1. Bill Tucker Austin Community College COSC 1315
Chapter 4 Lecture Notes
Bill Tucker
Austin Community College
COSC 1315
Copyright © 2002 W. A. Tucker
Copyright © 2002 W. A. Tucker

2. Scope of Names All names have a scope that is either
Global – the value stored in the memory location referred to by a global name is accessible for ALL of the program
Local – the value stored in the memory location referred to by a local name is accessible ONLY for the function where the value is declared
Copyright © 2002 W. A. Tucker

3. Example of Scope #include <iostream> using namespace std;
double calcDistance (double); // prototype
const double VELOCITY = 55; // global constant
double height; // global variable
int main() {
double distance, time;
time = 10;
distance = calcDistance (time);
cout << distance << endl;
return 0; }
double calcDistance (double time)
double distance = 0;
distance = VELOCITY * time;
return distance;
global constant OK
global variable NONO
local variables
local variable
NOTE: time and distance are local
Copyright © 2002 W. A. Tucker

4. Tracing the values Values when the program is loaded
GLOBAL main calcDistance
VELOCITY distance ??? Not loaded yet
time ???t
Both main and calcDistance have access to the global constant
Copyright © 2002 W. A. Tucker

5. Tracing the values Values after execution of instruction “time = 10;”
GLOBAL main calcDistance
VELOCITY distance ??? Not loaded yet
time
Both main and calcDistance have access to the global constant
Copyright © 2002 W. A. Tucker

6. Tracing the values Values after calling calcDistance (time);
GLOBAL main calcDistance
VELOCITY distance ??? distance
time time
Both main and calcDistance have access to the global constant
Copyright © 2002 W. A. Tucker

7. Tracing the values
Values after execution of instruction in calcDistance
“distance = VELOCITY * time;”
GLOBAL main calcDistance
VELOCITY distance ??? distance
time time
Both main and calcDistance have access to the global constant
Copyright © 2002 W. A. Tucker

8. Tracing the values Values after returning from calcDistance
GLOBAL main calcDistance
VELOCITY distance no longer loaded
time
Both main and calcDistance have access to the global constant
Copyright © 2002 W. A. Tucker

9. Style to Document Functions
Since communication to and from a function is key, documentation is important
Document the “contract” between the function and the calling function
// PRE: preconditions – what is true when the function is called
// POST: postconditions – what is true when the function finishes
Copyright © 2002 W. A. Tucker

10. Example Documentation
double calcDistance (double time) {
// PRE: time contains a valid value
// POST: returns a double value equal to the product of
// time and VELOCITY
double distance = 0;
distance = VELOCITY * time;
return distance;
} // end calcDistance
NOTE: The pre-conditions are the responsibility of the calling function
The post-conditions are the responsibility of the new function
Copyright © 2002 W. A. Tucker

11. Programming Structures
Recall that we have only talked about SEQUENTIAL programming structures where each instruction is executed in sequence, each and every time the program is executed
But what if we want to execute different instructions based upon the value of some variable that may change during execution?
Copyright © 2002 W. A. Tucker

12. Selection Logic
The SELECTION programming structure is used to alter the “flow of control” of instructions on a “real time” basis while the program is executing
These decisions will be based upon the evaluation of a logical expression as either TRUE or FALSE
Logical expressions can contain RELATIONAL or EQUALITY operators
Copyright © 2002 W. A. Tucker

13. Relational Operators < Less than > Greater than
<= Less than or equal to
>= Greater than or equal to
Which ones are compliments of each other ??
Copyright © 2002 W. A. Tucker

14. Equality Operators == Equal to != Not equal to
Note these are compliments of each other
Number one C++ programming error is to confuse equality operator (==) with assignment operator (=)
Copyright © 2002 W. A. Tucker

15. Logical Expressions
The data type of values that are compared should be the SAME or the compiler will make assumptions that you may not like!
What is the value of the following logical expressions?
Given:
a = 24, b = 36, a and b are integers
Expressions:
a > b a == b
a < b a != b
a > = b
a < = b
Copyright © 2002 W. A. Tucker

16. Logical Operators
There are three logical operators in C++ that are used to combine or modify logical expressions
AND &&
OR ||
NOT !
Copyright © 2002 W. A. Tucker

17. AND Logical Operator expression1 && expression2
both expressions must be true for the combined expression to be true
expression1 expression2 result
true true true
true false false
false true false
false false false
Copyright © 2002 W. A. Tucker

