| | | |---|---| | **nOWWeaReSiX** | (these last two names are considered the same, and refer to the same variable) | The following are *not* allowed to be the names of variables: | | | |---|---| | **pi** | **PI** is a keyword | | **2001** | (it begins with a digit) | | **A new variable** | (contains the separated keyword **NEW**) | | **3 bears** | (begins with a digit) | | **M\*A\*S\*H** | (\* is not a letter nor a digit) | | **Fotherington-Thomas** | (- is not a letter nor a digit) | Integer variables can only use the letters **A** to **Z** (again, case does not matter, so **a** to **z** are also acceptable) – as you can see below, for a variable to be treated as integer, a **%** symbol somewhere in the same expression must precede it. ### Scientific notation Numerical expressions can be represented by a number and exponent. Try the following to prove the point: ``` PRINT 2.34e0 PRINT 2.34e1 PRINT 2.34e2 ``` and so on up to: ``` PRINT 2.34e15 ``` You will see that after a while the computer also starts using scientific notation. Similarly, try: ``` PRINT 2.34e-1 PRINT 2.34e-2 ``` and so on. **PRINT** gives only eight significant digits of a number. Try: ``` PRINT 4294967295,4294967295-429e7 ``` This proves that the computer can hold the digits of **4294967295**, even though it is not prepared to display them all at once. The ZX Spectrum Next, unless integer variables are expressly used (see above), uses floating point arithmetic, which means that it keeps separate the digits of a number (its mantissa) and the position of the point (the exponent). This is not always exact, even for whole numbers. Type: ``` PRINT 1e10+1-1e10,1e10-1e10+1 ``` Numbers are held to about nine and a half digits accuracy, so **1e10** is too big to be held exactly right. The inaccuracy (actually about **2**) is more than **1**, so the numbers **1e10** and **1e10+1** appear to the computer to be equal. For an even more peculiar example, type: ``` PRINT 5e9+1-5e9 ``` Here the inaccuracy in **5e9** is only about **1**, and the **1** to be added on in fact gets rounded up to **2**. The numbers **5e9+1** and **5e9+2** appear to the computer to be equal.