course.

First Program - Improved

• Add an additional word, improved, into the string, after my
• Experiment with the backslash codes so that the output looks like

  Hello World.
  This is my improved
  first'C' program

• an additional line of code that will clear all the proceeding additional information that you have seen in the output screen. The function that does this follows and will require a ; at the end of the line as follows ...

        clrscr() ;

  • Some programmers like to leave a space before the ; this helps it to be seen more easily, and to distinguish it from the accidental use of : rather than ;

Main()
      {
	     cprintf("Hello World.\nThis is my\nfirst'C' program");
      }

• Therefore do the 3 things as 3 separate steps
• Do each step separately and make sure the program is working after each step, and before carrying on with the next one.
• This makes error finding much easier.

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Error Exercises

You will experience many error messages when you attempt to compile 'C' programs. Beginners are puzzled by these messages, because they can not see the error in there code that caused the message. In the next few exercises you are going to introduce very common errors, which sometimes the compiler will tell you exactly what is wrong.
At other times the message is not very helpful.

Many 'C' editors allow the display of line numbers, if you are using the OU (Open University) PFE (Programmer's File Editor) click the button marked.
1AB
2CL

TIP: The line number shown is where the compiler discovered there is an error. The error may be on that line, often it is on the line before, or may be several lines before. Experience helps with finding errors. As a beginner and therefore lacking experience note the next tip.

TIP: Make a list of errors messages, what actually caused the error, and where that error took place in relation to the line number quoted in the error message. You will experience the same error messages time and time again. You could copy and paste the following list as your starter for your own List of Compiler Error Messages

Exercise 4. Statement missing ;

Use the code in the alternative solution to exercise 3.

main()
      {
      clrscr();
      cprintf("Hello World.\r\n");
      cprintf("This is my improved");
      cprintf("\r\nfirst 'C'  program");
      }

Error \path\filename.c linenumber: Statement missing ; in function main

Error \path\filename.c linenumber: Statement missing } Compound statement missing }in function main

Exercise 5b. Declaration syntax error

Now delete the }
Save, compile, link, and run.
You know that you only made 1 error, but there is a list of errors, one for every line after the actual error.

Declaration syntax error.

This error is not as obvious as the others.

Tip. Printout the list of error messages, provided in Tut. 90. C Error Messages . Add to this list additional information as you correct your errors. You can guarantee that an error that causes many hours of frustration, will do so again in a week, month or whatever.
You require information such as

• What caused the error.
• How to rectify the error.
• Reference to exercises, chapters, pages etc.

Exercise 6a. Undefined symbol

Delete a start " in one of the strings such as Hello

Multiple error messages returned for a simple single error, look at the first one.

Undefined symbol Hello

The compiler thinks that Hello is a symbol, you know it is a string, and you can put guess the cause.

Another example see Exercise 7 below

Correct the error then continue with the next one.

Exercise 6a. Unterminated string or character constant.

Delete an end " in one of the strings

Unterminated string or character constant

Easy once you know the meaning.

Correct the error then continue with the next one.

Exercise 7. Undefined symbol

Delete the ( after line that clears the screen.

Undefined symbol 'clrscr'

Exercise 8. Linker errors

Misspell some function names to say clrscrn, cprint, mane etc.

There has been Linker errors

Exercise 9.
Misspell main

You can obtain some very unusual results.

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Times Table Project

7 times table
 1 x 7 =  7
 2 x 7 = 14
10 x 7 = 70

• should have the columns of numbers right aligned as shown in the proposed output window.
  • At this stage this will be achieved by inserting additional spaces where required.
• The right column that displays the answer is calculated within the program.

• Each stage being saved compiled, linked, and run.
• Correct any mistakes and , save compile, link, and run again. Repeat this step if required.
• When you save you can slightly amend the name of the file, using the 8^th spare character, so that you can return to an earlier version if required.
  • The earlier versions can be deleted when the project is complete.

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1. Start the project from scratch. If possible attempt to write the code without looking up the code shown earlier on this page. Only if required look the code up. You will soon remember this code. T223 students may like to use their reference manual if required.
2. Write the code that all programs require, if possible no cheating.
3. Write the code that will clear the output screen, if possible no cheating.
4. Write the code that will produce the first line of the output, if possible no cheating.

