NAME¶
Formula - A class for generating Excel formulas
SYNOPSIS¶
See the documentation for Spreadsheet::WriteExcel
DESCRIPTION¶
This module is used by Spreadsheet::WriteExcel. You do not need to use it
directly.
NOTES¶
The following notes are to help developers and maintainers understand the
sequence of operation. They are also intended as a pro-memoria for the author.
;-)
Spreadsheet::WriteExcel::Formula converts a textual representation of a formula
into the pre-parsed binary format that Excel uses to store formulas. For
example "1+2*3" is stored as follows: "1E 01 00 1E 02 00 1E 03
00 05 03".
This string is comprised of operators and operands arranged in a reverse-Polish
format. The meaning of the tokens in the above example is shown in the
following table:
Token Name Value
1E ptgInt 0001 (stored as 01 00)
1E ptgInt 0002 (stored as 02 00)
1E ptgInt 0003 (stored as 03 00)
05 ptgMul
03 ptgAdd
The tokens and token names are defined in the "Excel Developer's Kit"
from Microsoft Press. "ptg" stands for Parse ThinG (as in "That
lexer can't grok it, it's a parse thang.")
In general the tokens fall into two categories: operators such as
"ptgMul" and operands such as "ptgInt". When the formula
is evaluated by Excel the operand tokens push values onto a stack. The
operator tokens then pop the required number of operands off of the stack,
perform an operation and push the resulting value back onto the stack. This
methodology is similar to the basic operation of a reverse-Polish (RPN)
calculator.
Spreadsheet::WriteExcel::Formula parses a formula using a
"Parse::RecDescent" parser (at a later stage it may use a
"Parse::Yapp" parser or "Parse::FastDescent").
The parser converts the textual representation of a formula into a parse tree.
Thus, "1+2*3" is converted into something like the following,
"e" stands for expression:
e
/ | \
1 + e
/ | \
2 * 3
The function "_reverse_tree()" recurses down through this structure
swapping the order of operators followed by operands to produce a
reverse-Polish tree. In other words the formula is converted from in-fix
notation to post-fix. Following the above example the resulting tree would
look like this:
e
/ | \
1 e +
/ | \
2 3 *
The result of the recursion is a single array of tokens. In our example the
simplified form would look like the following:
(1, 2, 3, *, +)
The actual return value contains some additional information to help in the
secondary parsing stage:
(_num, 1, _num, 2, _num, 3, ptgMul, ptgAdd, _arg, 1)
The additional tokens are:
Token Meaning
_num The next token is a number
_str The next token is a string
_ref2d The next token is a 2d cell reference
_ref3d The next token is a 3d cell reference
_range2d The next token is a 2d range
_range3d The next token is a 3d range
_func The next token is a function
_arg The next token is the number of args for a function
_class The next token is a function name
_vol The formula contains a voltile function
The "_arg" token is generated for all lists but is only used for
functions that take a variable number of arguments.
The "_class" token indicates the start of the arguments to a function.
This allows the post-processor to decide the "class" of the ref and
range arguments that the function takes. The class can be reference, value or
array. Since function calls can be nested, the class variable is stored on a
stack in the @class array. The class of the ref or range is then read as the
top element of the stack $class[-1]. When a "_func" is read it pops
the class value.
Certain Excel functions such as
RAND() and
NOW() are designated as
volatile and must be recalculated by Excel every time that a cell is updated.
Any formulas that contain one of these functions has a specially formatted
"ptgAttr" tag prepended to it to indicate that it is volatile.
A secondary parsing stage is carried out by "parse_tokens()" which
converts these tokens into a binary string. For the "1+2*3" example
this would give:
1E 01 00 1E 02 00 1E 03 00 05 03
This two-pass method could probably have been reduced to a single pass through
the "Parse::RecDescent" parser. However, it was easier to develop
and debug this way.
The token values and formula values are stored in the %ptg and %functions
hashes. These hashes and the parser object $parser are exposed as global data.
This breaks the OO encapsulation, but means that they can be shared by several
instances of Spreadsheet::WriteExcel called from the same program.
Non-English function names can be added to the %functions hash using the
"function_locale.pl" program in the "examples" directory
of the distro. The supported languages are: German, French, Spanish,
Portuguese, Dutch, Finnish, Italian and Swedish. These languages are not added
by default because there are conflicts between functions names in different
languages.
The parser is initialised by "_init_parser()". The initialisation is
delayed until the first formula is parsed. This eliminates the overhead of
generating the parser in programs that are not processing formulas. (The
parser should really be pre-compiled, this is to-do when the grammar
stabilises).
AUTHOR¶
John McNamara jmcnamara@cpan.org
COPYRIGHT¶
X MM-MMX, John McNamara.
All Rights Reserved. This module is free software. It may be used, redistributed
and/or modified under the same terms as Perl itself.