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This section describes the PRINT statement which is used to put text on the screen or to a printer. It assumes that you understand that a variable (such as X) can be used to hold a number and that a string variable (such as A$) can be used to hold a line of text.
The following program will help to illustrate some of the ideas. Press BREAK and then type in the following program.
10 X=8 20 A$="HELLO" 30 PRINT X, X, X
When this is run it produces this:
>RUN 8 8 8
This shows that commas separating items in the print list (the print list is the list of things to be printed - X,X,X in this case) will force items to be printed in columns or 'fields' ten characters wide. Numbers are printed at the right hand side of each column whereas words are printed on the left hand side. You can see the difference if we add some lines to the program.
10 X=8 20 A$="HELLO" 30 PRINT X,X/2,X/4 40 PRINT AS,A$,A$ >RUN 8 4 2 HELLO HELLO HELLO
As we said above, the width of each 'field' is automatically set to ten characters when the computer is switched on.
Since the computer can operate in different screen modes, displaying either 20 or 40 or 80 characters to the line, clearly the number of fields which can be displayed on the screen will differ depending on the mode. So try typing in a new line and re running the program above.
5 MODE 5
or, if you have a Model B machine
5 MODE 0
|80 character modes||40 character modes||20 character modes|
|(MODES 0 and 3)||(MODES 1,4,6 and 7)||(MODES 2 and 5)|
Note: the widths of the fields can be altered by the use of a special command @% (see page 70).
So commas between items in the print list always put things in columns or 'fields'. On the other hand semi-colons between items in the print list cause items to be printed next to each other, without spaces:
30 PRINT A$; X; A$; X; X
Of course if the first item is a number it will be printed to the right of a 'field' unless it is preceded by a semi colon.
30 PRINT X; A$; A$
30 PRINT ;X;A$;A$
As well as printing variables and string variables as shown above the computer can print any characters placed in between pairs of inverted commas exactly as they have been typed in, providing they are in a print statement. The next program asks for your name and remembers it in the string variable N$.
10 PRINT "WHAT IS YOUR NAME ";
20 INPUT N$
30 PRINT "HELLO ";N$;". HOW ARE YOU?"
WHAT IS YOUR NAME ?JOHN
HELLO JOHN. HOW ARE YOU?
Notice the semi-colon at the end of line 10 that makes the computer stay on the same line while it waits for you to provide it with a value for N$. Without the semi-colon this happens:
WHAT IS YOUR NAME
HELLO JOHN. HOW ARE YOU?
Note also the space after the word HELLO and before the word HOW in line 30. Without these spaces the words run together to produce.
HELLOJOHN.HOW ARE YOU?
It is quite legitimate to do calculations in a print list - for example
10 X=4.5 20 PRINT X,X+2,X/3,X*X >RUN 4.5 6.5 1.5 20.25
but look what happens if instead of X = 4.5 we put X = 7
10 X=7 20 PRINT X,X+2,X/3,X*X >RUN 7 92.33333333 49
because x/3 is 2.33333333 it makes the number move left in the field until it immediately follows the previous field which contains a 9 and appears to give a result 92.33333333, which is misleading. For this reason, amongst others, the next section is important if you want to print out a lot of numbers.
It is often useful to be able to specify the width of the field when printing columns of figures or words and also to be able to specify the number of decimal places to which numbers will be printed.
On the BBC Microcomputer this can be done by setting a special 'variable' (called @%) in a particular way. For the moment this must be treated as a bit of 'magic' but, for example, if we write
then this statement tells the computer to print in a field 9 characters wide, and that numbers will be printed with a fixed number of decimal places - in this case, to 2 decimal places. The following program shows this being used:
5 @%=820209 10 X=7 20 PRINT X,X+2,X/3,X*X >RUN 7.00 9.00 2.33 49.00
@% is made up of a number of parts
|(Means Hexadecimal numbers follow)||Format|
number 2 i.e.
of decimal places
|2decimal places||field width of 9 characters|
@%=&20309 would give Format 2, 3 decimal places and field width of 9 characters.
5 @%=&20309 10 X=7 20 PRINT X,X+2,X/3,X*X >RUN 7.000 9.000 2.333 49.000
If you want 4 decimal places and a field width of 12 you would put the following
5 @%=&2040C 10 X=7 20 PRINT X,X+2,X/3,X*X >RUN 7.0000 9.0000 2.3333 49.0000
The & tells the computer that the numbers which follow are 'hexadecimal' numbers - that is, numbers based not on 10s but on 16s. For the sake of completeness, here is a list of hexadecimal numbers (which include the letters A to F)
|Decimal number||Hex number|
A few points:
As well as controlling the print layout by using the comma and semi-colon one can use the TAB statement to start printing at a particular place on the screen. You will remember that there can be 20,40 or 80 characters to the line depending on the MODE. MODE 7 has 40 characters. Try this:
10 PRINT "012345678901234567890" 20 F=16 30 REPEAT 40 PRINT TAB(10);F;TAB(15);2*F 50 F=F+1 60 UNTIL F=18 >RUN 012345678901234567890 16 32 17 34
TAB(10) prints the value of F ten spaces from the left and then TAB(15) prints the value of 2*F fifteen spaces from the left, on the same line. Note the semi-colon after TAB(10) - this causes the computer to begin printing at that point.
