1. Lecture 2A Data Types Richard Gesick
Figures from Lewis, “C# Software Solutions”, Addison Wesley

2. Topics Character Strings Variables and Assignments
Primitive Data Types

3. Character Strings
In C++, defined by either of 2 classes, cstring which is null terminated ‘\0’ and the string class which is not null terminated
String literal is 0 or more characters enclosed with double quotes
“The quick brown fox jumped.”
“x” “”
Can contain any valid character

4. string Concatenation Operator (+)
String literals cannot span lines
Combines string literals with other data types for printing
Example:
string hello = "Hello";
string there = "there";
string greeting = hello + ' ' + there;
cout<<greeting ;
Output is:
Hello there

5. The + Operator What it does depends on the order and the data types
String concatenation
addition

6. Escape Sequences
To include a special character in a string, use an escape sequence

7. Copyright © 2012 Pearson Education, Inc.
Declarations
A type declaration statement defines new identifiers and allocates memory.
Multiple variables can be created in 1 declaration
An initial value may be assigned to a memory location at the time an identifier is defined.
Syntax
[modifier] type specifier identifier [= initial value];
[modifier] type specifier identifier[(initial value)];
Examples
double x1, y1(0);
int counter=0;
const int MIN_SIZE=0;
bool error(false);
char comma(',');
Copyright © 2012 Pearson Education, Inc.

8. Initial Values C++ does not provide initial values for variables.
Thus using the value of a variable before it is initialized may result in ‘garbage’.

9. Literals and Variables
Literals are objects that store specific data that can not be modified.
10 is an integer literal
4.5 is a floating point literal
"The distance between the two points is" is a string literal
'a' is a character literal
Variables are named memory locations that store values that can be modified.
double x1(1.0), x2(4.5), side1;
side1 = x2 - x1;
x1, x2 and side1 are examples of variables that can be modified.

10. Copyright © 2012 Pearson Education, Inc.
Common C++ Data Types
Keyword Example of a literal
bool true
char '5'
int 25
double 25.0
string "hello" //#include<string>
Copyright © 2012 Pearson Education, Inc.

11. Copyright © 2012 Pearson Education, Inc.
Symbolic Constants
A symbolic constant is defined in a declaration statement using the modifier const.
A symbolic constant allocates memory for an object that can not be modified during execution of the program. Any attempt to modify a constant will be flagged as a syntax error by the compiler.
A symbolic constant must be initialized in the declaration statement.
Copyright © 2012 Pearson Education, Inc.

12. C++ Identifiers
Should be carefully chosen to reflect the contents of the object.
The name should also reflect the units of measurements when applicable.
Must be declared (and therefore typed) before they may be used.
C++ is a strongly typed programming language.
Avoid names similar to C++ reserved words

13. Assignment
An assignment statement changes the value of a variable
The assignment operator is the = sign
total = 55;
The expression on the right is evaluated and the result is stored in the variable on the left
The value that was in total is overwritten
You can only assign a value to a variable that is consistent with the variable's declared type

14. Assignment Operator
Syntax: target = expression; expression: operators and operands that evaluate to a single value --value is then assigned to target --target must be a variable (or constant) --value must be compatible with target's data type

15. Examples: The next statement is illegal
int numPlayers = 10; // numPlayers holds 10
numPlayers = 8; // numPlayers now holds 8
int legalAge = 18;
int voterAge = legalAge;
The next statement is illegal
int height = weight * 2; // weight is not defined
int weight = 20;

16. Declare a variable only once
Once a variable is declared, its data type cannot be changed.
These statements:
double twoCents;
double twoCents = .02;
generate a compiler error

17. Once a variable is declared, its data type cannot be changed.
These statements:
double cashInHand;
int cashInHand;
generate a compiler error

