Editing Sinclair BASIC (section)

===Data types===
One uncommon feature of Sinclair BASIC is the way it stores variables in memory. Typically, interpreters use a fixed-size entry to hold data, making it easy to scan the variable table. Due to the extremely limited memory of the ZX series, any wasted space had to be avoided, and this led to the use of a variable-length format. The data types included numbers stored in a 5-byte values, strings with a length and then the characters, and arrays of both of those types. The data was stored in the table itself, which contrasts with most BASICs of the era, where strings and array entries were stored in a separate [[Heap (data structure)|heap]].

The first byte for a variable entry always held the type in the first three bits, and the first character of the name in the next five bits. As was the case in most microcomputer dialects, <code>A</code>, <code>A$</code>, <code>A()</code> and <code>A$()</code> were all different variables and could store different values. Most variables could only have a single-character name; the exception are numeric variables (not arrays), where an alternate format held the first character of the name in those same five bits, but was then followed by additional characters ending with one with its high-bit set. Long variable names were whitespace-independent, and case-insensitive in later versions, so <syntaxhighlight lang="basic" inline>LET Number Of Apples = 5</syntaxhighlight> is the same as <syntaxhighlight lang="basic" inline>LET numberofapples = 5</syntaxhighlight> referred to the same variable.

The downside to this approach is that scanning the table to look up the value of a variable reference is more complex. In addition to testing whether the name matches using the subset of the first byte, if the entry is not the one that is being looked for, the type has to be read from the upper three bits and then the next location of a variable in storage calculated using the type. For instance, if the program encounters the variable "A" and the table starts with the entry for "B$", it fails to match A with B, then reads the type to see it is a string, and then has to read the following length byte and skip forward by that amount of bytes to find the next entry in the table. To make this somewhat easier, arrays also stored a two-byte length, so the entire structure could be skipped over more easily.

A unique feature was the "short float", or integer type. Any numeric variable could store either type, the storage itself did not change and used 5&nbsp;bytes in either case. Integers were indicated by setting the exponent byte to zero, while floating-point values were stored with an [[excess-128]] format exponent. This meant that it could not store zero as a float and lost one possible exponent magnitude. It also did not use any less memory, as the values were still 5-byte in memory. The advantage to this format is performance; the math library included tests to look for the zero exponent, and if it was seen, it would not attempt to perform various operations on the remaining 3&nbsp;bytes under certain conditions.