Spreadsheet::WriteExceUseroContributed PerlSpreadsheet::WriteExcel::Formula(3)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.
perl v5.14.22010-02-0Spreadsheet::WriteExcel::Formula(3)
