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CHAPTER 10 - Standard Input/Output During the course of this tutorial we have been using the WRITE and WRITELN procedures to display data, and it is now time to discuss them fully. Actually there is little to be said that has not already been said, but in order to get all of the data in one place they will be redefined here. As mentioned earlier, WRITE and WRITELN are not actually reserved words but are procedure calls. They are therefore merely identifiers that could be changed but there should never be a reason to do so. Lets get on to our first example program WRITELNX which has lots of output. MANY OUTPUT STATEMENTS Pascal has two output statements that are only slightly different in the way they work. The WRITELN statement outputs all of the data specified within it, then returns the cursor to the beginning of the next line. The WRITE statement outputs all of the data specified within it, then leaves the cursor at the next character where additional data can be output. The WRITE statement can therefore be used to output a line in bits and pieces if desired for programming convenience. The first example program for this chapter, WRITELNX, has many output statements for your observation. All outputs are repeated so you can observe where the present field ends and the next starts. Observe the INTEGER output statements. The first simply directs the system to output "index" twice, and it does with no separating blanks. The second statement says to output "index" twice also, but it instructs the system to put each output in a field 15 characters wide with the data right justified in the field. This makes the output look much better. This illustrates that you have complete control over the appearance of your output data. The REAL output statements are similar to the integer except that when the data is put into a field 15 characters wide, it is still displayed in scientific format. Adding a second field descriptor tells the system how many digits you want displayed after the decimal point. The next few lines illustrate the second field and its use. The BOOLEAN, CHAR, and STRING examples should be self explanatory. Notice that when the string is output, even though the string has been defined as a maximum of 10 characters, it has been assigned a string of only 8 characters, so only 8 characters are output. Page 46 CHAPTER 10 - Standard Input/Output NOW FOR SOME INPUT FROM THE KEYBOARD The example file READINT will illustrate reading some integer data from the keyboard. A message is output with an interesting fact that should be pointed out. Anyplace where Pascal uses a string variable or constant, it uses the apostrophe for a delimiter. Thus, anyplace where an apostrophe is used in a string, it will end the string. Two apostrophes in a row will be construed as a single apostrophe within the string and will not terminate the string. The term 'READ' within the string will therefore be displayed as shown earlier in this sentence. The variable "index" is used to loop five times through a sequence of statements with one READ statement in it. The three integer variables are read in and stored with the one statement. If less than three are input in the statement, only as many as are read in will be defined, the rest will be unchanged. Following completion of the first loop, there is a second loop that will be executed 5 times with only one minor change, the READ statement is replaced by the READLN statement. At this point it would be best run this program trying several variations with input data. When you run READINT, it will request three integers. Reply with three small integers of your choice with as many blank spaces between each as you desire, followed by a carriage return. The system will echo your three numbers back out, and request three more. Respond with only one number this time, different from each of the first three. You will get your new number followed by your previous second and third number indicating that you did not re-enter the last two integer variables. Enter all three again, this time including a negative number and observe the echo once again. Continue entering numbers until the system outputs the message indicating that it will now be using the 'READLN' for reading data. At this point enter the same numbers that you did in the previous section and notice the difference, which is only very slight. Each time you hit the enter key to cause the computer to process the data you have just given it, it will echo the carriage return to the display, and the "Thank you" message will be on a new line. When entering data from the keyboard, the only difference in READ and READLN is whether or not the carriage return is echoed to the display following the data read operation. It should not be a surprise to you that after you enter the data, the data is stored within the program and can be Page 47 CHAPTER 10 - Standard Input/Output used anywhere that integer data is legal for use. Thus, you could read in an integer, and use the integer to control the number of times through a loop, as a CASE selector, etc. TIME TO CRASH THE COMPUTER Crashing the computer will not hurt a thing. Rerun the above program and instead of entering integer data, enter some real data with decimal points, or even some character data. The computer should display some kind of message indicating that you have caused an I/O error (Input/Output), and most implementations of Pascal will probably abort operation (that simply means to stop the program and return control to the operating system). No harm has been done, simply start it again to enter more numbers or errors. READING REAL NUMBERS The example program READREAL will illustrate how to read real numbers into the computer. It will read an integer and three real numbers each time through the loop. It is perfectly fine to give the system a number without a decimal point for a real number. The computer will simply read it as a decimal number with zeros after the decimal point and consider it as a real number internally. As you found out in the last example program, however, it is not permissible to include a decimal point in the data if the computer is looking for an integer variable. Include some character data for a real number and crash the system in this program too. READING CHARACTER DATA The next example program, READCHAR, will read in one character each time through the loop and display it for you. Try entering more than one character and you will see that the extra characters will simply be ignored. It is not possible to crash this program because any character you enter will be valid. The next example, READARRY, will read in a string of characters and display them for you. Up to 10 characters will be read, and if less than 10 are read, the rest will be blank filled. Try entering 10 characters, then 4, to see that the residual 6 characters are blanked out before storing and printing. Since the array is fixed at ten characters, ten characters are always printed out, including trailing blanks. Page 48 CHAPTER 10 - Standard Input/Output Finally READSTRG will also read up to 10 characters, but since a string is a dynamic length variable, it will only print out the characters you input each time, up to the maximum of 10 as defined in the VAR declaration. It will display trailing blanks if you type them in because blanks are valid characters. BULLET PROOF PROGRAMMING It can be frustrating to be running a program and have it declare an I/O error and terminate operation simply because you have entered an incorrect character. The integer and real data inputs defined earlier in this chapter are fine for quick little programs to do specific calculations, but if you are writing a large applications program it is better to use another technique. Since the character and string inputs cannot abort operation of the program, it is best to use them to input the variable data and check the data internally under your own program control. An error message can then be given to the operator and another opportunity granted to input the correct data. All well written large application programs use this technique. HOW DO I PRINT SOMETHING ON THE PRINTER With all of the Pascal knowledge you now have, it is the simplest thing in the world to get data to the printer. The example file PRINTOUT will show you graphically how to do it. Every WRITE or WRITELN statement is required to have a device identifier prior to the first output field. If there is none, it is automatically defaulted to the standard output device, the display monitor. The example program has a few outputs to the monitor with the device identifier included, namely "output". This is only done to show you the general form of the WRITE statements. There are also many statements in this program with the display identifier "lst", which is the standard name for the "list" device or the printer. Compile and run this program with your printer turned on for some printer output. Just to supply you with a bit more information, every READ and READLN statement is also required to have a device identifier prior to the first input field. As you may suspect, it is also defaulted to "input" if none is specified, and the standard input device is the keyboard. Page 49 CHAPTER 10 - Standard Input/Output PROGRAMMING EXERCISE 1. Write a program containing a loop to read in a character string up to 60 characters long, then print the string on your printer. When you run the program, you will have the simplest word processing program in the world. Be sure to include a test to end the loop, such as when "END" is typed in. Page 50