The BBC and Master Computer Public Domain Library

Or

AC36 INSTR(

Submitted by Steve Fewell

Description:

Call &9D3B to obtain the result of the expression at BASIC Text pointer B. This value should be the String value to
search within for the search string value.
If the value of the expression is not a String then issue a 'Type mismatch' error, as a string to search value was not
found.

If the next non-space character at BASIC Text pointer B is not a comma ',', then issue 'Missing ,' error.
Otherwise, increment BASIC Text pointer B offset (to point to the character after the comma).

Push the SWA String value (the String to search within) to the BASIC Stack.
Call &9D3B to obtain the result of the expression after the comma. This value should be the String value to search
for. The INSTR function will search for this string occuring within the first String value specified (the value that is now
on the stack).
If the value of the expression is not a String then issue a 'Type mismatch' error, as a search string value was not found.

Set the IWA LSB byte value (&2A) to 1. This is the default search starting position within the String being searched.
Increment the BASIC Text pointer B offset (&1B) to skip the next non-space character, which should be either a comma
(if a search starting position is being specified) or a close bracket (to indicate the end of the INSTR statement).

If the next non-space character (the character that was skipped over) was not a ')' (close bracket) then we are not at the
end of the INSTR statement yet, so:

* Check that the skipped character was a comma (if not then issue 'Missing ,' error)

* Push the SWA String value (the string to search for) to the BASIC stack

* Get the Integer result of the expression (after the comma) and place the Integer value in the IWA

* Check that the next non-space character is a closing ')' (to terminate the INSTR statement), if not then issue a

     'Missing )' error

* Retrieve the second String value (the string to search for) from the BASIC stack.

If the characters do not match then try the next character of the String pointed to by &37-&38 as follows:

* [&ACA6] Increment &2A (the current position within the String pointed to by &37-&38)

* Decrement &2B (the number of characters of the &37-&38 String value at which to begin a search)

* If &2B has reached zero, then we have checked for the search String value at all required positions in the

    &37-&38 String value without finding a match, so exit with IWA = 0 (as end of String

    value was reached without finding a match).

* Increment the &37-&38 pointer to point to the next character of the String value

* Jump to &AC89 to check for the search SWA value at the new &37-&38 position within the String value

Disassembly for the INSTR routine