Chapter 2: pages 42-45 if statements
- Sequence
{steps in sequence
}
- Selection
- Two way Selections -- pick one of two branches
if ( condition )
{Steps executed if condition was true
}
else
}
Steps executed if condition was false
}
- One way Selections -- select of skip
if ( condition )
{Steps executed if condition was true
}
- Multi-way selections -- Below
- Two way Selections -- pick one of two branches
- Iteration -- Later
- ** page 42: a first example of a one way selection
- **** Box at top of page 43: Live by this advice. You won't regret it.
- ** page 43 -- you can omit the braces round a single statement.
- **** example of an if-else (two way selection).
- **** Box at top of page 44: Syntax of if statements
- +++ Keep It Simple -- always put in the brace!
- **** Page 44 -- the if-else-if-else structure
- +++ How to program a multiple choice situations:
if(condition1)
case1
else if(condition2)
case2
else if(condition3)
case3
else
case4
- ** Top of page 45 explains that if-else-if-else... is just putting and
if-else in the else of an if-else.
++ You can also put an if inside an if -- as deep as you need to express the logic of you solution.
+++ Braces are good for keeping track of complex nested ifs. - +++ Comments also help:
if(A)
{if(B)
{// A and B...
}
else
{// A and not B...
}//if(B)
}
else // not A
{// not Aif(B)
{// not A and B...
}
else
{// not A and not B...
}//if(B)
}//if(A)
Trap -- == vs =
Do not writeif( i = 1 )
when you want to test for equality. The above compiles and runs but (1) the condition is always true and (2) it changes the variable i to be 1! Instead writeif( i == 1 )
Questions on ifs
- Can make this shorter by starting off with an example.
- Example: If you spend more then $100, bought exactly 5 items, and it was
- on the weekend. Only then would you win the super prize.
- Would a nested If inside a nested If be the only way to chain these 3 If's
- together? Or is it possible to somehow link them together in the first If
- statement like you can with AND in Visual Basic?
- ammount > 100 and number == 5 and (day == SATURDAY or day==SUNDAY).
This is what the next readings and class will be about: [ 05.html ] Logic.
What is a literal
This is a value like 123, 1.23, or "Hello, World". It is a symbolic constant that represents itself. Numerical literals are normally typed using decimal notation (but C++ also does octal and hex if you need it). Strings like "Hello, World" as (currently) stored as a sequence of ASCII characters.When do I use if statements
Any time that you need to make a program choose between two (or more) alternative sequences of actions.You should expect to be using if in every program you write.
Can you use if statements for things other than sales
YES. We discussed calculations of date, pressures in veessels, tax rules, .... etc etc .The example discounts in the book are wrong
Quite possibly.Is there a limit on the number of if statements in a program
No. You can use as many as you need to make it work the way it should work.Is C++ used in Graphics, Games, Special Effects, Movies
Yes.However in Movies the C++ will be hidden inside an interface for movie makers to use.
Do you terminate an if with something like an ENDIF
No. An if-else ends at the end of the statement after the else ... which may be a sequence inside {braces}.This is historical and saves typing.
What does -= mean on page 45
This is the "subtract from" operator. It one of a series of operators:Short form Long form x -= e; x = x - e; x += e; x = x + e; x *= e; x = x * e; x /= e; x = x / e; etc. What syntax is used to construct if-statements
See above. WHat separates two independent if statements Nothing. For exampleif(A)
B
else
C
if(D)
E
else
F
If you have a } do you need a semicolon
Semicolons (;) in C++ are used to terminate expressions:x = e;
cin >> x...;
cout << e ....;
So sometimes that will be appear before a brace, but not always. Statements can also end with braces and so you can see things like this}
}
}
Sometimes I've even done this} } }
What does nesting mean
Here is the example from classif( day == THU )
{if( time == 12 )
{class = 201;
}
else if( time == 12 )
{class = 375;
}
else
{class = 0;
}
}
Isn't it not simpler to always use braces
YES. But when i a hurray, in class, or short of space I often just forget them. Just like this: Here is the example from classif( day == THU )
if( time == 12 )
class = 201;
else if( time == 12 )
class = 375;
else
class = 0;
Can you avoid complex nestings
Sometimes by writing complex conditions. See below.What are the logical operators on page 46 do in if statements
This is subject of the next question and the next class.What is bool
See the next class.Can I chain conditions together like I do in VBasic
YES you can chain these together, probably like thisExercise -- is this year a leap year
The 2008 class developed the following program [ 04ex.cpp ] which calculate whether a year was a leap year in the Julian (pre 1582?) calendar.Next -- Comparison and Logical Operators
[ 05.html ] with project 1 (straight forward program) due and quiz 2. - Can make this shorter by starting off with an example.