Micro­power

  

Volume 1 · Number 2 · September 1981

Page 25 of 33

example above for completeness. When writing programs FOR . . NEXT loops be sure not to change the value of the loop variable within the loop, or you may find the program gets ‘hung up’ in the loop. When the program exits from a loop the loop variable is one STEP greater then the value set by the TO limit. Try entering:

FOR A = 1 to 50:NEXT:PRINT A

when you will receive the answer 51.

Right, we can now analyse the operation of the title printing program. Line 10 clears the screen, defines Z$ as PROGRAM TITLE, and sets up a loop to scan this string letter by letter. Line 30 is merely the NEXT part of this loop. However, line 20 looks quite complex, and certainly refutes claims that Basic programs bear any relationship to simple English.

In fact the line consists of a single command of the form POKE X,Y, but here both X and Y are complex expressions. The memory location into which data is being inserted is 3017 + I + (48 – LEN(Z$))/2. Now 3017 is one less than the decimal address of the start of the top line on the Nascom screen, so as I is increased from 1 in the loop 3017 + 1 steps through the memory locations at the start of this line. To this value an offset (48 – LEN(Z$))/2 is added. This merely deducts the string length from the line length and halves the result, to ensure that the title is centred. If the number of characters in the string is odd the offset will not be an integer, but the POKE command used only the integral part of the expression, try entering POKE 2531.7,7.3. Thus as the program cycles round the loop, data is inserted into the central LEN(Z$) locations of the top line. The value inserted at the Ith point is ASC(MID$(Z$,​I,1)), that is, the ASCII code corresponding to the Ith character in the string.

So one by one the letters of the title appear at the top of the screen. Easy, isn’t it? Well, perhaps it is too easy that I can be accused of making a simple subject complicated, but the point of analysing this short program in such detail is that what you learn can be carried over to help you write your next program.

Finally, here is a simple program to print out the complete ASCII and graphics set, with a space between each character:

10 CLS:FOR A = 1 TO 255:PRINT CHR$(A);:NEXT

In the next article I shall cover further Basic commands, including the double PEEK and POKE commands DEEK and DOKE, and examine the syntax and use of the PRINT command.

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