Exercises
1. Imagine the space for one key symbol divided up into four quarters:
. Then if each quarter can be either black or white, there are 2*2*2*2 = 16 possibilities. Find them all in the character set.2. Imagine the space for one symbol divided into two horizontally:
. Then if each half can be black, white or grey, there are 3*3 = 9 possibilities. Find them all.3. The characters in exercise 2 are designed to be used in horizontal bar charts, using two colours, grey & black. Write a program that inputs two numbers A & B (both between 0 & 32), & draws a bar chart for them:
You will need to start off printing "
", & change to either "
" or "
", according as A is more or less than B.What does your program do if A & B are not whole numbers? Or if they are not in the range 0 to 32? a good - 'user friendly' is the fashionable term - program will do something sensible & useful.
4. There are two different all grey characters on the keyboard, on A & H. If you look at them very close up, you will see that the one on H is like a miniature chessboard, while the one on A is like a sideways chessboard. Print them next to each other, & you will see that they don't join up properly. The one on A is used to join up neatly with
& 
(on S & D), while the one on H joins up neatly with & 
(on F & G).5. Run this program:
10 INPUT A
20 PRINT CHR$A;
30 GOTO 10
If you experiment with it, you will find that for CHR$, A is rounded to the nearest whole number; & if A is not in the range 0 to 255 then the program stops with report B.
6. Using the codes for the characters, we can extend the concept of 'alphabetical ordering' to cover strings containing any characters, not just letters. If instead of thinking in terms of the usual alphabet of 26 letters we use the extended alphabet of 256 characters, in the same order as their codes, then the principle is exactly the same. For instance, these strings are in ZX81 alphabetical order.
" ZACHARY"
"
""(ASIDE)"
"123 TAXI SERVICE"
"AASVOGEL"
"AA
""ZACHARY"
"
RDVARK"Here is the rule. First, compare the first characters in the two strings. If these are different, then one of them has its code less than the other, & the string of which it is the first character is the earlier (lesser) of the two strings. If they are the same, then go on to compare the next characters. If in this process one of the strings runs out before the other, then that string is the earlier; otherwise they must be equal.
Type in again the program in exercise 4 of chapter 10 (the one that inputs two strings & prints them in order), & use it to experiment.
7. This program prints a screenful of random black & white graphics characters:
10 LET A=INT (16*RND)
20 IF A>=8 THEN LET A=A+120
30 PRINT CHR$ A;
40 GOTO 10
(How does it work?)