Be sure to place an open bracket immediately after the word TAB. If you leave a space between them, thus: TAB (10) the computer will not understand and will report
No such variable.
If you are beyond the place that you tell the computer to TAB to, for example in position 15 with a request to TAB(10), then the computer moves to the next line and then tabs ten spaces.
Type in this replacement line
40PRINT TAB(15);F;TAB(10);2*F >RUN 012345678901234567890 16 32 17 34
A useful extension of the TAB statement allows print to be placed at any specific character location anywhere on the screen. You will remember that in MODE 7 the text co-ordinates are
This program counts to 1000, printing as it goes
30 PRINT TAB(18,5);Q
50 UNTIL Q=1000
The two numbers in the brackets after TAB represent the X and Y text co-ordinates where printing should start (see also the program on page 132).
Using PRINT TAB(X,Y) allows text etc to be printed in any character 'cell' in the appropriate MODE. In MODE 5 there are 20 cells across the screen and 32 cells (lines) down the screen. Sometimes it is useful to be able to position characters on a much finer grid. The statement VDU 5 enables text to be printed at the exact position of the graphics cursor. The statement MOVE can be used to position text. Note that this will not work in MODE 7. You will remember that the graphics screen is addressed thus in all modes except MODE 7.
A few sums will show that each character cell is 32 graphic units high and, in a 40 character mode such as MODE 4, 32 units wide. Suppose we want to subscript a letter to produce for example the chemical formula H2 this can he done as follows
10 MODE 4
20 VDU 5
30 MOVE 500,500
40 PRINT "H"
50 MOVE 532,484
60 PRINT "2"
70 VDU 4
Note that the letter H is positioned with its top left corner at 500,500. The 2 is then printed one character to the right (532) and half a character down (484). Again the top left of 2 is at 532,484.
A neater way of achieving the same effect is to replace line 50 with
One further feature of the BBC computer which is not normally available on "personal" computers is the ability to superimpose
characters. One obvious use of this facility is to generate special effects such as accents and true underlining. The short program below prints the word role with the accent correctly placed.
10 MODE 4
20 VDU 5
50 MOVE X,Y
60 PRINT "role"
70 MOVE X+32,Y+16
90 VDU 4
Once in VDU 5 mode the screen will not scroll up if you reach the bottom of the page, instead the writing will start from the top of the screen again. In addition characters will be superimposed on those already on the screen. When in VDU 5 mode you can only print things in the graphics window and not in the text window, and colour is selected with the GCOL statement. VDU 5 will not work in text-only modes such as MODES 3, 6 and 7.
The text cursor is the flashing line on the screen which shows where text will appear if it is typed in on the keyboard. The text cursor also indicates where text will he printed on the screen by a PRINT statement. The cursor can be moved around the screen by a number of special "control codes" amongst which are
code effect 8 move cursor left 9 move cursor right 10 move cursor down 11 move cursor up
These code numbers can be used either with the VDU command e.g. to move left four spaces, use either
clearly the VDU command is simpler to type in in most cases.
In addition to the codes shown above the user can use the PRINT TAB(X,Y) statement to move the cursor directly to any character position on the screen. As we've seen in MODE 7 the screen can contain up to 25 lines (numbered 0 to 24) of up to 40 characters per line.
The position marked on the diagram above is 18 positions across and 6 lines down. The cursor could be moved directly there with the statement
Note that the opening bracket must immediately follow the word TAB thus TAB( and not TAB (.
Exactly the same effect can be obtained with the statement
The cursor can be moved to the "home" position at the top left of the screen with the statement
If the user wishes to clear the screen as well as move the cursor to the home position then he or she can use the statement
The last of the VDU commands directly to do with cursor control is
VDU 127 which moves the cursor left and deletes the character there. If you wish to delete the next four characters and then return the cursor to its initial place you could use
In some applications the flashing cursor can be a distraction. It can be turned off with the statement
and can be turned back on with the MODE statement
Note; In version 1.0 of the operating system
VDU 23,1,0;0;0;0; will turn the cursor off
VDU 23,1,1;0;0;0; will turn the cursor back on