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CSC100 :: Lecture Note :: Week 05
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Overview

Assignments:

Code: logicalops.cpp | ifcode.cpp | andor.cpp | NewAge.cpp
[loops] Adder.cpp | f10while.cpp | f10while2.cpp | DoAdder.cpp | f10do.cpp | ForAdder.cpp | f10for.cpp
narcissistic.cpp | narcissistic2.cpp | Elevator.cpp | GoToElevator.cpp


The while Statement

The while statement supports iteration (i.e. it is a repetition control statement). Iteration is the process of repeating an operation.

The idea of looping in a computer program can be attributed to Ada Lovelace (01815-01852). Many believe Lovelace wrote the first "computer program." On 10 December 1980 (Ada's birthday), the U.S. Defense Department approved the reference manual for its new computer programming language, called "Ada".

The while statement repeatedly executes a simple statement or block (i.e. compound statement) until a conditional EXPR becomes false (i.e. 0). Each loop iteration is called a cycle.

   while (EXPR)    // typically, you don't want a semicolon 
      statement;

   EXPR is the "conditional" test used to determine whether the loop 
   should continue for another cycle, and  statement  is to be repeated.
   If multiple  statements  need to be executed, then use a compound 
   statement (or block).

   EXPR is evaluated only at the beginning of a loop cycle.

   Pseudo-code for a while loop:
      
      top-of-loop
         evaluate the EXPR
         if  EXPR  is "true"      // i.e. a non-zero value
            execute statement
            go to top-of-loop
         else
            go to end-of-loop  (i.e. terminate the loop)
      end-of-loop
      statement
      ...

   //Example:  loop until user enters a positive number
   int n = -1;
   while (n <= 0)
      cin >> n;

   //Example:  print out the numbers 1 through 5
   int i = 1;
   while (i <= 5) {
      cout << i << endl;
      i = i + 1;
   }
Sentinel Values

One way to terminate (end) a loop is when a particular data value is entered (e.g. the user enters a -1 or end-of-file is encountered), a special value that is used to end the loop is called a sentinel value.

Sentinel values should be defined as manifest (symbolic) constants. Example:

   const int EXIT_VALUE = -1;
   int n = EXIT_VALUE + 1;
   while (n != EXIT_VALUE) {
      cout << "Enter a number (" << EXIT_VALUE << " to exit): ";
      cin >> n;
   }
Infinite Loops

When you use a loop in a program, it is important to make sure that the condition used to control the loop will eventually become false, so that the loop can end. A loop that never finishes is called an infinite loop.

To stop an infinite loop, a special command sequence must be typed on the keyboard to interrupt the program and forcibly cause it to quit. This command differs from machine to machine.

Sometimes infinite loops are used by design; therefore, existenance of an infinite loop does not necessarily imply a program defect.

   // the following are infinite loops that do nothing 
   while (1) 
      ;  //null statement

   int i = 99;
   while (i != 0) ;

   while (true) ;  //C++ only...

   while (!false) { }  //C++ only....

   while (43 == 43) ;

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The do while Statement

The do while statement supports iteration (i.e. it is a repetition control statement). Iteration is the process of repeating an operation.

The do while statement repeatedly executes a simple statement or block (i.e. compound statement) until a conditional EXPR becomes false (i.e. 0). Each loop iteration is called a cycle.

   do 
      statement;
   while (EXPR);  //semicolon must follow the while statement

   EXPR is the "conditional" test used to determine whether the loop 
   should continue for another cycle, and  statement  is to be repeated.
   If multiple  statements  need to be executed, then use a compound 
   statement (or block).  A compound statement is almost always used.

   EXPR is evaluated only at the end of a loop cycle.  

   Pseudo-code for a do-while loop:
      
      top-of-loop
         execute statement
         evaluate the EXPR
         if  EXPR  is "true"      // i.e. a non-zero value
            go to top-of-loop
         else
            go to end-of-loop  (i.e. terminate the loop)
      end-of-loop
      statement
      ...

   //Example:  print out the numbers 1 through 3
   int i = 1;
   do {
      cout << i << endl;
      i = i + 1;
   } while (i <= 3);


   Could the previous loop be rewritten:
      int i = 1;
      do cout << i++ << endl; while (i <= 3);

   How about:
      int i = 1;
      do cout << i << endl; while(i++ <= 3);

When you know for sure that a loop must cycle at least once, then a do-while is a good loop construct to use.

   set item counter to 0
   do {
      prompt user for data
      get data from user
      if (data equals sentinel value)
         break;
      if (data is junk)
         continue;  //get more data from the user
      process the data
      increment the item counter
   } while (1);
   print the item counter
Comment on Style

The do-while can be difficult to read; therefore, to help it "stick" out, I suggest the following.

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The for Statement

The for statement supports iteration (i.e. it is a repetition control statement). Iteration is the process of repeating an operation.

