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CS 105 Lecture 3

Constants & Expressions

Wed, Jan 26, 2011, 4:15 pm

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• Write programs in source code; preprocess, compile, and link them to get executable file we can run.

• Syntactic errors appear at compile-time; semantic errors (bugs) appear at run-time.

• Use test plan to to find bugs.

Review of Last Week

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• Identifiers are names used in programs• E.g., main, number, length

• Keywords can't be used as identifiers• E.g., int, float, double

• A variable is an identifier that holds a value that can change. We initialize a variable when creating it; we can use an assignment to change its values.

Review of Last Week

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• Can use letters: Remember that C++ is case sensitive (e.g., NumWidgets is not the same as numwidgets)

• Can use digits 0-9, and underscore• Cannot start with a digit• Cannot contain spaces or other

characters• Typical maximum of 32 characters• Cannot use C++ keywords

Names for Variables

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• Should use a meaningful, descriptive name so that variable’s use is easily understood: • Examples: counter, second,

minute, length, width • Be consistent with case; usually

lower case with upper case for second part of variable name. Examples: averageRainfall, totalStudentGrades, maxBuildingHeight, minPackageWeight;

Naming Variables (Cont)

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• Scope: Area of a program within which a variable can be referenced

• Variable definitions are recognized in the curly braces in which they were defined

• "Global" Variables declared outside of functions are recognized from the point declaration through the rest of the program

Variable Scope

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• Write a program that declares 2 integer variables, 1 double, 1 character, and 3 floats.

• 5 minutes: Go!

Try This!

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int main(){ int buildingHeight, buildingLength; double averageHeight; char firstLetter; float avgLength, avgSalary,avgHours; return(0);}

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• Named Constant: An identifier that is initialized to a value that cannot change

• Usually declared at top of program using keyword const

• Standard naming of constants is to use all upper case letter with or without underscore between words

• All constants must be initialized• Syntax: const int MAXHT = 100;

Named Constants

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• Easier to understand

• Easier to modify

• Example: Compare using 5000 in program versus constant MAXHT

Advantages of Constants

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const int MAXHT = 100;main(){ ... currentHeight > MAXHT ... ... bridgeHeight == MAXHT ... ... bridgeHeight + newAddition >= MAXHT ... // used MAXHT 223 times in this program}

Named Constants

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• Constants whose values are already known:• Characters (specified Inside Single

Quotes): 'A' , 'a' , '5' , ' ' , '\n' (newline) , '\0' (NULL Character)

• Integers: 10, 1345, -34• Float or double: 2.3, -45.18, 10.6e6 • String (Specified Inside Double

Quotes):• "HELLO", "What a great deal.", "5"

Literal Constants (Values)

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• Also known as I/O

• Output stream: cout << variable;

• Input stream: cin >> variable;

• New line: endl

• All C++ statements end in semicolon (;)

Input and Output

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#include <iostream>using namespace std;int main(){ int numEntered; cout << "Please enter an integer:"; cin >> numEntered; cout << "Thanks, you entered: " << numEntered << "." << endl; return 0;}

Input and Output Example

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• Standard Template Library (STL): Library of C++ functions used worldwide

• string type is NOT a primitive type in C++

• string type is defined in STL

Characters and Strings

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• To Use STL Strings:#include <iostream>#include <string>using namespace std;string lastName;

Characters and Strings

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• Declaration: string yourName;

• Assigning a value to a string:

• yourName = "A. Goose";

• String constants (string values) must be enclosed in double quotes.

STL Strings

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#include <iostream>#include <string>using namespace std;int main(){ string lastName; cout << "Enter your last name: "; cin >> lastName; cout << "Your last name is: " << lastName << endl; return(0);}

Input String Example

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Expressions

• Expression: A sequence of one or more operands and operators that evaluates to a value• Operator: A symbol expressing a

way to modify a value or values (e.g., + for addition)

• Operand: A value being used by an operator

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Expressions

• Example Expressions:

• 5

• currentHeight

• currentHeight + 10

• sqrt(3.14159)

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Arithmetic Expressions

• Standard arithmetic operations can be performed: addition, subtraction, multiplication, division

• Standard arithmetic operators can be used for these operations: +, -, *, /

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Arithmetic Expressions

• Others operators include

• % - "Modulo (Mod)" – Remainder after Integer Division

• -- (two hyphens) Decrement (subtract 1)

• ++ Increment (Add 1)

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Order of Operations

• Precedence: Level of importance of operations

• Multiplicative operators have higher precedence than additive operators:

• Higher: *, /, %

• Lower: +, -

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Order of Operations

• Associativity: Order of operation for equal level precedence.

• Most operators have left-to-right associativity

• Use parentheses to force different precedence for operations

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Know for the Quiz

• All terms (underlined items)• Also add: Variables, Identifiers,

Keywords

• Variable declaration, initialization, and assignment, constant declaration

• Expressions, operators

• Input (cin) and Output (cout)

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Be Ready!

• Quiz 1 Next Week

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Today’s Challenge

• Imagine you are in a room with 3 switches. In an adjacent room there are 3 bulbs (all are off at the moment), each switch belongs to one bulb. It is impossible to see from one room to another. How can you find out which switch belongs to which bulb, if you may enter the room with the bulbs only once?