4/03 lecture sketch

for next lecture
+ refamiliarize yourself with project solutions
+ we will convert some to java in lecture
+ you will convert others for P6

optional reading -- read/skim ahead of time or as we cover the material
+ JV/LL: skim 1
+ JV/LL: read 2, 3.1, 3.2, 3.4, 3.5, 3.6, 3.9
+ PL/GG: strongly recommended  -- see annotated table of contents

topics: matlab versus java
+ $;$ is statement terminator
+ $//$ and $/*$-$*/$ for comments
+ blocks, conditionals, loops
+ variable declaration
+ scalar datatypes and values
+ brief intro/motivation to references (more next time)
_______________________________________________________________________________

java differences from matlab
+ display/print not as convenient: $;$ is statement terminator
+ compiled, not interactive
+ $//$ and $/*$-$*/$ for comments
+ $%$ for remainder
+ control flow (conditionals, loops): 
  + condition must be parenthesized
  + bodies are *statement blocks*: sequence of code within braces ${$ and $}$
  + no $elseif$ -- use $else if$
  + $for$ is rather different -- seen next week or later
+ more types and more type-checking, e.g.
  + $int$ and $double$ are different types -- careful about mixing them
  + loop guard must be $boolean$
+ explicit variable declaration, which can be combined with initialization:

                    initialization
            ___________/\___________
           /                        \
   $<type> variable = <initial value>$
    \______ ______/
           V
     declaration
===============================================================================

below, comparisons are done side-by-side in two columns.



matlab in left column                   java in right column

_______________________________________________________________________________

comments

+ $%$ means "ignore rest of line"       + $//$ means "ignore rest of line"

                                          note: $%$ is an operator in java!

+ no equivalent                         + can put a comment within code
                                          if surrounded with $/*$ and $*/$
                                          note: cannot nest

example:

    x = x + 1; % increment x                x = x + /* this is a
                                                multi-line comment
                                                :) */ 1 ; // increment x
_______________________________________________________________________________

statements

+ optional semicolon: suppress output   + semicolon is statement *terminator*
                                          not needed for statement blocks 
                                          (see below), so not needed for
                                          an entire conditional or loop

sequencing
+ one line after the other              + one "line" after the other

  x = 2;                                  x = 2;
  y = 3;                                  y = 3;
  z = 4;                                  z = 4;

block with new scope
+ no equivalent                         + enclose 0 or more statements
                                          within braces ${$ and $}$

                                          { x = 3;
                                            y = x;
                                          }

                                          + variable declaration\ covered in
                                          + shadowing           / later lecture

conditionals and loops
+ conditions do not require parens      + conditions must be parenthesized
+ bodies are not blocks                 + bodies are blocks -- braces needed
+ $end$ terminates $else$, loop body    + no $end$

  if _______________                      if ( _______________ ) {
                                                                
      +-------------+                         +-------------+   
      |             |                         |             |   
      +-------------+                         +-------------+   
                                                                
  elseif ___________                      } else if ( ___________ ) {
                                                                
      +-------------+                         +-------------+   
      |             |                         |             |   
      +-------------+                         +-------------+   
                                                                
  elseif ___________                      } else if ( ___________ ) {
                                                                
      +-------------+                         +-------------+   
      |             |                         |             |   
      +-------------+                         +-------------+   
  else                                    } else {
                                                                
      +-------------+                         +-------------+   
      |             |                         |             |   
      +-------------+                         +-------------+   
  end                                     }


  while ________                          while ( ________ ) {
                                                                
      +-------------+                         +-------------+   
      |             |                         |             |   
      +-------------+                         +-------------+   
  end                                     }


$if$ example:

  if x > 3                                if (x > 3) {
      y = 2;                                  y = 2;
  elseif y > 7 & z < 2                    } else if (y > 2 & z < 2) {
      w = 3;                                  w = 3;
      z = 4;                                  z = 4;
  else                                    } else {
      x = x + w;                              x = x + w;
      z = w;                                  z = w;
  end                                     }

$while$ example:

  while x > y                             while (x > y) {
      x = x-1;                                x = x - 1;
      y = 1.1 * y;                            y = 1.1 * y;
  end                                     }