18. Constants Constants are useful for three important reasons
First, they give meaning to otherwise unclear literal values
For example, MAX_LOAD means more than the literal 250
Second, they facilitate program maintenance
If a constant is used in multiple places, its value need only be updated in one place
Third, they formally establish that a value should not change, avoiding inadvertent errors by other programmers

19. Conventions
Use all capital letters for constants and separate words with an underscore:
Example:
const double TAX_RATE = .05;
Declare constants at the top of the program so their values can easily be seen
Declare as a constant any data that should not change during program execution

20. Data Types For all data, assign a name (identifier) and a data type
Data type tells compiler:
How much memory to allocate
Format in which to store data
Types of operations you will perform on data
Compiler monitors use of data
C++ is a "strongly typed" language

21. Primitive Data Types Everything else is an object
3 subsets of integers
short, int, long (signed or unsigned)
3 subsets of floating point numbers
float, double, long double
character
bool
Everything else is an object

22. Why so many types?
Difference is in amount of memory reserved for each (and hence the size of the value stored
float only has 7 significant digits
Signed numbers have both positive and negative values
Unsigned numbers are >= 0

23. Literals All numeric values without a decimal point are considered int
All numeric values with decimal point are considered double
int testGrade = 100;
long cityPopulation = L;
float salesTax = .05F;
double interestRate = 0.725;
double avogadroNumber = E23;

24. char Data Type One ASCII character (8 bits - 1 byte)
Example declarations:
char finalGrade = ‘A’;
char newline, tab, doubleQuotes;

25. bool Data Type Two values only:
true
false
Used for decision making or as "flag" variables
Example declarations:
bool isEmpty;
bool passed, failed = false;

26. C-style Character Strings
A C style strings is defined as a sequence of characters, terminated by the null character.
When declaring a character array to store a C style string, memory must be allocated for the null character ('\0').
Literal string constants are enclosed within double quote marks: "a string".

27. C-style String Input
Recall that the input operator (>>) skips whitespace .
To input strings with embedded whitespace , the getline() function can be used as illustrated:
char phrase[SIZE];
cin.getline(phrase, SIZE);
The getline() function reads up to SIZE-1 characters from the input stream and will insert the null character.
getline() is a member function of what class?

28. C-style String Functions
The Standard C++ library contains a set of predefined functions that operate on C style strings.
These functions are defined in the header file: cstring
Commonly used string functions:
strlen()
strcpy()
strcat()
strcmp()

29. C-style String Example
#include <iostream> #include <cstring> //strcmp(), strcpy(), strcat() uses namespace std; void main() { char str1[30] = "John", str2[30] = "Johnson"; char phrase[20] = "'s shirt was green", sentence[30]; if (strcmp(str1,str2) < 0) strcpy (sentence, str1);//puts "John" into sentence else strcpy (sentence,str2);//puts "Johnson” into sentence strcat(sentence, phrase);//append phrase to sentence cout << "Sentence is: " << sentence << endl; }

30. Avoiding Bugs
The null character ( ‘\0’ ) must always mark the end of the string.
If there is no null character, functions will produce invalid references that can be difficult to identify (remember that C-style strings are 1D arrays of characters).
String sizes are fixed.
Input operator does NOT ‘know’ what the array size is; input may result in overflow of the array (invalid references).

31. The String Class
The string class implements the concept of a character string.
A string object can increase and decrease its size dynamically.
Numerous operators and methods are defined in the string class.

32. Common Methods of the string Class
size( )
empty( )
substr (int start, int len)
c_str()

33. Operators Overloaded for the string Class
relational operators
< > == <= >=
concatenation
+ +=
assignment =

34. String Class Example
#include <iostream> #include <string> //string class uses namespace std; void main() { string str1 = "John", str2 = "Johnson"; string phrase = "'s shirt was green", sentence; if ( str1< str2 ) sentence = str1;//puts "John" into sentence else sentence = str2;//puts "Johnson” into sentence sentence += phrase; //append phrase to sentence cout << "Sentence is: " << sentence << endl; }