course-c

Introduction

A C structure is a collection of one or more variables of the same or different types
Structures permit convenient handling of complicated data as a single unit
Similar to records in Pascal
Copying, assigning to, recovering address using & and accessing members are all legal operations on structures

Creating a structure

The struct keyword is used to create structures

struct address {
  char * street;
  char * city;
  int zip;
} a, b;

The tag address is optional but useful for identifying the struct so new variables can be created
```
struct address a, b;
```

Initializing structures

Structures can be initialized just like arrays

struct address a = {"street", "recife", 123456}, b;

An automatic structure can also be initialized by assignment or by calling a function that returns the structure of the right type
```
a.zip = 123456;
a.street = "street";
a.city = "recife";
```

Copying and assigning to structures

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Example of an address structure being assigned to another

struct address a, b;
a.zip = 123456;
a.street = "street";
a.city = "recife";
b = a;
b.zip = 654321;
printf("%d, %s, %s\n",
  a.zip, a.street, a.city);

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Outputs
```
123456, street, recife
```
Note that changing the copy has not affected the original structure :::

::::

Structures and functions

Structures can be passed as parameters to a function
Structures are passed by value i.e. copying their content
Large structures should be passed by reference, by passing their pointers as parameters to functions

Pointers to structures

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Example of structure manipulation using its pointer

struct address *b;
b = (struct address *)
  malloc(sizeof(struct address));
b->street = "street";
b->city = "recife";
(*b).zip = 654321;
printf("%d, %s, %s\n",
  (*b).zip, b->street, b->city);

:::

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The . operator has higher precedence than the * operator
C provides the -> operator to facilitate accessing members of structures through their pointers
A structure can point to itself e.g. in a tree structure

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::::

Arrays of structures

Arrays can store structure types
```
struct address a[10];
```

Arrays can be initialized at time of declaration using literal values and variables

struct address a[] = {"street1", "recife", 4123456,
  "street2", "salvador", 654321};
// or
struct address a[] = { {"street1", "recife"},
  {"street2", "salvador", 654321} };

Typedef

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New scalar types can be created using the typedef keyword
New complex types can be created using structures

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  typedef unsigned short UCHAR;
  typedef struct address {
    char * street;
    char * city;
    int zip;
  } Address;

  b = (Address*)
    malloc(sizeof(Address));
  b->street = "street";
  b->city = "recife";
  b->zip = 654321;

:::

::::

Unions

Look like a structure but store only one type at any given time
```
union number {
  int ival;
  float fval;
} n;
```
The compiler assigns a union a size large enough to store the widest type
Unions can be nested within structures
Unions support the same operations as structures

Bit fields

Useful for conveniently handling several option flags as a single entity
Each flag field can only be an int

The fields cannot be arrays, nor be pointed to; thus the & operator cannot be applied to them

struct bit_fields {
  unsigned int is_keyword : 1;
  unsigned int is_extern : 1;
  unsigned int is_static : 1;
} f;

Exercise

Write a program to count the occurrence of each word in a given string. Use a binary search tree to store the words along with their counts.
Print the words with their count to standard output in an ascending order by traversing the binary search tree in-order.