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CS149D Elements of Computer Science. Ayman Abdel-Hamid Department of Computer Science Old Dominion University Lecture 16: 10/24/2002. Outline. Variables and Data types Assignment statement Arithmetic operators Increment/Decrement operators. Variables Names.
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CS149D Elements of Computer Science Ayman Abdel-Hamid Department of Computer Science Old Dominion University Lecture 16: 10/24/2002 CS149D Fall 2002
Outline • Variables and Data types • Assignment statement • Arithmetic operators • Increment/Decrement operators CS149D Fall 2002
Variables Names • Program variables correspond to memory spaces reserved for storage • A variable name is called an identifier • An identifier in C++ can be up to 255 characters long • Can not begin with a digit (Invalid: 1First) • Can not contain blanks (Invalid: num elements) • Can not contain a hyphen, underscore is OK (Invalid: num-elements) • Special symbols are not allowed (Invalid: cost$, cost!) • Reserved words can not be used as identifiers (Invalid: int, const, float) • A reserved word is a word that has a special meaning in C++. It can not be used as a programmer-defined identifier • C++ is case sensitive, so count is different than Count • To declare a variable, need to identify its data type CS149D Fall 2002
Data Types • C++ Data types (built-in data types) • Integers • Floating-point numbers • Characters • And more • The data type determines how data is represented in the computer and the kind of processing that the computer can perform on it • The number of bytes that a data type occupies in memory is system dependent CS149D Fall 2002
Variable Declaration • DataType identifier; • int x; • DataType identifier, identifier, …; • int x,y,z; • The initial value stored in a variable is not know unless you initialize it with the declaration • int x = 10; • Can mix declaration with declaration/initialization • int x=10, y, z = 5; • Use const keyword to indicate that the value of this variable can not change • const float PI = 3.141593f; CS149D Fall 2002
Numeric Data Types • Integer numbers • short, int, long Can use unsigned keyword (unsigned short) • Numbers with fractions • float, double, long double (double is “double precision”) • Can use scientific notation float x = 5.0e6; which means x 5 * 106 • const float x = 2.3; //x is considered a double constant • const float x = 2.3f; //or const float x = 2.3F; • Number of bytes and range of values is system dependent. In Microsoft VC++ The textbook lists 2 bytes for int on a Borland Turbo C++ 3.0 compiler See Table 2.2 on page 29 CS149D Fall 2002
Scientific Notation • Rewrite the floating-point number as a mantissa times a power of 10 • Mantissa: absolute value greater than or equal to 1.0 and less than 10.0 • 25.6 2.56 * 101 • Letter e is used to separate mantissa from exponent • 25.6 2.56e1 • Precision: number of digits allowed for decimal portion of mantissa • Exponent range: number of digits allowed for for exponent • Float double long double • Precision 6 15 19 see Table 2.2 CS149D Fall 2002
Alphanumeric Data Types • Single alphanumeric character • char x = `a`;char y = `1`; is different than int y =1; • Most programming languages use ASCII to represent the English alphabet and other symbols (1 byte/character) • Sequence of characters • string name = “CS149 Lecture”; CS149D Fall 2002
Assignment statement • Identifier = expression • Where expression can be a constant, another variable, or the result of an operation • int x, y; • x = 10; // x 10, x is assigned the value of 10 • y = x; • x = x –10; • Multiple assignments x = y = z = 0; • Look out for the data types of both sides of the assignment • int a; • a = 12.8; // actually a is assigned 12 • Numeric conversion with possibility of information loss CS149D Fall 2002
Arithmetic Operators1/3 • Operators • Unary operator: One operand • Binary operator: two operands • + Unary Plus + Addition • - Unary minus - Subtraction • * Multiplication • / floating-point division or integer division (no fractional part) • % modulus operator (remainder of integer division) • Examples • Area_triangle = 0.5 *base * height; • Y = -x; CS149D Fall 2002
Arithmetic Operators2/3 • Examples of integer division and modulus • 7/2 = 3 7%2 = 1 • 2/7 = 0 2%7 = 2 • 7.0/2.0 = 3.5 5 % 2.3 = Error (both operands must be integers) • Note that the result of integer division is a truncated result, not a rounded result. 5/3 = 1 5.0/3.0 = 1.66666 • Operation between different types is a mixed operation • Value with lower type is converted or promoted to the higher type • int sum = 18, count = 5; • float average; • average = sum/count; // average assigned 3.0 and not 3.6 (Why?) • The correct way to compute average would be • average = (float) sum / (float) count; // explicit type casting CS149D Fall 2002
Arithmetic Operators3/3 Practice problems page 34 int a = 27, b= 6; float c; c = a/(float)b; int b = 6; float a, c = 18.6; a = (int) c/b; CS149D Fall 2002
Increment and Decrement Operators • Unary operators for incrementing and decrementing variables ++, -- • Can be used in a prefix (++count) or postfix (count++) position • int x; • x++; is equivalent to x = x +1; is equivalent to ++x; • How about • x = y++ * 3; • x = ++y *3; //is the value assigned to x the same in both cases: NO • Will see what is the difference next lecture when we talk about operator precedence and compound arithmetic expressions CS149D Fall 2002
