Tutorial 011 Siren and Print Format


Again the programs of this tutorial are developments of some found  in Alan Thomas'  "Further Programming for the BBC Micro", where I have rewritten them to replace GOTO statements and line numbers by means of  REPEAT...UNTIL loops and in some cases procedures (PROCs).

Siren
Turn up the sound on your system then run:

      REM Siren after Alan Thomas
      REM FuPrFBBCMicro p.37
      MODE8:OFF
      a=100
      b=135
      s=1
      v=1
      incv=1
      REPEAT
        FOR i=a TO b STEP s
          SOUND 1,-v,i,1
          COLOUR ABS(v)
          PRINTTAB(i)"£"
        NEXT
        v = v + incv
        IF ABS(v-8)=7 THEN incv=-incv
        c=a  : a=b : b=c
        s=-s
      UNTIL FALSE

When you find a keyword such as ABS in the above, you can find out about it by using the "context sensitive help" in the BB4W editing pane - this is activated from the icon on the top tool bar which has a symbol containing a balck arrow and a large question mark. In this case drag the cursor with its question mark over ABS and click, wherupon you should see a box appear which tells you that ABS converts its argument (value in brackets) in this case v-8 into a postive number if it is negative and leaves it positive if it is already positive.

Change some parameters and discover for yourself how it works!
Field width

@% enables you to set the "field width"which applies differently to the printing of numbers and strings.

 In the following program :

      MODE8
      @%=4
      FOR i=1 TO 20
        PRINT RND(999);
      NEXT
      PRINT

To see how it works alter the value of @% from 4 to 5,8,10,20

In the following you can compare the effects more easily:

      MODE8:OFF
      REPEAT
        READ k : PRINT"@% = ";k
        PRINT
        @%=k
        FOR i=1 TO 20
          n=RND(999)
          PRINT n;
        NEXT
        PRINT'
      UNTIL k=20
      END
      DATA 4,5,8,10,20

Incidently, you can copy the text from your screen to the clipboard by Holding down the Control key then pressing th TAB key (the one just above the Caps Lock Key on the left of the keyboard).This is what I did to get the output shown below:

@% = 4

 152 446 519 344 935 445 495 500 107 633 459 691 406 618 494 445 199 579 774  19


@% = 5

  264  834    9  420  800  536  456  189  247  868  340  222  640  721   48  957
  418  481  718  479

@% = 8

     764     806     626     118     485     933     917     306     309     925
     763     995     653     980     848     965       7     718     246      58


@% = 10

       952       942       493       286       269       301       322       258
       734       524       720       286       864       221        92       341
       124       796       813       891

@% = 20

                 742                 749                 573                 541
                 727                 914                 662                 606
                 300                 529                  62                 469
                 826                 793                 899                 161
                 339                 559                 614                 712

For example, where @%=10, above, there are seven spaces before the three digits of 952
where @%=20 there are 17 spaces before the 742 in the first column.

FOR strings the effect is shown by the following:

      MODE8:OFF
      REPEAT
        READ k : PRINT"@% = ";k
        PRINT
        @%=k
        PRINTSTRING$(7,"1234567890")
        PRINT"Mary","had","a","little","lamb"'''
      UNTIL k=10
      DATA 5,8,10

Which gives the output :

@% = 5

1234567890123456789012345678901234567890123456789012345678901234567890
Mary had  a    little    lamb



@% = 8

1234567890123456789012345678901234567890123456789012345678901234567890
Mary    had     a       little  lamb



@% = 10

1234567890123456789012345678901234567890123456789012345678901234567890
Mary      had       a         little    lamb

Note that here the strings are started at the beginning of the fields, whereas the numbers were fitted in at the end of the fields.

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