CS100A, Fall 1998

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CS100A, Fall 1998

• After discussing the notion of an algorithm and
  (computer) program briefly, the first lecture
  will introduce method calls, since method calls
  are mandatory when doing any I/O or graphics and
  since they are one of the most frequently used
  statements.
• First, we will discuss the heading of a method
  --the precise comment that specifies what the
  method does, prefix modifiers, name, and
  parameters. Throughout, the need for a precise
  specification will be emphasized, for without a
  precise specification, one cant easily tell what
  the method does. Then, we will discuss method
  calls (invocations), using a substitution
  semantics the method call does what the
  specification of the method says, but with
  parameters replaced by arguments of the call.
• We will discuss the fact that THEY (the students)
  could execute something like drawOval(5,5,4,3),
  if given the right color crayon and the graph
  paper on which to draw it. Something like
  drawOval(5,5,4,3) is simply shorthand for the
  comment of the method, with the parameters
  replaced by the arguments.
• We will illustrate this on the computer, using a
  small program that draws circles/squares and
  titles for them in a window. We will also step
  through execution of the program using the
  debugger, so they get the idea of sequential
  execution.
• A nice feature of using the debugger for this is
  that we can show what happens when the Drawing
  window is first covered by another window and
  then uncovered by moving the other window away
  the Drawing window is now all messed up --but the
  system calls method paint in order to redraw that
  window! Thus, the students get a bit of a feel
  for what is going on behind the scenes when they
  move windows around.
• Necessarily, we will also mention integer
  arithmetic, but weleave a full explanation of
  integer expressions to the first section.
• We will explain carefully that many parts of a
  Java program will not be understandable at this
  point --prefix modifiers, classes, the g. on
  g.drawOval(), and so forth. These will all
  become clearer as the course progresses.

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CS100A, Fall 1998

• Goal learn to write intelligible and correct
  computer programs effectively and efficiently.
• Main Language Java (object oriented, cleaner and
  easier to use than C).
• Software Metrowerks CodeWarrior, on the
  Macintosh or the PC.
• Instructors Gries Cardie

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• Main Concepts today
• Algorithm, program.
• Method and method call.
• Graphics.
• Execution of a program.
• Executing using the debugger
• Reading
• Holmes 2, 12-19, 25, 28-30
• Handout for assignment 0, Sect. 2-4
• First assignment due Tuesday, 8 Sept. at
  beginning of lecture!!!

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• Algorithm A set of instructions for performing
  some task. An algorithm is
• precise
• unambiguous
• effective (always works)
• abstract
• Examples
• A recipe for cooking soup.
• Heres a list of integers tell me whether each
  one is even or odd.
• Play a game of tic-tac-toe in a way that you
  never lose.

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• Program An algorithm that is written in some
  language English, Sanskrit, C, Java, FORTRAN,
  Pascal.
• Computer program a program in a computer
  language, e.g. Java.

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• An algorithm is usually given as a sequence of
  instructions to execute (in the order given).
• In a programming language like Java, each of
  these instructions is called a statement.
• One of the most frequent kinds of statement used
  in Java is the call of a method call, or method
  invocation.
• To understand a method call, we need to know a
  bit about what a method is.
• Terms sometimes used interchangeably
• method procedure function

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• // Draw a line from point (x1,y1) to
• // point (x2,y2)
• public void drawLine
• (int x1, int y1, int x2, int y2)
• Heading of a method
• Comment explains precisely what the method is to
  do. Written as a command to do it.
• Some prefix modifiers (public and void)
  --explained later
• The name of the method drawLine.
• The parameters x1, y1, x2, y2 (along with their
  types (int)).

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• HEADING
• // Draw a line from point (x1,y1) to
• // point (x2,y2)
• public void drawLine
• (int x1, int y1, int x2, int y2)
• CALL
• drawLine(3, 5, 1, 1)
• COMMAND TO BE EXECUTED
• Draw a line from point (3, 5) to
• point (1, 1).

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• HEADING
• // Print the largest of x, y, and z
• public void printMax
• (int x, int y, int z)
• CALL
• printMax(20, 5, 20-54)
• COMMAND TO BE EXECUTED
• Print the largest of 20, 5, and 20-54.
• Note the method name, printMax, has no meaning
  for Java. We could as well have used the name
  aaa, or w.

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• HEADING
• // In the output window, skip a line and
• // print Hello.
• public void printHello( )
• CALL
• printHello( )
• COMMAND TO BE EXECUTED
• In the output window, skip a line and
• print Hello.

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• Important points
• 1. A method can have zero parameters.
• 2. Syntax of a method call
• a name
• (
• list of zero or more arguments (names),
  separated by ,
• )
• 3. To find out what a method call does, make a
  copy of the specification of the method (the
  comment in the heading) and replace all
  occurrences of the parameters by the arguments of
  the call.

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2. For this process to be effective, the
specification (the comment) must be precise
understandable correct (it must say
exactly what the method actually
does Rest of lecture Demonstrate calls with a
small Java program, using both run and debug.