Computational Science I

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Computational Science I

• Lecture 7
• CAAM 420, Fall 2004
• Instructor Tim Warburton

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struct

• It is sometimes convenient to gather more than
  one variable type in a user defined type of
  variable, known as a struct
• For example if I wish to beef up an array by
  keeping some information with it, perhaps its
  length then I can create my own array struct

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Example of a struct
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Here I define theivec struct
Here I create an ivec named ivecA
Here I assign the contents of ivecA
Here I fill up the contentsof the array pointed
atfrom ivecA
Here I access the contents of the array pointed
at from ivecA
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Creating a Pointer to a Variable

• We can use the operator to find out what the
  memory address of a variable is
• We can use to dereference a pointer to examine
  the memory at that pointer address
• We can also use 0 to do the same thing.

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Using and 0
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Pointer to a struct

• Suppose we dereference ivecA with
• i.e. we could create a pointer to where ivecA
  starts in memory
• ivec ivecptrA ivecA
• Then we can access the contents with -gt
• Accessing ivecptrA-gtiNentries
• And ivecptrA0.iNentries
• Are the same as ivecA.iNentries

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Array of structs

• We can also create an array containing multiple
  versions of a struct
• Again we can use calloc.
• So for an array of 10 ivec structs we can use
• ivec ivecptrs (ivec) calloc(10, sizeof(ivec)
  )
• each of the 10 ivecs and their contents can be
  accessed
• ivecptrs3.iNentries gives the value of
  iNentries in the 4th ivec in the allocated
  memory.
• We could also have used
• (ivecptrs3)-gtiNentries

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Here I create space for 10 ivecs
Here I initialize each of the 10 ivecAs
Here I derefernce the pointers in two different
ways
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cont

• When we run we see that both methods of
  dereferencing give the same answer.

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Functions

• The C math library includes a large number of
  math functions.
• But first we should examine how a function
  operates in C.
• A function is an encapsulated piece of code which
  has its own scope.

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Example of a Function

• Here I define a function myfunction which takes
  an integer argument and returns an integer value.
• myfunction will be visible (i.e. callable) by all
  functions lower than it in the file

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Scope of Function

• The scope of a function is loosely defined as
  the set of variables visible inside the function.
  This includes
• variables passed into the function as arguments
• data pointed to by pointers passed in as
  arguments
• variables created inside the function
• variables created by calling other functions from
  inside the function (e.g. calloc, realloc)
• variables defined in header files (ok, but not
  great)
• variables which are defined above this function
  in the file but not in a function (dont do this)
• variables added to the scope with the extern
  command (do not do this)

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Example of Scope

• Here we define a function which takes as
  arguments an integer as well as a pointer and
  length of the data pointed to.
• We can not only use the int value passed in but
  also the contents of the int array passed in by
  pointer.

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Example of Scope cont

• To call that function, we create the input data
• This involves initializing the input variable and
  the input array.

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Important Note

• When you call a function with
• ib myfunction(ia)
• myfunction actually receives a copy of ia, not
  the original.
• If myfunction changes ia, it will not change in
  the function calling myfunction

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However

• If you pass a pointer into a function
• myfunction1(ia)
• Then myfunction1 can access the variable pointed
  to by the pointer. E.g.
• void myfunction1(int iptrA)
• iptrA 2
• main()
• int iA 3
• myfunction1(iA)
• will change the value of the iA from 3 to 2.

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Class Exercise J

• Write a function which accepts an int as an
  argument variable and adds one to the variable
• Write a function which accepts a pointer to an
  int as an argument, and adds one to the variable
  pointed to by the pointer.
• Compare the before and after values of the
  variable you are trying to change using printf.
• For entertainment, in function for 1) print out a
  pointer to the argument before the function and
  in the function.

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Extending the Visibility of a Function

• Normally a function is only visible to functions
  below it in the source code.
• However, we can introduce a prototype which
  describes the calling convention of the function.
  
• If the prototype is introduced between functions,
  then all subsequent functions in the file will be
  able to use this function.
• If the prototype is introduced inside a function
  then all lines of code until the next will be
  able to call the function which is prototyped.

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Prototype
This is a prototype formyfunction
This is a call tomyfunction
This is a definition formyfunction
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Passing struct to a Function

• We can also pass a user defined struct to a
  function
• For instance, we can create a function which
  elongates our ivec array.

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Recall the ivec struct

• Here we need to pass in a pointer to the struct
  as we are going to change its contents.
• Notice how I update the affected members of the
  struct when I change the vector length.

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cont

• Here I show how I passed in the pointer to the
  original ivecA

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cont

• What happened at ivecA.iptr10 ???

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Class Exercise K

• Take myextendarray.c from the SourceCode
  directory online.
• Change the code so you pass in ivecA as a
  variable and not as a pointer.
• Access and change the contents of the input ivec
  using ivecA.iptr and ivecA.iNentries
• Before and after the ivecextend function call
  print out the contents of ivecA
• What is going on?