Working with Structures

1
Working with Structures
2
Structure Declaration

• Creates a user-defined data type.
• Gives the compiler a heads up as to what the
  structure contains.
• Does NOT create an instance of a structure.
• Does NOT allocate any memory.

3
Structure Declaration Example

• struct pt3d
• int pt
• double x, y, z

4
Structure Definition

• Creates an instance of a structure.
• Allocates memory.
• Associates a variable name with that instance.
• Directly analogous to defining a variable of any
  of the pre-defined data types.

5
Structure Definition Example

• int main(void)
• int i
• struct pt3d pt1, pt2 struct pt3d pt5
• ....
• return 0

6
Typedef simplifies the syntax

• Almost identical to define statement.
• Performs text replacement.
• distributes any pointer de-reference operators.
• Syntax
• typedef data-type alias
• Example
• typedef unsigned int size_t
• typedef struct pt3d PT3D

7
Everything to this point
Typedef can appear before structure declaration

• typedef struct pt3d PT3D
• struct pt3d
• int pt
• double x, y, z
• int main(void)
• int i
• PT3D pt1
• PT3D pt5
• ....
• return 0

Structure declaration outside any function to
make it have global scope
8
Accessing Elements

• Similar to accessing elements of an array.
• Array Follow the name of the array with the
  structure access operator (square brackets)
  surrounding the offset of the element to be
  accessed.
• Structure Follow the name of the structure with
  the member access operator (a period) followed
  by the name of the element to be accessed.
• Use the accessed element just like a plain
  variable of that same type.

9
Accessing Elements Examples

• int j
• int k12
• PT3D point
• j 5
• kj 42
• point.pt kj
• j point.pt 3

10
Pointers to Structures

• Work just like pointers to other data types.
• Use the indirection operator (asterisk) to
  de-reference) the pointer in order to get at the
  value being pointed to.
• Treat the dereferenced pointer the same as the
  name of a variable of the type being pointed to.

11
Structure Pointer Examples

• int j, m
• int k12
• PT3D point, p
• m j
• p point
• m 5 // same as j 5
• kj 42
• (p).pt kj // same as point.pt kj
• j (p).pt 3 // same as j point.pt 3

12
Why (p).pt and not just p.pt ?

• Highest precedence operators (), , ., -gt
• De-reference is second in precedence.
• The code fragment
• p.pt compiles as (p.pt)
• The operand on the left side of the member access
  operator must be a variable of structure type.
• But p is not a variable of structure type - it
  points to a variable of structure type.
• So we must override the precedence and force the
  pointer to be de-referenced first using
  parentheses.

13
Structure Indirection

• De-referencing structure pointers very common.
• Failing to override operator precedence very
  common.
• A different operator is available specifically
  for this purpose.
• If the left operand is a structure to a pointer,
  then the structure indirection operator (a
  hyphen followed immediately by a greater-than
  symbol) will first de-reference the pointer and
  then access the indicated element.
• The code p-gtpt compiles as (p).pt

14
Primitive and Utility Functions

• Structure elements frequently change.
• Sloppy use of structures becomes a code
  maintenance nightmare.
• Primitive and Utility functions provide a
  localized interface to a structures elements.
• ONLY the primitive function should ever directly
  access the structure elements.
• ONLY the utility functions should ever call the
  primitive functions.
• The programmer should only use utility (and
  higher) functions.

15
Get()/Set() Primitive Functions

• One Get()/Set() Pair for each structure element.
• Get() functions read the current value.
• Set() functions write a new value.
• Both return the final value.
• Work best if structures passed by reference.

16
Get()/Set() list for PT3D

• int GetPT3D_pt(PT3D p)
• int SetPT3D_pt(PT3D p, int v)
• double GetPT3D_x(PT3D p)
• double SetPT3D_x(PT3D p, double v)
• double GetPT3D_y(PT3D p)
• double SetPT3D_y(PT3D p, double v)
• double GetPT3D_z(PT3D p)
• double SetPT3D_z(PT3D p, double v)

17
Get()/Set() Primitive Functions
Naming convention GetSTRUCT_element

• int GetPT3D_pt(PT3D p)
• if (NULL p)
• return -1
• return p-gtpt
• int SetPT3D_pt(PT3D p, int v)
• if (NULL p)
• return -1
• p-gtpt v
• return GetPT3D_pt(p)

18
Utility Function - set all the point coordinates
at once.

• PT3D NewPoint(int pt, double x, double y, double
  z)
• PT3D temp
• SetPT3D_pt(temp, pt)
• SetPT3D_x(temp, x)
• SetPT3D_y(temp, y)
• SetPT3D_z(temp, z)
• return temp

19
Utility Function - subtract two points (vector
between points).

• PT3D SubtractPT3D(PT3D ptA, PT3D ptB)
• PT3D temp
• SetPT3D_pt(temp, 0)
• SetPT3D_x(temp, GetPT3D_x(ptA) -
  GetPT3D_x(ptB) )
• SetPT3D_y(temp, GetPT3D_y(ptA) -
  GetPT3D_y(ptB))
• SetPT3D_z(temp, GetPT3D_z(ptA) -
  GetPT3D_z(ptB))
• return temp

20
Utility Function - Distance between two points

• double DistanceBetweenPT3D(PT3D ptA, PT3D ptB)
• double distance
• distance LengthPT3D(SubtractPT3D(ptA, ptB))
• return distance

21
Utility Function - Distance between two points

• double LengthPT3D(PT3D ptA)
• double length
• int i
• for (i 0, length 0.0 i lt 3 i)
• length pow(GetPT3D_dimensionN(ptA, i),2)
• return sqrt(length)

22
Utility Function - Get coordinate of a point
based on index

• double GetPT3D_dimensionN(PT3D p, int i)
• if (NULL p)
• return -1
• switch (i)
• case 0 return GetPT3D_x(p) break
• case 1 return GetPT3D_y(p) break
• case 2 return GetPT3D_z(p) break
• return -2

23
Simple main()

• define PTS (5)
• int main(void)
• int i
• PT3D ptPTS1
• double theta
• for (i 0 i lt PTS i)
• theta i (2.0PI/(double) PTS)
• pti NewPoint(i, cos(theta), sin(theta),
  0.0)
• fprintPT3D(stdout, pti)
• return 0

24
Utility function to print structure to a file.

• void fprintPT3D(FILE fp, PT3D pt)
• int i
• fprintf(fp, i, GetPT3D_pt(pt))
• for (i 0 i lt 3 i)
• fprintf(fp, , f, GetPT3D_dimensionN(pt,
  i))
• fprintf(fp, \n)