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For 2021 - online Python 3 training - see ((here)).

Our plans were to retire in summer 2020 and see the world, but Coronavirus has lead us into a lot of lockdown programming in Python 3 and PHP 7.
We can now offer tailored online training - small groups, real tutors - works really well for groups of 4 to 14 delegates. Anywhere in the world; course language English.

Please ask about private 'maintenance' training for Python 2, Tcl, Perl, PHP, Lua, etc.
Just pass a pointer - do not duplicate the data

When you want to pass a big bundle of data - such as an array - into a function, it's far more efficient to pass in just a single value in the form of a pointer to the array. That's the address in memory at which the array starts and that single value can provide the key to all the data without havig to duplicate it. By passing in the address rather than a copy of the data, it also means that the C function can - if it wishes - modify the original copy of the data and not just a duplicate which would be lost when the operation of the function completes.

From yesterday's C Course, I've added an example of passing in the address of the start of an array to a function to our source examples - see [here]. The parameter is passed in as
  &stuff[0]
and picked up by the declaration
  int * options

You'll want to do this so often that you can use a shorthand - if you just pass in the array, it automatically assumes that you mean the address of the first element, and you can also collect the parameter using array notation:
  int options[]
See source code [here].

If you don't know at compile time how big you need your array to be, you can allocate memory to be used as an array with a function such as calloc; the resultant memory does not have its own variable name, so it has to be accessed only via pointers. I've extended the above examples into an equivalent using calloc - source code [here]. There are further examples in the memory management module too - see [here].




This is really a "C" example ... but a similar principle applies in Perl. If you pass in @abc to a sub, then you're duplicating the data. If you pass in \@abc then you're passing in a reference - that's in effect a sophisticated pointer.
(written 2010-06-30)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
P217 - Perl - More than Simple Lists and Hashes!
  [43] Hash of lists in Perl - (2004-09-09)
  [293] Course follow-ups - (2005-04-27)
  [1514] Autovivification - the magic appearance of variables in Perl - (2008-01-21)
  [2241] Perl references - $$var and \$var notations - (2009-06-15)
  [2877] Further more advanced Perl examples - (2010-07-19)
  [2996] Copying - duplicating data, or just adding a name? Perl and Python compared - (2010-10-12)
  [3007] Setting up a matrix of data (2D array) for processing in your program - (2010-10-21)
  [3072] Finding elements common to many lists / arrays - (2010-11-26)
  [3105] Adventure with references to lists and lists of references - (2010-12-26)
  [3118] Arrays of arrays - or 2D arrays. How to program tables. - (2011-01-02)
  [3399] From fish, loaves and apples to money, plastic cards and BACS (Perl references explained) - (2011-08-20)
  [3406] Not multidimentional arrays - but lists of lists. Much more flexible. Perl! - (2011-08-26)
  [3444] Take the dog on a lead - do not carry her. Perl references. - (2011-09-17)
  [3577] How to do multidimensional arrays (or rather lists and hashes) in Perl - (2012-01-14)
  [3906] Taking the lead, not the dog, for a walk. - (2012-10-28)

C205 - C and C based languages - Arrays
  [1614] When an array is not an array - (2008-04-17)
  [2002] New C Examples - pointers, realloc, structs and more - (2009-01-20)
  [3121] New year, new C Course - (2011-01-05)
  [3144] Setting up arrays in C - fixed size at compile time, or dynamic - (2011-01-24)
  [3245] Collections in C and C++ - arrays, vectors and heap memory blocks - (2011-04-12)
  [4338] Passing arrays into functions in C - (2014-12-02)
  [4566] C - why is slow to write and debug) but fast to run? - (2015-11-01)


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Learning about Regular Expressions in C through examples
String functions in C
Staring a C course with Hello World - why?
C Course exercise and sample answer - source in 2 files
Just pass a pointer - do not duplicate the data
Software versions used - June 2010
Respecting our customers anonimity
Lorry Parking in Melksham
Perl - the duplicate key problem explained, and solutions offered
A course is more than just a chap giving a lecture
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