Sub-Strings – Details

The results of sub-string functions (LEFT$, MID$ and RIGHT$) will use the same character array as the argument string. Exactly how this is accomplished depends on whether the argument string is a reference string or a temporary string.

For a reference string, the pointer and length of the character array are copied to the evaluation stack. This means that the pointer and length on the stack can be modified without affecting the actual pointer and length of the variable or array element. For LEFT$, only the length on the stack needs to be changes. For MID$ and RIGHT$, both the pointer and length need to changed depending on the integer arguments.

For a temporary string, the sub-string operation is a little more involved because the pointer to the character array on the stack cannot be modified – it is needed to delete the character array when the temporary string is no longer needed. The length is not needed to delete the character array, so it can be modified. Since the pointer cannot be modified, the portion of the sub-string needs to be moved to the beginning of the character array. Again for LEFT$, only the length needs to be changed, the sub-string is already at the beginning of the array. But for MID$ and RIGHT$, the sub-string result needs to be moved to the beginning of the array.

For reference strings, there is no allocation, copying or deletion required for the sub-string operation. For temporary strings, there is still a moving of characters, but there is no allocation of a new character array or deletion of the old character array. After the expression is evaluated, the temporary string will be deleted. There could be some unused space in the character array from a sub-string operation when the temporary string is assigned and used as it. Enough of discussing how sub-strings will work at run-time, next, what impact sub-strings have on the Translator...