CISC181 Introduction to
Computer Science
Dr. McCoy
Lecture 3 (2) & 4
September 8 & 10, 2009
1
Programming Gets Tougher
•
•
•
Need rules for thinking about more
difficult programming problems
Take your time – think first.
Make sure you understand what it is you
are trying to do before you try to do it.
2
Rules
1. Think before you code
– Use some abstract short-hand design
•
•
Flowcharts/activity diagrams
Pseudocode – informal language for writing
“algorithms”
– Set of actions to be executed
– Specified order for the actions
3
More Rules for Thinking
2. Know the tools available to you
– Control structures of the language
4
5
2.4
Control Structures
• Sequential execution
– Statements executed in order
• Transfer of control
– Next statement executed not next one in sequence
• 3 control structures (Bohm and Jacopini)
– Sequence structure
• Programs executed sequentially by default
– Selection structures
• if, if/else, switch
– Repetition structures
• while, do/while, for
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6
2.4
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Control Structures
7
2.5
if Selection Structure
• Flowchart of pseudocode statement
A decision can be made on
any expression.
zero - false
nonzero - true
Example:
3 - 4 is true
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8
2.6
if/else Selection Structure
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9
2.7
The while Repetition Structure
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Control Structures and
Programming
• Each C++ Program made up of these 7
control structures combined appropriately
(turns out that we can make the last of
these more specific, but we’ll see that
later)
– Sequentially
– Nested
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11
2.5
if Selection Structure
• Selection structure
– Choose among alternative courses of action
– Pseudocode example:
If student’s grade is greater than or equal to 60
Print “Passed”
– If the condition is true
• Print statement executed, program continues to next statement
– If the condition is false
• Print statement ignored, program continues
– Indenting makes programs easier to read
• C++ ignores whitespace characters (tabs, spaces, etc.)
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12
2.5
if Selection Structure
• Flowchart of pseudocode statement
A decision can be made on
any expression.
zero - false
nonzero - true
Example:
3 - 4 is true
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13
2.5
if Selection Structure
• Translation into C++
If student’s grade is greater than or equal to 60
Print “Passed”
if ( grade >= 60 )
cout << "Passed";
• Diamond symbol (decision symbol)
– Indicates decision is to be made
– Contains an expression that can be true or false
• Test condition, follow path
• if structure
– Single-entry/single-exit
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14
2.6
if/else Selection Structure
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15
2.6
if/else Selection Structure
• if
– Performs action if condition true
• if/else
– Different actions if conditions true or false
• Pseudocode
if student’s grade is greater than or equal to 60
print “Passed”
else
print “Failed”
• C++ code
if ( grade >= 60 )
cout << "Passed";
else
cout << "Failed";
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if-else Statement Syntax
• Formal syntax:
if (<boolean_expression>)
<yes_statement>
else
<no_statement>
• Note each alternative is only
ONE statement!
• To have multiple statements execute in
either branch  use compound statement
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2-16
Branching Mechanisms
• if-else statements
– Choice of two alternate statements based
on condition expression
– Example:
if (hrs > 40)
grossPay = rate*40 + 1.5*rate*(hrs-40);
else
grossPay = rate*hrs;
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2-17
if-else Statement Syntax
• Formal syntax:
if (<boolean_expression>)
<yes_statement>
else
<no_statement>
• Note each alternative is only
ONE statement!
• To have multiple statements execute in
either branch  use compound statement
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2-18
Compound/Block Statement
• Only "get" one statement per branch
• Must use compound statement { }
for multiples
– Also called a "block" stmt
• Each block should have block statement
– Even if just one statement
– Enhances readability
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2-19
Compound Statement in Action
• Note indenting in this example:
if (myScore > yourScore)
{
cout << "I win!\n";
wager = wager + 100;
}
else
{
cout << "I wish these were golf scores.\n";
wager = 0;
}
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2-20
Common Pitfalls
• Operator "=" vs. operator "=="
• One means "assignment" (=)
• One means "equality" (==)
– VERY different in C++!
– Example:
if (x = 12) Note operator used!
Do_Something
else
Do_Something_Else
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2-21
The Optional else
• else clause is optional
– If, in the false branch (else), you want "nothing" to happen,
leave it out
– Example:
if (sales >= minimum)
salary = salary + bonus;
cout << "Salary = %" << salary;
– Note: nothing to do for false condition, so there is no else
clause!
– Execution continues with cout statement
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2-22
Nested Statements
• if-else statements contain smaller statements
– Compound or simple statements (we’ve seen)
– Can also contain any statement at all, including another ifelse stmt!
– Example:
if (speed > 55)
if (speed > 80)
cout << "You’re really speeding!";
else
cout << "You’re speeding.";
• Note proper indenting!
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2-23
Multiway if-else
• Not new, just different indenting
• Avoids "excessive" indenting
– Syntax:
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2-24
Multiway if-else Example
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reserved.
2-25
26
2.6
if/else Selection Structure
• Ternary conditional operator (?:)
– Three arguments (condition, value if true, value if false)
• Code could be written:
cout << ( grade >= 60 ? “Passed” : “Failed” );
Condition
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Value if true
Value if false
What is wrong, if anything, with the
following?
int a, b, c, max, med, min;
if (a < b);
min = a;
27
What is wrong, if anything, with the
following?
int a, b, c, max, med, min;
if (a < b);
min = a;
else
min = b;
28
What is wrong, if anything, with the
following?
int a, b, c, max, med, min;
if (a < b)
min = a;
max = b;
else
min = b;
max = a;
29
What is wrong, if anything, with the
following?
Go To File ex2-26-and-more.cc
Exercise 2.26 – Dangling-Else Problem
Decide what prints
Also, another messed-up if statement!
30