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CSC100 :: Lecture Note :: Week 06
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Overview

Assignment(s):

Code: DoAdder.cpp | f10do.cpp | ForAdder.cpp | f10for.cpp | narcissistic.cpp | narcissistic2.cpp | Elevator.cpp | GoToElevator.cpp

### 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;
```

### 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
// the 1st executable statement after the
// body of the  while  statement (/*B*/)
...
}
/*A*/ ...
}
/*B*/ ...
```

### 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 */
...
}
}
```

### 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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