Nested Loops

A loop can be nested inside of another loop. C++ allows at least 256 levels of nesting.

Syntax:

The syntax for a nested for loop statement in C++ is as follows:

for ( init; condition; increment )
{
   for ( init; condition; increment )
   {
      statement(s);
   }
   statement(s); // you can put more statements.
}

The syntax for a nested while loop statement in C++ is as follows:

while(condition)
{
   while(condition)
   {
      statement(s);
   }
   statement(s); // you can put more statements.
}

The syntax for a nested do...while loop statement in C++ is as follows:

do
{
   statement(s); // you can put more statements.
   do
   {
      statement(s);
   }while( condition );

}while( condition );

Example:

The following program uses a nested for loop to find the prime numbers from 2 to 100:

#include <constream.h>
 
int main ()
{
   int i, j;
   
   for(i=2; i<100; i++) {
      for(j=2; j <= (i/j); j++)
        if(!(i%j)) break; // if factor found, not prime
        if(j > (i/j)) cout << i << " is prime\n";
   }
   return 0;
}

This would produce the following result:

2 is prime
3 is prime
5 is prime
7 is prime
11 is prime
13 is prime
17 is prime
19 is prime
23 is prime
29 is prime
31 is prime
37 is prime
41 is prime
43 is prime
47 is prime
53 is prime
59 is prime
61 is prime
67 is prime
71 is prime
73 is prime
79 is prime
83 is prime
89 is prime
97 is prime

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For Loop

A for loop is a repetition control structure that allows you to efficiently write a loop that needs to execute a specific number of times.

Syntax:

The syntax of a for loop in C++ is:

for ( init; condition; increment )
{
   statement(s);
}

Here is the flow of control in a for loop:

• The init step is executed first, and only once. This step allows you to declare and initialize any loop control variables. You are not required to put a statement here, as long as a semicolon appears.
• Next, the condition is evaluated. If it is true, the body of the loop is executed. If it is false, the body of the loop does not execute and flow of control jumps to the next statement just after the for loop.
• After the body of the for loop executes, the flow of control jumps back up to the incrementstatement. This statement allows you to update any loop control variables. This statement can be left blank, as long as a semicolon appears after the condition.
• The condition is now evaluated again. If it is true, the loop executes and the process repeats itself (body of loop, then increment step, and then again condition). After the condition becomes false, the for loop terminates.

Flow Diagram:

Example:

#include <constream.h>
 
int main ()
{
   // for loop execution
   for( int a = 10; a < 20; a++ )
   {
       cout << "value of a: " << a << endl;
   }
 
   return 0;
}

When the above code is compiled and executed, it produces the following result:

value of a: 10
value of a: 11
value of a: 12
value of a: 13
value of a: 14
value of a: 15
value of a: 16
value of a: 17
value of a: 18
value of a: 19

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Do-While Loop

Unlike for and while loops, which test the loop condition at the top of the loop, the do...while loop checks its condition at the bottom of the loop.

A do...while loop is similar to a while loop, except that a do...while loop is guaranteed to execute at least one time.

Syntax:

The syntax of a do...while loop in C++ is:

do
{
   statement(s);
}while( condition );

Notice that the conditional expression appears at the end of the loop, so the statement(s) in the loop execute once before the condition is tested.

If the condition is true, the flow of control jumps back up to do, and the statement(s) in the loop execute again. This process repeats until the given condition becomes false.

Flow Diagram:

The do-while loop The do-while loop is similar to the while loop, except that the test condition occurs at the end of the loop.  Having the test condition at the end, guarantees that the body of the loop always executes at least one time.  The format of the do-while loop is shown in the box at the right. The test condition must be enclosed in parentheses andFOLLOWED BY A SEMI-COLON.  Semi-colons also follow each of the statements within the block.  The body of the loop (the block of code) is enclosed in braces and indented for readability.  (The braces are not required if only ONE statement is used in the body of the loop.) The do-while loop is an exit-condition loop.  This means that the body of the loop is always executed first.  Then, the test condition is evaluated.  If the test condition is TRUE, the program executes the body of the loop again.  If the test condition is FALSE, the loop terminates and program execution continues with the statement following the while. /*The following program fragment is an input routine that insists that the user type a correct response -- in this case, a small case or capital case 'Y' or 'N'.  The do-while loop guarantees that the body of the loop (the question) will always execute at least one time.*/ char ans; do {        cout<< "Do you want to continue (Y/N)?\n";        cout<< "You must type a 'Y' or an 'N'.\n";        cin >> ans; } while((ans !='Y')&&(ans !='N')&&(ans !='y')&&(ans !='n')); /*the loop continues until the user enters the correct response*/

FORMAT: do {      block of code; } while (test condition); exit condition loop

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End Program OR Loop Early

Ending a Program or Loop Early On occasion, it may be necessary to end a program (or section of a program) earlier than its normal termination.  Below are examples of methods of interrupting or terminating programs or loops.

return 0; -  it is possible to end a program (or function) prior to its normal termination by using an "extra" return 0; statement.  The drawback to this strategy is that it ends the entire program (or function) at the point where the return 0; statement is placed.  This may not always be what you had in mind.  Oftentimes, the programmer may wish to simply end a "process" within the program, but continue with the remainder of the programming code. return 0; returns a value of 0 to the IDE indicating that the program reached normal termination.

exit( ); - the exit( ) function also ends a program before its normal termination.  It requires the Standard Library header file, stdlib.h.  The format is                                          exit(value);   where value is an integer variable or value.  Using exit(1); returns a value of 1 to the IDE indicating that an error must have occurred.  This process is often used for error trapping.

break; - the break statement gets you out of a loop.  No matter what the loop's ending condition, break immediately says "I'm outta here!"  The program continues with the next statement immediately following the loop.  Break stops only the loop in which it resides.  It does not break out of a "nested loop" (a loop within a loop).

continue; - continue performs a "jump" to the next test condition in a loop.  The test condition is then evaluated as usual, and the loop is executed as long as the test condition remains true.  The continue statement may be used ONLY in iteration statements (loops).  It serves to bypass, or jump over, certain iteration sections of a loop.

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While Loop

A while loop statement repeatedly executes a target statement as long as a given condition is true.

Syntax:

The syntax of a while loop in C++ is:

while(condition)
{
   statement(s);
}

Here, statement(s) may be a single statement or a block of statements. The condition may be any expression, and true is any non-zero value. The loop iterates while the condition is true.

When the condition becomes false, program control passes to the line immediately following the loop.

Flow Diagram:

Here, key point of the while loop is that the loop might not ever run. When the condition is tested and the result is false, the loop body will be skipped and the first statement after the while loop will be executed.

Example:

#include <constream.h>
 
int main ()
{
   // Local variable declaration:
   int a = 10;

   // while loop execution
   while( a < 20 )
   {
       cout << "value of a: " << a << endl;
       a++;
   }
 
   return 0;
}

When the above code is compiled and executed, it produces the following result:

value of a: 10
value of a: 11
value of a: 12
value of a: 13
value of a: 14
value of a: 15
value of a: 16
value of a: 17
value of a: 18
value of a: 19

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