CS100A, Fall 1997
This lecture covers the declaration of variables, assignment, and the if statement.
While discussing the declaration of variables, it will be important to mention that variables have meaning
and that writing down that meaning at the point of declaration is extremely important for developing correct
programs and for understanding programs.
At this point in the course, the only types the students have seen is int, float, long, and the like; they don’t
know what a class is.
We will mention the modifier final, which is used for constants.
The debugger will be used in the lecture to demonstrate execution of the assignment. For example, after
slide 5 is discussed, we’ll illustrate execution of that program within the debugger, showing carefully how
variable timeOfDay changes during execution. The debugger will be used to illustrate assignment, declaration
of a final variable (a constant), and the if statement.
CS100A, Fall 1997.
Lecture 2
1
CS100A, Fall 1997
Concepts for this lecture:
• Variables and constants
• Declaration of a variable
• Assignment to variable
• Declaration of a constant
• The conditional (if) statement
Reading:
Lewis/Loftus 75-82,
Lewis/Loftus 92-93, 97-100
CS100A, Fall 1997.
Lecture 2
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Variable: a placeholder for a
value; a box into which a value
can be placed.
Examples of declarations
int x;
int timeOfDay;
x
-49
TimeOfDay
1115
CS100A, Fall 1997.
Lecture 2
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Variable names (identifiers).
• Begin with a letter, _, or $
• Contain letters, digits, _ and $
Generally, we don’t use _ or $
Convention for variable names: first
letter small; capitalize words within
the identifier.
timeOfDay
f12
f0
restOfInput
CS100A, Fall 1997.
Lecture 2
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New statement (or command):
Assignment to a variable.
int timeOfDay = 3;
// timeOfDay
3
timeOfDay= 1200;
// timeOfDay
1200
timeOfDay= 3 + timeOfDay + 2;
// timeOfDay
1205
CS100A, Fall 1997.
Lecture 2
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Form of assignment:
<variable> = <expression> ;
Here, = does not mean equality! In Java,
Read x= y; as
“Assign y to x” or “x becomes y”
Read “x==y as “x equals y”
x==y (yields true if x and y contain the
same value and false otherwise)
CS100A, Fall 1997.
Lecture 2
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= introduced as a sign for equality by Robert
Recorde (1510-1558). He chose = because
nothing could be more equal than two parallel
lines. = for equality has been a worldwide
convention for over 100 years.
The use of = for assignment was started in
FORTRAN in the early 1950s. C (about 1970)
and then C++ (1980s) continued to use = for
assignment and introduced == for equality;
thus bringing great confusion.
This use of = for assignment and requirement
of == for equality has caused more confusion
and waste of time in the whole world than any
other notational convention.
Algol 60 used := for assignment, = for equality
CS100A, Fall 1997.
Lecture 2
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• Short names make program shorter,
perhaps more manageable.
• Long names can be used to convey
something about the meaning of a
variable.
• However, a name can never (almost)
give the full, precise meaning, so it is
best to give a comment at the
declaration of a variable to give its
full meaning. You’ll see many
examples of this as the course
progresses.
CS100A, Fall 1997.
Lecture 2
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The conditional statement
(if statement). Allows a choice of
command based on some condition.
// Store the maximum of x,y in z
z= x;
if (y>x)
z:= y;
syntax: if ( <boolean expression> )
statement
CS100A, Fall 1997.
Lecture 2
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Using a block statement:
// if x != y, store 0 in x and store y in z
if (y!=x)
{x= 0; z= y;}
or
// if x != y, store 0 in x and store y in z
if (y!=x) {
x= 0;
z= y;
}
CS100A, Fall 1997.
Lecture 2
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The block statement
{<statement 0>
<statement 1>
<statement 2>
…
}
{ and } are used to delimit a sequence of
statements that are to act together as a
single statement, in the same way that
parentheses are used to aggregate in
arithmetic expressions.
CS100A, Fall 1997.
Lecture 2
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Second form of if statement
// store maximum of x and y in z
if (x > =y)
{z= x;}
else {z= y;}
The rest of the lecture will demonstrate
the use of the assignment and if
statements in Codewarrior on the a
Macintosh.
CS100A, Fall 1997.
Lecture 2
12