Programming Language Principles

Control Structures

Sequence -- one statement after another

Selection -- take one path or another

if
if...else
switch

Iteration -- loop

while
do...while
for

if (Decision) Statements

if statement

if (expression)
  {
   statement-1;
   statement-2;
   ...
   statement-n;
  }

expression usually must be in parentheses

braces on separate lines

indent 3-4 spaces

if expression is true, execute block

if expression is false, skip block

if (sum == size)
  {
   result = total / size;
   write result;
  }

What if only 1 statement in block?

OK ---

if (sum == size)
   result = total / size;

Preferred ---

if (sum == size)
  {
   result = total / size;
  }

if/else statement

if (expression)
  {
   statement-1;
   ...
   statement-m;
  }
else
  {
   statement-1;
   ...
   statement-n;
  }

if expression is true, execute 1st block, skip 2nd

if expression is false, skip 1st block, execute 2nd

if (sum == size)
  {
   result = total / size;
   write result;
  }
else
  {
   total += voltage;
   sum++;
  }

Again, still use {...} if only one statement in either block

switch statement

if statement has one branch

if/else statement has two branches

switch statement has multiple branches

Case statement

Selector variable

Compared to set of possible matches

switch (selector variable)
{
 case value-1:
      statement-1;
      ...
      break;
 case value-2:
      statement-1;
      ...
      break;
 ...
 case value-n:
      statement-1;
      ...
      break;
}

Here is an example:

switch (size)
{
 case 5:
      x = s - z;
      break;
 case 10:
      x = s * z;
      --y;
      break;
 case 15:
      x = s + z;
      break;
}

Do you need the last break statement? Not really. But what if you add a new case???

Several values can go to same set of statements

switch (size)
{
 case 3:
 case 5:
      x = s - z;
      break;
 case 8:
 case 10:
 case 13:
      x = s * z;
      --y;
      break;
 case 15:
      x = s + z;
      break;
}

What if selector variable matches two cases? Normally goes to first match.

What if selector variable does not match any cases? Skips all statements.

default may be used if no other matches

switch (size)
{
 case 5:
      x = s - z;
      break;
 case 10:
      x = s * z;
      --y;
 case 15:
      x = s + z;
      break;
 default:
      write "Incorrect size.";
      break;
}

put default last because it matches anything

Loops

Pretest -- test a condition each time BEFORE the loop is executed (while)

Posttest -- test a condition each time AFTER the loop is executed (do...while)

Fixed Repetition -- execute the loop a pre-determined number of times (for)

The while loop

while (expression)
  {
   statement-1;
   statement-2;
   ...
   statement-n;
  }

if expression is true, execute block and return to while statement

if expression is false, skip block

If expression false first time encountered, statement block not executed even once

while (melt < final_score)
  {
   read plenum;
   melt += plenum;
   discrep = mass * accel;
  }

The do/while loop

do
  {
   statement-1;
   statement-2;
   ...
   statement-n;
  }
while (expression);

If expression false first time encountered, statement block still executed once

if expression is true, execute block again and return to while statement

if expression is false, do not execute block again

do
  {
   pay = base - deduct + overtime;
   hours -= base_hours;
  }
while (hours > 0);

The for loop

for (initialization; expression; increment)
  {
   statement-1;
   statement-2;
   ...
   statement-n;
  }

Initialization -- give initial value to variable

Expression -- test variable against some limit

Increment -- Add (or subtract) something (usually 1) to (from) variable

for (count = 1; count <= 12; ++count)
  {
   read temp;
   write "The temperature at ", count;
   write ":00 is ", temp;
  }

Loop variable (count) can be used in loop, but probably should not be altered...

