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Variable Initialization
- A variable has no meaningful value unless assigned #include <iostream.h> int main() { int i; int j; int b; j = i + 1; // <- problem cout << b; // <- problem return 0; }
- Rule: Set each variable before its value is used! GARBAGE, otherwise! (The container has no predictable value)
- One way of avoiding uninitialized variables: initialize at the time of declaration int your_sum = 20; double rate = 0.1, balance = 0.00;
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Output
cout << "Hello, World" ; cout << "Enter the Number:\n" ;
Syntax cout << exp 1 << exp 2 << ... << exp (^) n;
Note
- << is the insertion operator
Semantics
- Expressions exp 1 through expn are displayed to screen Expressions are normally variables and literals Newline
- Just another character Symbolic representation: \n Alternative version endl (p. 49)
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Input
Syntax cin >> var 1 >> var 2 >> ... >> var (^) n ;
Semantics
- Variables var 1 through var (^) n are assigned the n values from standard input var 1 first
- Input values are separated by whitespace (space, tab, CR)
Examples cin >> fahrenheit; cin >> x_coordinate >> y_coordinate; BETTER: Issue a prompt for input. cout << "Current temperature? "; cin >> fahrenheit;
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Types
Literals and variables have types
- Literal: compiler can figure out type e.g. "This is a string" is a string e.g. 243 ----- compiler knows it is an int literal
- Variable: must tell compiler type Done through declaration Helps compiler determine efficient storage Helps ensure type safety (advanced topic)
- Remember, literals can be numeric, character or string
- Variables can be any basic or user-defined type
Basic types of variables:
- character ( char )
- numeric (See table in textbook p. 57 [ int range is wrong!!]) focus on int and double for now
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Numeric Types
5, -3.14159, 3.125267E+3, 2147483648
Why so many?
- e.g. int, long int, float, double, long double
- efficiency vs convenience
- int ’s much more efficient than long ints
- but, long ints can hold bigger integer values
- similar for float , double , long double
What to do?
- learn about int and double for now
- but recognize all of the numeric types in text (p. 57)
Beware!
- Don’t count on range specified in book. Why?
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Numerical Literals: Rules
Common sense, for the most part
- If you've used a calculator, you know most of the rules
Integers ( int , long , ..)
- Use only digits 0..9 (and + or - or nothing) 31 -1729 32767
- Beware of size limitations --- in Borland C++ the values which can be represented in a variable of type int (16 bits) range from -32768 to +32767 (again: error in Savitch, p. 57)
Reals ( float , double ...)
- Digits and embedded decimal point (and sign)
- 9.323 0.179 -2.0 91700.
- Scientific notation
- 9.17E+
- Beware the common comma and $ errors
- Don’t use "," or "$" in numeric literals
- Data should be, e.g. 1729.56, not $1,729.
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Characters and Strings
Remember:
- Character literals: ’A’ ’5’ ’?’ ’\n’
- Character variables: char first; char last; first = ’P’; last = ’W’;
- String literals: "Balance due: " "1011 High Street"
- String variables? — No
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Type Compatibilities
General rule: you cannot store a value of one type in a variable of a different type --- type mismatch - convert if possible int distance, rate; distance = 33.9; - stores 33 rate = 2.0; - stores 2
- assigning int value to double variable double growth_rate; growth_rate = 2; growth_rate will be assigned 2.
Textbook error: bottom of page 59 - what he says is wrong. One can always assign a real value to an integer variable - it will always be truncated
- Need to be careful if you mix types and values on assignment statements or arithmetic expressions
- especially when performing arithmetic operations
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Division
Need to recognize the very distinct difference between division of two real values and the division of two integer values.
Remember the rule: in A op B if any of the operands are real, the result is real. Putting this another way, the integer operand value is first converted to an equivalent real value, and then the opera- tion (+, -, *, or /) is performed.
so, if we have 9.0/2.0, we obviously get 4.50000 as the result
if we have 9.0/2, then 2 is converted to 2.0000, gives 4.
however, if we have: 9/2, then neither operand is real, so we have to do INTEGER DIVISION --- result is 4 (with a remainder of 1)
The operator ’%’ (called "modulus") yields the remainder after division:
9/4 has value 2 (with remainder of 1) 9%4 has value 1 (the remainder after 9/4)
10/4 has value 2 10%4 has value 2
11/4 has value 2 11%4 has value 3 Trick question: value of val*(1/4)?
-11%4 has value -3.........
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Formatting Numbers for Output
double price; price = 78.5; cout << "The price is $" ; cout << price << endl;
Output The price is $78. The price is $78.500000 WHICH????? The price is $7.8500000e
Output we want The price is $78.
- Need to tell the computer how to format the numbers
Magic formula (textbook p.51) cout.setf(ios::fixed); cout.setf(ios::showpoint); cout.precision(2); Output The price is $78.
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Arithmetic Operators
Common
- Addition +
- Subtraction -
- Multiplication *
- Division / — overloaded
- Modulo %
Note
- No exponentiation operator
- Single division operator
Precedence Rules
- Determines the order in which operators are performed
- Similar to the ones used in mathemathics and algebra
- A complete list is in Appendix 2 of the textbook (p. 771)
- Order can be changed by inserting parentheses () (x + y) * z
