There may be a situation, when you need to execute a block of code several number of times. In general statements are executed sequentially: The first statement in a function is executed first, followed by the second, and so on.
Programming languages provide various control structures that allow for more complicated execution paths.
A loop statement allows us to execute a statement or group of statements multiple times and following is the general from of a loop statement in most of the programming languages:
C++ programming language provides the following types of loop to handle looping requirements. Click the following links to check their detail.
| Loop Type | Description |
|---|---|
while loop
| Repeats a statement or group of statements while a given condition is true. It tests the condition before executing the loop body.
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. Thecondition 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 <iostream> using namespace std; 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 |
for loop
| Execute a sequence of statements multiple times and abbreviates the code that manages the loop variable.
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:
Flow Diagram: Example:#include <iostream> using namespace std; int main () { // for loop execution for( int a = 10; a < 20; a = a + 1 ) { 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 |
do...while loop
| Like a while statement, except that it tests the condition at the end of the loop body
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: Example:#include <iostream> using namespace std; int main () { // Local variable declaration: int a = 10; // do loop execution do { cout << "value of a: " << a << endl; a = a + 1; }while( a < 20 ); 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 |
nested loops
| You can use one or more loop inside any another while, for or do..while loop.
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 <iostream> using namespace std; 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 |
Loop Control Statements:
Loop control statements change execution from its normal sequence. When execution leaves a scope, all automatic objects that were created in that scope are destroyed.
C++ supports the following control statements. Click the following links to check their detail.
| Control Statement | Description |
|---|---|
break statement
| Terminates the loop or switch statement and transfers execution to the statement immediately following the loop or switch.
The break statement has the following two usages in C++:
If you are using nested loops (i.e., one loop inside another loop), the break statement will stop the execution of the innermost loop and start executing the next line of code after the block.
Syntax:
The syntax of a break statement in C++ is:
break; Flow Diagram: Example:#include <iostream> using namespace std; int main () { // Local variable declaration: int a = 10; // do loop execution do { cout << "value of a: " << a << endl; a = a + 1; if( a > 15) { // terminate the loop break; } }while( a < 20 ); 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 |
continue statement
| Causes the loop to skip the remainder of its body and immediately retest its condition prior to reiterating.
The continue statement works somewhat like the break statement. Instead of forcing termination, however, continue forces the next iteration of the loop to take place, skipping any code in between.
For the for loop, continue causes the conditional test and increment portions of the loop to execute. For the while and do...while loops, program control passes to the conditional tests.
Syntax:
The syntax of a continue statement in C++ is:
continue; Flow Diagram: Example:#include <iostream> using namespace std; int main () { // Local variable declaration: int a = 10; // do loop execution do { if( a == 15) { // skip the iteration. a = a + 1; continue; } cout << "value of a: " << a << endl; a = a + 1; }while( a < 20 ); 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: 16 value of a: 17 value of a: 18 value of a: 19 |
goto statement
| Transfers control to the labeled statement. Though it is not advised to use goto statement in your program.
A goto statement provides an unconditional jump from the goto to a labeled statement in the same function.
NOTE: Use of goto statement is highly discouraged because it makes difficult to trace the control flow of a program, making the program hard to understand and hard to modify. Any program that uses a goto can be rewritten so that it doesn't need the goto.
Syntax:
The syntax of a goto statement in C++ is:
goto label; .. . label: statement;
Where label is an identifier that identifies a labeled statement. A labeled statement is any statement that is preceded by an identifier followed by a colon (:).
Flow Diagram: Example:#include <iostream> using namespace std; int main () { // Local variable declaration: int a = 10; // do loop execution LOOP:do { if( a == 15) { // skip the iteration. a = a + 1; goto LOOP; } cout << "value of a: " << a << endl; a = a + 1; }while( a < 20 ); 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: 16 value of a: 17 value of a: 18 value of a: 19
One good use for the goto is to exit from a deeply nested routine. For example, consider the following code fragment:
for(...) { for(...) { while(...) { if(...) goto stop; . . . } } } stop: cout << "Error in program.\n";
Eliminating the goto would force a number of additional tests to be performed. A simple break statement would not work here, because it would only cause the program to exit from the innermost loop.
|
The Infinite Loop:
A loop becomes infinite loop if a condition never becomes false. The for loop is traditionally used for this purpose. Since none of the three expressions that form the for loop are required, you can make an endless loop by leaving the conditional expression empty.
#include <iostream> using namespace std; int main () { for( ; ; ) { printf("This loop will run forever.\n"); } return 0; }
When the conditional expression is absent, it is assumed to be true. You may have an initialization and increment expression, but C++ programmers more commonly use the for(;;) construct to signify an infinite loop.
NOTE: You can terminate an infinite loop by pressing Ctrl + C keys.