18. OR Logical Operator expression1 || expression2
either expression must be true for the combined expression to be true
expression1 expression2 result
true true true
true false true
false true true
false false false
Copyright © 2002 W. A. Tucker

19. ! (NOT) Operator ! expression
after evaluating the “expression” the value of true or false is FLIPPED
! (a == b) is the same as (a != b)
! (condition1 && condition2) is the same as
(condition1 || condition2)
Copyright © 2002 W. A. Tucker

20. Logical Expression Examples
What is the value (result) of the following logical expressions?
Given: letter1 = ‘A’ letter2 = ‘a’ string1 = “Testing” string2 = “TESTING”
value1 = value2 =
bool raining = true
letter1 == letter letter1 > letter2
string1 == string2 string1 > string2
value1 == value2 value1 > value2
raining ! raining
Copyright © 2002 W. A. Tucker

21. More Examples
What is the value (result) of the following logical expressions?
Given: letter1 = ‘A’ letter2 = ‘a’ string1 = “Testing” string2 = “TESTING”
value1 = value2 =
bool raining = true
(letter1 < letter2) && raining
(string1 == string2) || ( value1 < value2)
((value1 == value2) || raining) && (value1 > value2)
Copyright © 2002 W. A. Tucker

22. Selection Structure
One of two possible alternatives is taken, depending upon a condition
Condition?
False
True
Alternative A
Alternative B
Copyright © 2002 W. A. Tucker

23. Selection Structure
A selection structure may have only a single alternative - the true alternative
Condition?
True
Alternative
False
Copyright © 2002 W. A. Tucker

24. if / else Statement
The two alternative selection in C++ is coded by using an if / else statement
false
true
if (a > b)
largest = a;
else
largest = b;
a > b ?
largest
= b
largest
= a
NOTE: if and else are reserved words
Copyright © 2002 W. A. Tucker

25. if Statement
A single alternative selection is coded in C++ using the if statement
largest = b
largest = b;
if (a > b)
largest = a;
a > b ?
True
largest
= a
False
NOTE:
if is a reserved word
Copyright © 2002 W. A. Tucker

26. More on the if statement
The if statement has two generic forms
if (condition)
a single statement or block of code;
else
Copyright © 2002 W. A. Tucker

27. Block of code
A “block of code” is like a paragraph and is written inside a set of curly braces { }
if (a > b) {
largest = a;
smallest = b;
} // end if
else
largest = b;
smallest = a;
} // end else
Copyright © 2002 W. A. Tucker

28. Nested if statements
More than two alternatives may be coded by using “nested if statements”
true
Display
“a is largest”
a > b?
if (a > b)
cout << “a is largest”;
else
if (a < b)
cout << “b is largest”;
cout << “a = b”;
false
true
false
a < b?
Display
“b is largest”
Display
“a = b”
Copyright © 2002 W. A. Tucker

29. Nested if statements
More than two alternatives may be coded by using “nested if statements”
false
true
a >= b?
if (a >= b)
if (a > b)
cout << “a is largest”;
else
cout << “a = b”;
cout << “b is largest”;
Display
“b is largest”
false
true
a > b?
Display
“a = b”
Display
“a is largest”
Copyright © 2002 W. A. Tucker

30. Common Errors An else always refers to the last if statement
What is the output of the following code?
(Try temp = 40, 50 and 60)
if (temp < 60)
if (temp <= 40) cout << “wear a jacket”; else cout << “no jacket required”;
Does this make sense? What is wrong?
Copyright © 2002 W. A. Tucker

31. Common Errors
The following program fragment is supposed to print out the values 25, 60 and 8 in that order. Instead, it prints out 50, 60 and 4, why?
length = 25;
width = 60;
if (length = 50)
height = 4;
else
height = 8;
cout << length << ‘ ‘ << width << ‘ ‘ << height
<< endl;
Copyright © 2002 W. A. Tucker

32. In Class Exercise
Write a function that will return a letter grade given an integer grade as input
char convertGrade (int grade) {
//PRE: grade is a positive value <= 100
//POST: returns a letter grade as follows:
// – 100 = A; 80 – 89 = B
// – 89 = C; 60 – 69 = D: <59 = F
char letter;
// PUT SELECTION LOGIC HERE
return letter;
} // convertGrade
Copyright © 2002 W. A. Tucker

33. Common Algorithms Minimum (smallest) Maximum (largest) Swap Odd Even
Average (will cover later)
Copyright © 2002 W. A. Tucker