cs201/05 -- if-then-else etc

Previous -- objects

04.html

Study Pages 99-115 if--then--else Selections

3.1 The if Statement

Syntax 3.1 if statement

if_statement::= "if" "(" condition ")" statement O( "else" statement ), an if_statement starts with an "if" and a parenthesis, then a condition and a close parenthesis and a statement, if may end with an optional "else" and another statement.
```
 		if(x<0) x= -x;
```
Semantics
```
 		if(A) B else C
```
A is evaluated and if true then B is executed but if A was false then C is executed.
```
 		if(A) B
```
A is evaluated and if true B is executed otherwise nothing is done.
Syntax 3.2 Block Statement
A vital tool -- combines a sequence of statements into a single statement. Allows an "if" to select a complex piece of a program.
block::= "{" # statement "}", A block starts with an open brace and ends with a matched close brace and contains a sequence of C++ statements.
Most "if"s and "else"s have braces after them.
Quality Tip 3.1 -- Layout braces correctly
Each open brace should be vertically above it's closing brace.
Productivity Hint 3.1 Tabs
Experiments indicate you should indent the statements in a block 3 spaces
Advanced topic 3.1 -- The selection operator
Later
3.2 Relational operators
Take note of "<=", ">=", "==", "!=".
Common Error 3.1 -- Confusing = with ==

Quality tip 3.2 -- Zero Warnings

Common Error 3.2 -- Comparing Floating point numbers
Never use "==" with real numbers.
3.3 Multiple Alternatives

Advanced topic 3.2 -- the switch statement
Later
Common Error 3.3 -- Dangling Else

3.4 Nested branches
Take great care and use indenting!
Testing for failed input
Notice that if cin.fail() is true that any data that was input is suspect -- and will probably be garbage. So never write code like this:
```
 	cin >> a;
```
```
 	cout << sqrt(a);
```
```
 	if(cin.fail()) cout << "input error";
```
Instead, something like this is more likely to work.
```
 	cin >> a;
```
```
 	if(cin.fail()) cout << "input error";
```
```
 	else cout << sqrt(a);
```

Review Questions

Review Exercise -- Syntax

variable = expression;
What is the syntax of an if-else statement?
What is the syntax of an if statement with no else?
What is the syntax of a block?
Simple Exercise -- common error
What is wrong with [ 05ex4.cpp ]
Tricky Exercise -- Leap years
What is wrong with [ 05ex5.cpp ]
Exercise -- solving a quadratic equation
[ 05ex3.cpp ]
Exercise on if-else-if-else
[ 05ex0.cpp ]
Exercise on if-if-if
[ 05ex1.cpp ]
Note -- how to diagram complex logic
[ 05Flow.gif ]
-- an Activity Diagram of the following code
```
 	if(A) B else C
```
```
 	if(D) E else F
```
Project 2 Due next time
[ projects.html#P2 ]
Quiz 2 next time on sequences of assignments
1. Cheat sheet OK.
2. Closed book.
3. Calculator OK.
4. No electronic communication.
5. There will be no if's in this quiz.
6. Follow the instructions: find the mistakes and correct them first.
7. Walk through/trace the corrected program, one assignment statement at a time.
8. Record the values of the variables, after each line is executed, in the given table. [ 03.html#Sequences of Assignments ]
9. Demo Generating the start of the Fibbonaci series [ 05ex.cpp ]
. . . . . . . . . ( end of section Quiz 2 next time on sequences of assignments) <<Contents | End>>
Next -- Boolean Operations
[ 06.html ]

O::="Optional".
Abbreviations
Algorithm::=A precise description of a series of steps to attain a goal, [ Algorithm ] (Wikipedia).
Class::=A description of a type of object that includes the data it knows and the functions it can execute.
Function::programming=A selfcontained and named piece of program that knows how to do something.
Gnu::="Gnu's Not Unix", a long running open source project that supplies a very popular and free C++ compiler.
OOP::="Object-Oriented Programming", Current paradigm for programming.
Semantics::=Rules determining the meaning of correct statements in a language.
SP::="Structured Programming", a previous paradigm for programming.
Syntax::=The rules determining the correctness and structure of statements in a language, grammar.
Q::software="A program I wrote to make software easier to develop",
TBA::="To Be Announced", something I should do.
TBD::="To Be Done", something you have to do.
UML::="Unified Modeling Language", industry standard design and documentation diagrams.
void::C++Keyword="Indicates a function that has no return value".

( End of document ) <<Contents | Top