1Spreadsheet::WriteExcelU:s:eFrorCmounltar(i3b)uted PerlSDporceuamdesnhteaetti:o:nWriteExcel::Formula(3)
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NAME

6       Formula - A class for generating Excel formulas
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SYNOPSIS

9       See the documentation for Spreadsheet::WriteExcel
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DESCRIPTION

12       This module is used by Spreadsheet::WriteExcel. You do not need to use
13       it directly.
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NOTES

16       The following notes are to help developers and maintainers understand
17       the sequence of operation. They are also intended as a pro-memoria for
18       the author. ;-)
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20       Spreadsheet::WriteExcel::Formula converts a textual representation of a
21       formula into the pre-parsed binary format that Excel uses to store
22       formulas. For example "1+2*3" is stored as follows: "1E 01 00 1E 02 00
23       1E 03 00 05 03".
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25       This string is comprised of operators and operands arranged in a
26       reverse-Polish format. The meaning of the tokens in the above example
27       is shown in the following table:
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29           Token   Name        Value
30           1E      ptgInt      0001   (stored as 01 00)
31           1E      ptgInt      0002   (stored as 02 00)
32           1E      ptgInt      0003   (stored as 03 00)
33           05      ptgMul
34           03      ptgAdd
35
36       The tokens and token names are defined in the "Excel Developer's Kit"
37       from Microsoft Press. "ptg" stands for Parse ThinG (as in "That lexer
38       can't grok it, it's a parse thang.")
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40       In general the tokens fall into two categories: operators such as
41       "ptgMul" and operands such as "ptgInt". When the formula is evaluated
42       by Excel the operand tokens push values onto a stack. The operator
43       tokens then pop the required number of operands off of the stack,
44       perform an operation and push the resulting value back onto the stack.
45       This methodology is similar to the basic operation of a reverse-Polish
46       (RPN) calculator.
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48       Spreadsheet::WriteExcel::Formula parses a formula using a
49       "Parse::RecDescent" parser (at a later stage it may use a "Parse::Yapp"
50       parser or "Parse::FastDescent").
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52       The parser converts the textual representation of a formula into a
53       parse tree. Thus, "1+2*3" is converted into something like the
54       following, "e" stands for expression:
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56                    e
57                  / | \
58                1   +   e
59                      / | \
60                    2   *   3
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62       The function "_reverse_tree()" recurses down through this structure
63       swapping the order of operators followed by operands to produce a
64       reverse-Polish tree. In other words the formula is converted from in-
65       fix notation to post-fix. Following the above example the resulting
66       tree would look like this:
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68                    e
69                  / | \
70                1   e   +
71                  / | \
72                2   3   *
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74       The result of the recursion is a single array of tokens. In our example
75       the simplified form would look like the following:
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77           (1, 2, 3, *, +)
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79       The actual return value contains some additional information to help in
80       the secondary parsing stage:
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82           (_num, 1, _num, 2, _num, 3, ptgMul, ptgAdd, _arg, 1)
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84       The additional tokens are:
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86           Token       Meaning
87           _num        The next token is a number
88           _str        The next token is a string
89           _ref2d      The next token is a 2d cell reference
90           _ref3d      The next token is a 3d cell reference
91           _range2d    The next token is a 2d range
92           _range3d    The next token is a 3d range
93           _func       The next token is a function
94           _arg        The next token is the number of args for a function
95           _class      The next token is a function name
96           _vol        The formula contains a voltile function
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98       The "_arg" token is generated for all lists but is only used for
99       functions that take a variable number of arguments.
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101       The "_class" token indicates the start of the arguments to a function.
102       This allows the post-processor to decide the "class" of the ref and
103       range arguments that the function takes. The class can be reference,
104       value or array. Since function calls can be nested, the class variable
105       is stored on a stack in the @class array. The class of the ref or range
106       is then read as the top element of the stack $class[-1]. When a "_func"
107       is read it pops the class value.
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109       Certain Excel functions such as RAND() and NOW() are designated as
110       volatile and must be recalculated by Excel every time that a cell is
111       updated. Any formulas that contain one of these functions has a
112       specially formatted "ptgAttr" tag prepended to it to indicate that it
113       is volatile.
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115       A secondary parsing stage is carried out by "parse_tokens()" which
116       converts these tokens into a binary string. For the "1+2*3" example
117       this would give:
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119           1E 01 00 1E 02 00 1E 03 00 05 03
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121       This two-pass method could probably have been reduced to a single pass
122       through the "Parse::RecDescent" parser. However, it was easier to
123       develop and debug this way.
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125       The token values and formula values are stored in the %ptg and
126       %functions hashes. These hashes and the parser object $parser are
127       exposed as global data. This breaks the OO encapsulation, but means
128       that they can be shared by several instances of Spreadsheet::WriteExcel
129       called from the same program.
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131       Non-English function names can be added to the %functions hash using
132       the "function_locale.pl" program in the "examples" directory of the
133       distro. The supported languages are: German, French, Spanish,
134       Portuguese, Dutch, Finnish, Italian and Swedish. These languages are
135       not added by default because there are conflicts between functions
136       names in different languages.
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138       The parser is initialised by "_init_parser()". The initialisation is
139       delayed until the first formula is parsed. This eliminates the overhead
140       of generating the parser in programs that are not processing formulas.
141       (The parser should really be pre-compiled, this is to-do when the
142       grammar stabilises).
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AUTHOR

145       John McNamara jmcnamara@cpan.org
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148       X MM-MMX, John McNamara.
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150       All Rights Reserved. This module is free software. It may be used,
151       redistributed and/or modified under the same terms as Perl itself.
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155perl v5.12.0                      2010-01-21Spreadsheet::WriteExcel::Formula(3)
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