© Janice Regan, CMPT 128, Jan CMPT 128: Introduction to Computing Science for Engineering Students Data representation and Data Types Variables and Constants

© Janice Regan, CMPT 128, Jan Tokens  Token: smallest individual unit of a program  Types of tokens include  Reserved words  Variables, Constants, Literals  Functions  Structures and classes  Operators  We will discuss each of these later

Why variables and constants?  Calculate the sum of two numbers  First you need the two numbers to add On paper write the numbers down to record them (store them) In a program those two numbers will be put into variables or constants to record them  Each variable or constant (Each number) has a name (an identifier) so we can refer to it a value (the number being stored) © Janice Regan, CMPT 128, Sept

3 Constants and Variables  Each constant or variable represents a specific item in the problem the program solves and is associated with  a memory cell used to hold its value  a unique identifier or name  a data type  The value of a constant does not change during execution of the program  The value of a variable may be changed during the execution of a program.

© Janice Regan, CMPT 128, Jan C++ Identifiers  Names of program components:  Variables, Constants  Structures, functions, enumerations, … (more later)  IMPORTANT!!!.C++ is case sensitive  The following identifiers (names ) are NOT the same Toy toy TOY

© Janice Regan, CMPT 128, Jan Legal C++ Identifiers  Must contain only:  Letters (upper and lower case)  Digits (0-9)  The underscore character ( _ )  Identifiers must begin with a letter or an _  Identifiers beginning with an underscore are used for specific purposes and should be avoided in general use

Reserved words  In a computer language there are always a number of special words that are used as instructions and definitions in the language. These are called reserved words  Reserved words cannot be used as identifiers © Janice Regan, CMPT 128, Sept

© Janice Regan, CMPT 128, Jan Reserved words in C++  A reserved word is a special word that gives the compiler a specific instruction it can understand (summary p489 text)  asm, auto, bool, break, case, catch, char, const, class, continue  default, delete, do, double, else, enum, explicit, export, extern  false, float, for, friend, goto, if, inline, int, long, mutable, throw  namespace, new, operator, private, protected, public, register  return, short, signed, sizeof, static, struct, switch, template, this  true, try, typedef, union, unsigned, using, virtual, void, volatile, while

© Janice Regan, CMPT 128, Jan More About C++ Identifiers  Can divide identifiers into two groups  Standard Identifiers: Identifiers defined by the compiler that are not parts of the C++ language definition This include identifiers used as names of functions in the standard C++ libraries  User (programmer) defined Identifiers: Names the programmer chooses for quantities, functions, etc. used in their own programs

© Janice Regan, CMPT 128, Jan Program - sum two numbers #include using namespace std; int main() { // Declare variables. double number1, number2; double sum; // Initialize variables number2 = 45.7; number1 = 473.9; // Calculate and print the sum. sum = number1 + number2; cout << “The sum is ” << sum << endl; // Return successful termination indicator return 0; } Standard identifiers

© Janice Regan, CMPT 128, Jan Illegal Identifiers  Prog#1The # is not a letter digit or _  1stprogramcannot begin with a digit  autocannot be a keyword

© Janice Regan, CMPT 128, Jan Which Are Legal C++ Identifiers?  cycle  A!star  int  Score  Y-Z  Trial#2  This_One  $MyProgram  FOR  3constant_values  “MaxScores”  _External_Prog  Mary’s  StarChart

© Janice Regan, CMPT 128, Jan More About C++ Identifiers  An identifier should be descriptive. It makes your code much easier to understand and maintain  sideLength is much clearer than sl  But keep your names a reasonable length  The maximum number of significant characters in an identifier is determined by the operating system and compiler used.

© Janice Regan, CMPT 128, Sept Components of a C++ Program  More types of tokens (smallest individual units of a program) include:  String Literal Constants or constant labels used in expressions Bob in expression Name = ” Bob”;  Literal Constants or fixed numbers used in expressions 3 in expression y = x + 3;  Variables and Constants

© Janice Regan, CMPT 128, Sept Constants and Variables  Each constant or variable represents a specific item in the problem the program solves and is associated with  a memory cell used to hold its value  a unique identifier or name  a data type  The value of a constant does not change during execution of the program  The value of a variable may be changed during the execution of a program.

© Janice Regan, CMPT 128, Sept Types of Constants and Variables  A Data Type is  A set of values plus a set of operations on those values  A crucial concept on modern programming  The data type of a variable or constant also determines how the variable’s value will be represented in memory  Variables of several types can have numerical values  Variables of other types have values that are characters, or logical values.

