Chapter 4 - The Sinclair ZX81 as a calculator
Turn the computer on. You can now use it as a calculator, along the lines of chapter 2: type
PRINT
, then
whatever it is that you want working out, & then
NEWLINE
. (We shan't usually bother to tell you to type
NEWLINE
.)
As you would hope, the ZX81 can not only add, but also subtract, multiply using a star * instead of the
usual times sign - this is fairly common on computers) & divide (using / instead of ). Try these out.
+, -, * and / are operations
, & the numbers they operate on are their operands.
The computer can also raise one number to the power of another using the operation
**
(Shifted H. Do
not type * - shifted B - twice): type
PRINT
2
**
3 (Remember the
NEWLINE
.)
& you will get the answer 8 (2 raised to the power 3, or 2
3
, or 2 cubed)
The ZX81 will also work out combinations of the operations. For instance.
PRINT
20-2*3
**
2+4/2*3
gives the answer 8. It goes all round the houses to get this, because first it works out all the powers (
**
) in
order from left to right, & then all the multiplications & divisions (* & /), again from left to right, & then the
additions & subtractions (+ & -), yet again from left to right. Thus our example is worked out in the following
stages:
We formalize this by giving each operation a priority
, a number between 1 & 16. The operations with
highest priority are evaluated first, & operations with equal priority are evaluated in order from left to right.
When - is used to negate something, as when you write -1, then it has priority 9. (This is unary
minus, as
opposed to the binary
minus in 3-1: a unary operation has one operand, while a binary operation has two.
Note that on the ZX81 you cannot use + as a unary operation.)
This order is absolutely rigid, but you can circumvent it by using brackets: anything in brackets is
evaluated first & then treated as a single number, so that
PRINT
3*2+2
gives the answer 6+2 = 6, but
**
has priority 10
* and / have priority 8
+ & - have priority 6
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