The for statement repeatedly executes a simple statement or block (i.e. compound statement) until a control-EXPR becomes false (i.e. 0). Each loop iteration is called a cycle.

   for ([initialization-step]; [conditional-step]; [increment-step])
      statement;

   The optional "initialization-step" is executed once before the loop 
   is ever executed (and before the "conditional-step" is evaluated
   for the first time).

   The optional "conditional-step" is an EXPR that is evaluated at
   the top of the loop.  If it is true, then the body of the
   for  statement is executed; otherwise, flow control jumps to
   the first executable statement after the  for  loop body.  If
   no conditional-step is specified, then  it is taken as
   permanently true.

   After the body of a  for  loop has been executed, the optional
   "increment-step" is executed.

   After the increment-step is executed; the conditional-step
   is re-evaluated.

   Example:

      //print the numbers 1 through 3

      int i;
      for (i = 1; i <= 3; i = i + 1)
         cout << i << endl;

If the body of the for contains multiple statements, then a compound statement is needed.

All three steps for a for loop are EXPRs. Most commonly, the initialization-step and the increment-step are assignments or functions calls, and the conditional-step is a relational EXPR.

The for is frequently used when there are simple initialization and increment steps because it keeps the loop control statements close together and visible at the top of the loop.

The initialization and increment steps often take advantage of the sequence operator.

   for (i = 0, j = 99; i < n; i = i + 1, j = j - 1)
      do_something;

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The break Statement

The break statement is a jump control statement that is used inside of repetition control statements and the switch selection control statement.

The break statement causes an immediate exit from the innermost enclosing switch or loop statement.

The break statement allows a loop to be exited from some point other than at the top or bottom of the loop.

Structured programming purist are not overly fond of the break statement.

   //
   // enter into an infinite loop reading integers entered by
   // the user... the loop terminates when a sentinel value of
   // -1 is entered... 
   //
   int cnt = 0;
   const int SENTINEL = -1;
   while (1) {
      cout << "Enter an integer (" << SENTINEL << " to exit): ";
      cin >> input;
      if (SENTINEL == input)
         break;  //flow control jumps to the statement A
      cnt = cnt + 1;
      ...
   }
   /*A*/ cout << cnt << " numbers entered" << endl;

You must be careful when you use the break.

   while (...) {
      ...
      if (...) {
         ...
         if (...) 
            break;  // the programmer wants to  break  out of the
                    // body of the enclosing  if  statement,
                    // but instead the  break  will cause the
                    // flow control of the program to jump to
                    // the 1st executable statement after the
                    // body of the  while  statement (/*B*/)
         ...
      }
      /*A*/ ...
   }
   /*B*/ ...

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The continue Statement

The continue statement is a jump control statement that is used inside of repetition control statements.

The continue statement causes the next iteration of the enclosing loop statement to begin. In the while and do this means that the control-EXPR is executed immediately; in the for control passes to the step-EXPR.

The continue statement is often used when the part of the loop that follows is complicated, so that reversing a test and indenting another level would nest the program too deeply.

Structured programming purist are not overly fond of the continue statement.

   while (input != SENTINEL) {
      /* Part A */
      ...

      if (EXPR)
         continue;

      /* Part B */
      ...
   }

      When  if (EXPR)  evaluates to  true,  the  continue  statement
      is executed and the flow control of the program jumps to 
      control expression of the  while  (i.e.  input != SENTINEL)
      is re-evaluated.

Here is the same loop written without using a continue statement.

   while (input != SENTINEL) {
      /* Part A */
      ...

      if (!EXPR) {
         /* Part B */
         ...
      }
   }

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The goto Statement

The goto statement is a jump control statement that is used to alter the flow control of a program.

Statements can labeled. Label names conform to the naming syntax that is applied to variables. Syntax of a labeled statement.

   some_label_name:  statement;

Statement labels are used with the goto statement.

   goto some_label_name;

When a goto is executed, the flow control of the program immediately jumps to the labeled statement.

Although use of the goto should be minimized, there are a few situations where they may be useful.

   for (EXPR) {
      ...
      for (EXPR) {
         ...
         if (EXPR)
            goto end_of_loops;  /*want to break from both loops*/
         ...
      }
      ...
   }
   end_of_loops:
   ...

An aside: In Java, goto is not a keyword, but it is a reserved word. In addition, to handle the aforementioned example, Java supports labeled break statements.

The goto statement is frequently found in code that is generated by other programs.

If you want to go somewhere, goto is the best way to get there.
-- Ken Thompson (creator of Unix)

Edsger Dijkstra (01930-02002) was a Dutch computer scientist. Dijkstra won the ACM Turing Award in 01972. According to the Wikipedia, Dijkstra was also known for his "low opinion of the GOTO statement in computer programming, culminating in the 1968 article 'A Case against the GO TO Statement', regarded as a major step towards the widespread deprecation of the GOTO statement and its effective replacement by structured control constructs such as the while loop. This methodology was also called structured programming." {Wikipedia.org::Edsger W. Dijkstra}

Update::2016.12.19

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