© Janice Regan, CMPT 128, Sept Types with Numerical Values  Two classes of numerical values  Integer and floating point  Integer values are whole numbers  No fractional part  1, 12354, 68, -123  C++ Types: int, unsigned int, long int, short int, long unsigned int, …

© Janice Regan, CMPT 128, Sept Types with Numerical Values  Two classes of numerical values  Integer and floating point  Floating point values have a fractional part  987.4, , 0.123,  C++ types: float, double, long double  Integers and floating point numbers are represented differently inside the computer

© Janice Regan, CMPT 128, Sept Declaring the variables you need  A first step in building the body of your main function is the declaration of the variables you will need to execute your algorithm.  To declare a variable we need to  choose an identifier as the name of the variable.  choose what the type of the variable is.

© Janice Regan, CMPT 128, Sept Declaration of Variables  Programming style for this course:  Declare one variable or constant per statement  Declare all variables at the start of the function.  Examples:  int myintegervalue;  double scalefactor;  float temperature;

© Janice Regan, CMPT 128, Sept Program - sum two numbers #include using namespace std; int main( ) { // Declare variables. double number1; double number2: double sum; // Initialize variables number2 = 45.7; number1 = 473.9; // Calculate and print the sum. sum = number1 + number2; cout << “The sum is ” << sum << endl; // Return successful termination indicator return 0; }

© Janice Regan, CMPT 128, Sept Initialization of Variables  Memory locations associated with variables should have their values defined before the start of program execution.  Using uninitialized variables often produces unpredictable results  Programming style for this course:  Initialize all variables at the start of your executable code  Initialize when or just after your variables have been declared

© Janice Regan, CMPT 128, Sept Program - sum two numbers #include using namespace std; int main(void) { // Declare variables. double number1; double number2; double sum; // Initialize variables number2 = 45.7; number1 = 473.9; sum = 0.0; // Calculate and print the sum. sum = number1 + number2; cout << “The sum is ” << sum << endl; // Return successful termination indicator return 0; }

© Janice Regan, CMPT 128, Sept Program - sum two numbers #include using namespace std; int main(void) { // Calculate and print the sum. sum = number1 + number2; cout << “The sum is ” << sum << endl; // Return successful termination indicator return 0; } // Declare and initialize variables. double number1 = 473.9; double number2 = 45.7; double sum=0.0;

© Janice Regan, CMPT 128, Sept Literal Data  Literals  Examples of literal constants: 2/*Literal constant int*/ 5.75/* Literal constant double*/  Examples of string literal constants "Z"/* String literal constant char array*/ "Hello World"/* String literal constant string*/  Cannot change values during execution  Called "literals" because you "literally typed" them in your program!  Literals do not have identifiers (names), they are used directly in the C++ code

© Janice Regan, CMPT 128, Sept Program - sum two numbers #include using namespace std; int main(void) { // Declare variables. double number1, number2; double sum; // Initialize variables number2 = 45.7; number1 = 473.9; // Calculate and print the sum. sum = number1 + number2; cout << “The sum is ” << sum << endl; // Return successful termination indicator return 0; } String literal constants literal constants

© Janice Regan, CMPT 128, Sept Named Constants  Naming your constants  Literal constants are "OK", but provide little meaning e.g., seeing 45.7 in a program, tells nothing about what it represents  Use named constants instead  Meaningful name to represent data const double FIRST_NUMBER = 45.7; Called a "declared constant" or "named constant" Now use it’s name wherever needed in program Added benefit: changes to value need only be made once

Programming style: summary  Descriptive identifiers  Declare and initialize all variables at the start of each program (function)  Easier to keep track of variables, they will always be available for the whole program (function)  You will not have problems with undeclared variables  Use named constants, not literal constants  Easier to keep track of what the constants mean  Easier to keep track of what the constants values are © Janice Regan, CMPT 128, Sept

© Janice Regan, CMPT 128, Sept Components: C, C++ Programs  Operators  actions used to combine variables and or constants  Example  = and + in the expression y = x + THREE;

© Janice Regan, CMPT 128, Sept Binary Arithmetic Operators  A Binary Operator operates on two arguments  + addition  -subtraction  * multiplication  / division  % modulus or remainder (only for integers, 5%2=1)  Evaluated left to right (C and C++)

© Janice Regan, CMPT 128, Sept Unary Arithmetic Operators in C++  A Unary Operator operates on one argument  + positive  -negative  ++ increment  --decrement  ~ones complement  Evaluated right to (C and C++)

© Janice Regan, CMPT 128, Sept Expressions in C and C++  An expression can be a single variable, or can be a series of variables combined using operators  Arithmetic expressions manipulate numeric variables following the rules of algebra  The two variables or constants combined using a binary arithmetic operator should have the same type  Otherwise conversions are needed (more later)