// Caution - this does not do what you want...
for (count = 1; count <= 12; ++count)
  {
   read temp;
   write "The temperature at ", count;
   write ":00 is ", temp;
   count = count +1;
  }

for (count = 0; count <= table_size; 
     count = count + 5)
  {...}

Suppose table_size is -9. Loop statements will not be executed even once (pretest loop).

for (count = 25; count > 0; --count)
  {...}

for (count = 25; count; --count)
  {...}

Nested Loops

for (hours = 0; hours < 24; ++hours)
  {
   for (mins = 0; mins < 60; ++mins)
     {
      for (secs = 0; secs < 60; ++secs)
        {
         ...
        }
     }
  }

While and do...while loops can be nested as well

break statement -- immediately terminates loop

Usually used with if statement within loop to terminate if some condition develops

while (radiation > lowest)
  {
   ...
   if (radiation > highest)
     break;
   ...
  }

Some loops are un-ending on purpose...

char code;
char stop = 'q';
...
while (true)
  {
   read code;
   if (code == stop)
     break;
   ...
  }

continue statement -- skips this iteration

Usually used with if statement within loop to avoid some problems but to continue the looping mechanism

while (input_flag == more_data)
  {
   ...
   if (group_size == 0)
     continue;
   average = sum / group_size;
   ...
  }

Functions

Function is block of statements to perform specific task

Functions eliminate need for duplicate statements -- same function can be invoked from multiple locations

Use of functions also enhances program readability

Function Format

return_type function_name ([parameter_list])
{
 // declare local variables

 statement-1;
 ...
 statement-n;
 return [(return_value)];
}

void main ()
{
 ...
 s = max (y, z);
 ...
 if (max (a,b) > ROOF) {...}
 ...
}

int max (int num1, int num2)
{
 int part;
 part = num1;
 if (num2 > num1)
   {
    part = num2;
   }
 return (part);
}

Function Header is first line

return_type function_name ([parameter_list])

May have none, one, or more arguments (parameters)

Actual arguments and function parameters should be same number and types

int max (int num1, int num2)
{...}

int a;
int b;
int c;
float d;
char e;
...
a = max (b,c); //OK
a = max (a,b,c); // wrong number 
a = max (c,d); //wrong type
d = max (a,b); //OK

Return statement should be last statement of function

Arrays

Array is indexed data structure used to store data elements

Can have arrays of ints, floats, chars, ...

Zero-based arrays -- indices are 0,1,2,....

Array of ints (age)

 5  11   7   4   0  21   6   2  14   7
[0] [1] [2] [3] [4] [5] [6] [7] [8] [9]

age[5]
array name is age
array index is 5
array element is 21

All elements in array must usually be same type

Declaring Arrays

data_type array_name [number_elements];

int age [10];
float temp [24];
char name [20];

Array of characters is usually "character string"

Accessing Arrays

age [i] = 24;
weight [i] = pounds;
name [k] = 'B';

Arrays and Loops

for (i=0; i<10; ++i)
  {
   weight [i] = 0.0;
  }

for (i=0; i<10; ++i)
  {
   read weight [i];
  }

warm = 0.0;
for (time=0; time<24; ++time)
  {
   write temp [time];
    warm += temp [time];
  }

Classes and Objects

Class is a way of providing additional data types

Class defines data members (Fields), constructor functions (Constructors), and associated functions (Methods)

public  class  Button
{
     // Fields
 public String label;
 public Color shade;
 
     // Constructors
 public Button ();
 public Button (String label);
 public Button (String label, Color shade);
 
     // Methods
 public void setLabel (String label);
 public String getLabel ();
 public void setColor (Color shade);
 public Color getColor ();
}

String and Color are also classes. A Button has two Fields -- a label (which is a character String) and a shade (which is a Color).

// public Button ();
// creates a button with no label
// and a standard gray shade
Button pushme = new Button ();

// public Button (String label);
// creates a button with the indicated label 
// and a standard gray shade
Button pushme = 
  new Button ("Press here to reset");

// public Button (String label, Color shade);
// creates a button with the indicated label 
// and the indicated shade
Button pushme = 
  new Button ("Press here to reset", red);

// public void setLabel (String label);
// changes the label to the one indicated
pushme.setLabel ("Push to reset");
// the button's label is now "Push to reset

// public String getLabel ();
// returns the label on the button
String whatsay = pushme.getLabel ();
// the String whatsay is now "Push to reset

// public void setColor (Color shade);
// changes the shade to the one indicated
pushme.setColor (green);
// the button's label shade is now green

// public Color getColor ();
// returns the shade on the button
Color whathue = pushme.getColor ();
// the Color whathue is now green