   7 times table

5. Save as t02ex10a.c compile, link, and run.
6. Correct any errors. Save, compile, link, and run. Repeat as required.
7. Go to solution 10a.

1. Starting with the answer given in solution 10a.
   1. Add the following line of code. Note you will insert the formatting in the next step

          cprintf("1 x 7= %i" ,1*7) ;

   2. Save as t02ex10b.c compile, link, and run.
   3. Correct any errors. Save, compile, link, and run. Repeat as required.

Code explanation

• Is a place holder
  • It shows the place within the string where the result of the calculation will be placed.
  • Compare the position of %i in the code, with the position of the final 7 in the output.
• Is also another type of formatting code, the i stating the format is for an integer (whole number, i.e. no decimal point.)
• Much more on % codes later.

• After the end of the string is a comma which is used as a separator.
• This is followed by the actual calculation 1*7
• The result of this calculation is placed within the string of the output at the location of the place holder.
• More than 1 place holder can be used in a string, you will be using additional place holders in the next lesson.

7 times table 1 x 7=7

1. Starting with the answer given in solution 10b.
2. Sort out the formatting. Alter the code to ...
   1. Place the output on separate line

      7 times table
      1 x 7=7

   2. Insert a blank line between these 2 lines.
   3. Insert a few spaces as required.
   4. Save as t02ex10c.c compile, link, and run.
   5. Correct any errors. Save, compile, link, and run. Repeat as required.

1. Copy the line containing the code that outputs the following to the screen

    1 x 7 =  7

2. Paste this line twice into the source code.
   1. You should now have 3 identical lines of code, following each other.
   2. Amend the second of these lines to correctly output the 2 times line of the table.
   3. Aimed the third of these lines to correctly output the 10 times line of the table.
   4. Save as t02ex10d.c compile, link, and run.
   5. Correct any errors. Save, compile, link, and run. Repeat as required.
   6. Check that the output is vertically aligned with the numbers aligned right.
      1. This right alignment is achieved by inserting/ deleting spaces.
      2. Later in the course you will see how this can be achieved by using % formatting.

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Comments

/*  This is an example of a comment */

/* Large comments
   can be spread
  over several lines */

/***********************
  *  This is a comment    *
  *     embellished to        *
  *   make it stand out     *
 *****************************/

//  This comment does not need an end marker.

//  It goes as far as the end of the line

Compilers that do not allow this form will create errors if used. The PFE is one of them.

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Differences between printf( ) and cprintf( )

Most courses on C use the function printf(), the Open University course T223 uses the function cprintf( ), basically the 2 functions produce the same results but there are subtle differences listed in the table below. Table: Differences between cprintf() and printf() put device.

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Solutions to Exercises

main()
      {
      clrscr();
      cprintf("Hello World.\r\nThis is my improved\r\nfirst C program");
      }

• This way is useful if the cprintf() line becomes rather long, and as you will shortly find out more complicated.
• Note how the \r\n is placed at the end of one of the lines, and at the start of another.
• When splitting lines up it is usual place the \r\n at either the start or end of a line, but not required.
• It is usual for the \r\n to be placed at the end of most cprintf() lines.
• Note. It is the \r\n that places the output onto 3 lines, it is not

main()
      {
      clrscr();
      cprintf("Hello World.\r\n");
      cprintf("This is my improved");
      cprintf("\r\nfirst C program");
      }

Main()
      {
      clrscr();
      cprintf("7 times table") ;
       }

Main()
      {
      clrscr();
      cprintf("7 times table") ;
      cprintf("1 x 7=%i",1*7) ; 
       }

Main()
      {
      	clrscr();
 	     cprintf("7 times table\r\n\r\n");
	      cprintf("1 x 7 =  %i\r\n", 1 * 7) ;               /* the spacing between , 1 * 7 is white space */
      }

Main()
      {
      clrscr();
 	    cprintf(" 7 times table\r\n\r\n");
	     cprintf(" 1 x 7 =  %i\r\n", 1 * 7) ; 
	     cprintf(" 2 x 7 = %i\r\n", 2 * 7) ; 
	     cprintf("10 x 7 = %i\r\n", 10 * 7) ; 
      }