© Janice Regan, CMPT 128, Sept Precedence of operators in C++  ( ) []. innermost first static_cast  (pre) + - ! ~ & *(unary operators)  * / %  + -  = += -= *= /= %=  Evaluate 2 and 5 right to left  Evaluate 1, 3, and 4 left to right

© Janice Regan, CMPT 128, Sept Expressions: arithmetic operators  A + B + C X + C  Let A=10, B=5, C=2 + A B X C A B C + + Value of expression

© Janice Regan, CMPT 128, Sept Expressions: arithmetic operators  Order of operations is determined by operator precedence rules / before +  A + B / C A + X  Let A=10, B=5, C=2 + A B X C / A B C / + Value of expression

© Janice Regan, CMPT 128, Sept Importance of order of operations  Order of operations is determined by operator precedence rules () before /  (A + B) / C X / C  Let A=10, B=5, C=2 2 A B C + / + A B X C Value of expression

© Janice Regan, CMPT 128, Sept Expressions: arithmetic operators  Types of operands: float vs. integer divide  An operator always operates on two operands of the same type  A + B / C A + X  Let A=23.7, B=55.4, C=1.2 + A B X C / Value of expression A B C / +

© Janice Regan, CMPT 128, Sept Expressions: arithmetic operators  Let A=27, B=5, C=2, D=3, E=8  ((A + B) / C) – D % E ( X / C) – D % E Y – D % E Y – Z + A B X C / Y D 3 8 % Z E A B C + % D E / _

© Janice Regan, CMPT 128, Sept Expressions: arithmetic operators  Order of operations is determined by operator precedence rules unary -, before /, before +  -A + B / C X + B / C X + Y  Let A=10, B=5, C=2 + X B Y C / A B C / + - A 10 -

© Janice Regan, CMPT 128, Sept Expressions: arithmetic operators  ++A – B pre increment  --B + C pre decrement  Pre-increment: increment variable then use in expression  A * C++ post increment  A / C-- post decrement  Post increment: use variable in expression then increment the variable

© Janice Regan, CMPT 128, Sept  Order of operations is determined by operator precedence rules unary ++ and --, before /, before +  ++A + B / --C ++A + B / C (C=1) A + B / C (A=11) A + Z  Let A=10, B=5, C=2 Expressions: arithmetic operators + A B C C / A B C / + ++ A Z 5

© Janice Regan, CMPT 128, Sept  Order of operations is determined by operator precedence rules unary ++ and --, before /, before +  A-- + B / --C A-- + B / C (C=1) A-- + Z A (A=9)  Let A=10, B=5, C=2 Expressions: arithmetic operators + B C C / A B C / + -- A Z 5 A A

© Janice Regan, CMPT 128, Sept  Q = (++A + B / --C);  This is a shorthand way of writing C = C – 1; A = A + 1; Q = (A + B / C); Why use increment / decrement

© Janice Regan, CMPT 128, Sept  Q = (A-- + B / --C);  This is a shorthand way of writing C = C – 1; Q = (A + B / C); A = A - 1; Why use increment / decrement

© Janice Regan, CMPT 128, Sept Assignment Statements  Basic statement used to perform calculations  Form: result = expression;  Example: A = B + C * D;  NOT the same as an = in an equation  Each variable is associated with a location in memory  Evaluate the expression on the right (B+C*D) Multiply the value in the memory location associated with variable C by the value in the memory location associated with variable D Add the product to the value of the in the memory location associated with variable B The sum is the value of the expression  The value of the expression on the right hand side is placed in the memory location associated with variable A

© Janice Regan, CMPT 128, Sept Expressions: arithmetic operators  Order of operations is determined by operator precedence rules * before + before =  A = B + C * D A = B + X A = Y  A=10, B=5, C=2, D=12 + X C B D * A B C * + = D Y A =

© Janice Regan, CMPT 128, Sept Assignment Statements:  Form: result = expression;  Example: X = X * Y;  Each variable is associated with a location in memory  Evaluate the expression on the left (X*Y) Multiply the value in the memory location associated with variable X by the value in the memory location associated with variable Y The product is the value of the expression  The product is placed in the memory location associated with variable X overwriting the previous value of X

© Janice Regan, CMPT 128, Sept Assignment operators  A = B assign value of expression B to variable A, store value of expression B in A  A += B add the value of expression B to the value of variable A, store result in A  A -= B subtract the value of expression B from the value of variable A, store result in A  A *= B multiply the value of variable A by the value of expression B, store result in A  A /= B divide the value of variable A by the value of expression B, store result in A