1 Types The type of a variable represents a set of values that may be assigned to the variable. For example, an integer variable is one that may take the values –2 31 to 2 31 – 1 What if we want a variable that may take values from a non-numerical set? For example, variable day may take values from the set {Mon, Tue, Wed, Thu, Fri, Sat, Sun}

2 Enumerations An enumerated type is a set of values defined by the programmer. Each element of an enumerated type is given a unique name by the programmer. Example: enum {Mon, Tue, Wed, Thu, Fri, Sat, Sun} Each element of an enumerated type is internally represented by an integer. Unless the programmer explicitly specifies the integers, the values are consecutive starting at 0. This allows us to perform iterate from one variable to another.

3 Enumerations In order to be able to declare enumerated variables, an enumeration must be given a name. Example: enum dayT {Mon, Tue, Wed, Thu, Fri, Sat, Sun}; int main () { dayT today, tomorrow; today = Mon; cout << today; // this will print 0 tomorrow = (today + 1) % 7; if (tomorrow == Sat) cout << “It’s the weekend!” << endl;...

4 Enumerations enum dayT {Mon, Tue, Wed, Thu, Fri, Sat, Sun}; Mon, Tue, etc. are called “enumeration constants” CAUTION! When you declare dayT today;, today is essentially an integer. There are no restrictions on its value. Example: today = 135; is allowed (but meaningless)

5 Structures We have seen that we can use arrays when we want several similar variables of the same type. But what if we need to store several pieces of information that have different types but refer to the same entity? Example: a student roster may contain the name and id number of each student. We’d like one variable to somehow contain two components, the name and the id. The answer is a structure: a collection of named components (fields) that may have different types. (Sneak preview: C++ classes are very similar to C structures)

6 Structures Example: struct studentInfoT { char name[30]; int id_number; }; int main () { studentInfoT student1; // a variable of type studentInfoT studentInfoT roster[15]; // an array of 15 elements of type // studentInfoT.

7 Structure To access the individual fields of a structure we use the dot operator. Example 1: struct pointT { double xcoord; double ycoord; }; int main () { pointT origin; origin.xcoord = 0.0; origin.ycoord = 0.0;... }

8 Structure To access the individual fields of a structure we use the dot operator. Example 2: struct studentInfoT { int id_number; char grade; }; int main () { studentInfoT classlist[15]; classlist[0].id = ; classlist[0].grade = ‘A’;... }

9 Pointers A pointer variable is a variable whose value is an address. Some programmers use the terms pointer and address interchangeably. Pointers allow us to manipulate data through the address This makes pointers very useful but also very dangerous.

10 Pointers Example: address variable name, value 0xbfffc290:px, 0xbfffc294 x, -10 px is a pointer variable that contains the address of an integer variable, x. The value of px is address 0xbfffc294 The value of x is integer -10 We say that px points to x Because x is an int, we say that the base type of px is int 0xbfffc294: 0xbfffc298:

11 Pointers A pointer variable is declared as follows: base_type * variable_name ; the type of the object pointed to by the pointer the name of the pointer variable. It follows the same rules as regular variables, and usually starts with a p the asterisk signifies that this is a declaration of a pointer variable

12 Pointers Examples All of these variables are currently unitilialized int *pnum; // pnum will contain the address of an // integer float *ptemp; // ptemp will contain the address of a // floating point number char *pword, letter; // CAUTION! pword is a pointer // to char but letter is a char.

13 Pointers A pointer variable is initialized by assigning an address to it. Example 1: int list[5] = {10, 20, 30, 40, 50}; int *pnum; pnum = list; /* pnum now contains the address where the list begins. In other words, it contains the address of the first element of the list. */

14 Pointers A pointer variable is initialized by assigning an address to it. We can access the address of a variable by using the & operator. int *pnum, number; pnum = &number; /* pnum now points to number */

15 Pointers char *pletter, *pgrade; char grades[4] = {'A', 'B', 'C', 'D'}; char ch; pgrade = grades; /* pgrade points to the beginning of grades */ pletter = &ch; /* now, pletter points to variable ch */ pletter = pgrade; /* now, pletter points to the beginning of grades */ grades = pletter; /*ILLEGAL! The address of an array is constant! You cannot change its value */ pletter = 100; /* ILLEGAL! 100 is an integer, pletter's value should be an address */ pletter = &pgrade /* ILLEGAL! pletter should be the address of a char, but &pgrade is the address of a pointer (or, to be more specific, it's the address of the address of a char). The types do not match. */