Steffen Müller > Math-Series > Math::Series
Module Version: 1.01

# NAME

Math::Series - Perl extension dealing with mathematic series

# SYNOPSIS

```  use Math::Series;
my \$x_n = Math::Series->new( formula       => 'n*x',
start_value   => 1,
iteration_var => 'n',
previous_var  => 'x',
start_index   => 0,
cached        => 1      );

print \$x_n->next(), "\n" foreach 0..5;
# prints 1, 2, 6, 24...

print \$x_n->at_index(3);
# prints 24```

# DESCRIPTION

Math::Series defines a class for simple mathematic series with a recursive definition such as `x_(n+1) = 1 / (x_n + 1)`. Such a recursive definition is treated as a sequence whose elements will be added to form a series. You can refer to the previous sequence element as well as to the current index in the series. Creation of a Math::Series object is described below in the paragraph about the constructor.

Math::Series uses Math::Symbolic to parse and modify the recursive sequence definitions. That means you specify the sequence as a string which is parsed by Math::Symbolic. Alternatively, you can pass the constructor a Math::Symbolic tree directly.

Because Math::Series uses Math::Symbolic for its implementation, all results will be Math::Symbolic objects which may contain other variables than the sequence variable and the iterator variable.

Each Math::Series object is an iterator to iterate over the elements of the series starting at the first element (which was specified by the starting element, the second argument to the new() constructor). It offers facilities to cache all calculated elements and access any element directly, though unless the element has been cached in a previous calculation, this is just a shortcut for repeated use of the iterator.

Every element in the series may only access its predecessor, not the elements before that.

## CLASS DATA

Math::Series defines the following package variables:

\$Math::Series::Parser

This scalar contains a Parse::RecDescent parser to parse formulas. It is derived from the Math::Symbolic::Parser grammar.

\$Math::Series::warnings

This scalar indicates whether Math::Series should warn about the performance implications of using the back() method on uncached series. It defaults to true.

## METHODS

new()

The constructor for Math::Series objects. It takes named parameters. The following parameters are required:

formula

The formula is the recursive definition of a sequence whose elements up to the current element will be summed to form the current element of the series. The formula may contain various Math::Symbolic variables that are assigned a value elsewhere in your code, but it may also contain two special variables: The number of the current iteration step, starting with 0, and the previous element of the series.

The formula may be specified as a string that can be parsed by a Math::Symbolic parser or as a Math::Symbolic tree directly. Please refer to the Math::Symbolic and Math::Symbolic::Parser man pages for details.

start_value

This parameter defines the starting value for the series. It used as the element in the series that is defined as the lowest series element by the start_index parameter. The starting value may be a string that can be parsed as a valid Math::Symbolic tree or a preconstructed Math::Symbolic tree.

The following parameters are optional:

iteration_var

The iteration variable is the name of the variable in the Math::Symbolic tree that refers to the current iteration step. It defaults to the variable 'n'.

It must be a valid Math::Symbolic variable identifier. (That means it is `/[A-Za-z][A-Za-z0-9_]*/`.)

previous_var

The previous_var parameter sets the name of the variable that represents the previous iteration step. It defaults to the name 'x' and must be a valid Math::Symbolic variable identifier just like the iteration variable.

cached

This parameter indicates whether or not to cache the calculated series' elements for faster direct access. It defaults to true. At run-time, the caching behaviour may be altered using the cached() method.

start_index

The lower boundary for the series' summation. It defaults to 0, but may be set to any positive integer or zero.

next()

The next() method returns the next element of the series and advances the iterator by one. This is the prefered method of walking down a series' recursion.

cached()

Returns a true value if the series is currently being cached, false if it isn't. By default, new objects have caching enabled. It is suggested that you only disable caching if space is an issue and you will only walk the series uni-directionally and only once.

cached() can be used to change the caching behaviour. If the first argument is true, caching will be enabled. If it is false, caching will be disabled.

current_index()

Returns the index of the current element. That is, the index of the element that will be returned by the next call to the next() method.

This method also allows (re-)setting the element that will be next returned by the next() method. In that case, the first argument shoudl be the appropriate index.

Returns undef and doesn't set the current index if the argument is below 0.

at_index()

This method returns the series element with the index denoted by the first argument to the method. It does not change the state of the iterator. This method is extremely slow for uncached series.

Returns undef for indices below the starting index.

back()

This methods returns the series element previously returned by the next() method. Since it is extremely slow on uncached series, it warns about this performance hit by default. To turn this warning off, set the \$Math::Series::warnings scalar to a false value.

This method decrements the current iterator series element.

Returns undef if the current index goes below the starting index.

# AUTHOR

Steffen Mueller, <series-module at steffen-mueller dot net<gt>