2.4. Loops

C# provides four iteration statements: while, do, for, and foreach. The first three are the same constructs you find in C, C++, and Java; the foreach statement is designed to loop through collections of data such as arrays.

while loop

Syntax:

while ( boolean expression ) { body }

The statement(s) in the loop body are executed until the boolean expression is false. The loop does not execute if the expression is initially false.

Example:

byte[] r = {0x00, 0x12, 0x34, 0x56, 0xAA, 0x55, 0xFF};

int ndx=0;

int totVal = 0;

while (ndx <=6) 

{

   totVal +=    r[ndx];   

   ndx += 1;

}

do loop

Syntax:

do { do-body } while ( boolean expression );

This is similar to the while statement except that the evaluation is performed at the end of the iteration. Consequently, this loop executes at least once.

Example:

byte[] r = {0x00, 0x12, 0x34, 0x56, 0xAA, 0x55, 0xFF};

int ndx=0;

int totVal = 0;

do 

{

   totVal += r[ndx];

   ndx += 1;

}

while (ndx <= 6);      

for loop

Syntax:

for ( [initialization]; [termination condition]; [iteration] )

     { for-body }

The for construct contains initialization, a termination condition, and the iteration statement to be used in the loop. All are optional. The initialization is executed once, and then the condition is checked; as long as it is TRue, the iteration update occurs after the body is executed. The iteration statement is usually a simple increment to the control variable, but may be any operation.

Example:

int[] r = {80, 88, 90, 72, 68, 94, 83};

int totVal = 0;

for (int ndx = 0; ndx <= 6; ndx++) {

   totVal += r[ndx];

}

If any of the clauses in the for statement are left out, they must be accounted for elsewhere in the code. This example illustrates how omission of the for-iteration clause is handled:

for   (ndx = 0; ndx < 6; )

{

   totVal += r[ndx];

   ndx++;          // increment here

}

You can also leave out all of the for clauses:

for (;;) { body   }   // equivalent to while(true) { body }

A return, goto, or break statement is required to exit this loop.

foreach loop

Syntax:

foreach ( type identifier in collection )  { body }

The type and identifier declare the iteration variable. This construct loops once for each element in the collection and sets the iteration variable to the value of the current collection element. The iteration variable is read-only, and a compile error occurs if the program attempts to set its value.

For demonstration purposes, we will use an array as the collection. Keep in mind, however, that it is not restricted to an array. There is a useful set of collection classes defined in .NET that work equally well with foreach. We look at those in Chapter 4, "Working with Objects in C#."

Example:

int totVal = 0;

foreach (int arrayVal in r)

{

   totVal += arrayVal;

}

In a one-dimensional array, iteration begins with index 0 and moves in ascending order. In a multi-dimensional array, iteration occurs through the rightmost index first. For example, in a two-dimensional array, iteration begins in the first column and moves across the row. When it reaches the end, it moves to the next row of the first column and iterates that row.

Transferring Control Within a Loop

It is often necessary to terminate a loop, or redirect the flow of statements within the loop body, based on conditions that arise during an iteration. For example, a while (true) loop obviously requires that the loop termination logic exists in the body. Table 2-7 summarizes the principal statements used to redirect the program flow.

Table 2-7. Statements to Exit a Loop or Redirect the Iteration

Statement	Description	Example
break	Redirects program control to the end point of a containing loop construct.	while (true) { ndx+=1; if (ndx >10) break; }
continue	Starts a new iteration of enclosing loop without executing remaining statements in loop.	while (ndx <10) { ndx +=1; if(ndx %2 =1) continue; totVal += ndx; }
goto identifier; goto case exp; goto default;	Directs program control to a label, a case statement within a switch block, or the default statement within a switch block. The goto may not transfer control into a nested scope—for example, a loop.	public int FindMatch(string myColor) { string[] colorsAvail("blueaqua", "red", "green","navyblue"); int loc; int matches=0; foreach (colorType in colorsAvail) { loc = colortype.IndexOf(myColor); if (loc >=0) goto Found; continue; Found: matches += 1; } return(matches); }
return [expression] ;	Returns program control to the method that called the current method. Returns no argument if the enclosing method has a void return type.	public double Area(double w, double l) { return w * l; }

There are few occasions where the use of a goto statement improves program logic. The goto default version may be useful in eliminating redundant code inside a switch block, but aside from that, avoid its use.