$if$-and-$while$ example: 

  given integer variables $n, d, x$, with $n$ positive, set $x$
  to the sum of all proper divisors of $n$.

    matlab:
    
      d = 1;
      x = 0; % sum of proper divisors of $n$ up to, but excluding $d$
      while d < n
          if rem(n, d) == 0
              x = x+ d;
          end
          d = d + 1;
      end
    
    java:
    
      d = 1;
      x = 0; // sum of proper divisors of $n$ up to, but excluding $d$
      while (d < n) {
          if (n % d == 0) {
              x = x+ d;
          }
          d = d + 1;
      }
_______________________________________________________________________________

scalar variables, datatypes, values

+ identifiers                           + pretty much the same
+ $=$ for assignment                    + the same
+ implicit declaration & initialization + explicit declaration & initialization

    $nan$                                   no equivalent
    $d = -2.3e7;$ % $double$                $double d = -2.3e7;$
        $i = 7;$    % integer               $int i = 7;$
        $b = logical(1); % $logical$        $boolean b = true;$
        $bb = logical(0); % $logical$       $boolean bb = false;$
    $c = 'h';$ % length 1 string            $char c = 'h';$
_______________________________________________________________________________

references -- no matlab equivalent

+ *value* = data that can be stored in a variable
+ values are "small", "can be written on a scrap of paper"
+ arrays and objects are "too big" to be values
+ a *reference* (aka *address* or *pointer*) is "small enough" to be a value
+ box scope diagram: drawn as an arrow
+ copying the reference does not copy what it points to!
+ analogy: 
  + the room $Upson 322$ is too big to fit on a piece of paper
  + the address $Upson 322$ is small enough to fit on a piece of paper
  + writing/copying down address $Upson 322$ does not copy/duplicate the room!
+ operator $new$: create "big" object and return reference to it
+ value $null$: no reference (kind of like $nan$ in matlab)
+ two references are *aliases* if they point to the same object
+ *aliasing* is when two or more references point to the same object

example: suppose $x$ is or points to an array and $y = x$ is executed:

matlab                                  java

      +---+---+---+                       +---+     +---+---+---+
    x | 3 | 7 | 9 |                     x | *-+---->| 3 | 7 | 9 |
      +---+---+---+                       +---+     +---+---+---+
                                                     ^
      +---+---+---+                       +---+      |
    y | 3 | 7 | 9 |                     y | *-+------+
      +---+---+---+                       +---+     

y$ gets a copy of the value in $x$:     $y$ gets a copy of the value in $x$:
    $y$ gets a copy of the array            $y$ gets a copy of the *reference*

modifying $y(2)$ has no effect on $x$   modifying $y(2)$ also modifies $x(2)$
                                        since $x$ and $y$ point to the same
                                        array
_______________________________________________________________________________

questions asked in lecture.

Q: can you put semicolons in comments?

A: yes.

Q: how is indentation done for statement blocks?

A: the code inside the block should be indented.
A: i generally recommend putting the opening brace on the previous line.


Q: why isn't there a semicolon for the entire $if$ statement 
Q: or after each statement block?

A: the statement terminator $;$ is needed only to tell java where a statement
A: ends.  since a statement block clearly indicates its end with the close
A: brace $}$, it does not need a semicolon.  an $if$ statement is done either
A: if there is no $else$ part or when the $else$ part is done: the $else$
A: part is either a statement block or another $if$, and thus does not need
A: a semicolon.


Q: where is $n$ declared and/or initialized in the $if$-and-$while$ example?

A: the assumption is that it is *given* earlier in some code that is not shown.

Q: in the $if$ example above, the $else$ is on the same line as closing and 
Q: opening braces ($} else {$), but in the $if$-and-$while$ example, the 
Q: statement $d = d+1;$ isn't on the same line as the closing brace -- why not?

A: the statement blocks are part of the conditional, so it is fine for the
A: opening and closing braces to be on the same line as the $if$ and $else$
A: keywords.  however, $d = d+1;$ is not part of the conditional, and thus
A: should not be on the same line as any part of the conditional.


Q: in the example of a reference, is the pointer to just one element, 
Q: part of the array, or the entire array?

A: in java, a reference always points to an entire array or object 
A: (or is $null$).  it is not possible to point to part of an array or object,
A: e.g. just one element within an array.