Iteration

2017-11-03 09:05:06

C++ provides three iteration structures.

while while ( loopCondition ) { loopBody }
1. Evaluate loopCondition first.
2. Execute loopBody repeatedly until loopCondition is false.

do..while: do { loopBody } while ( loopCondition ) ;
1. Execute loopBody first.
2. Evaluate loopCondition.
3. Execute loopBody repeatedly until loopCondition is false.

for loop: for ( initStatement; loopCondition; incrStmt ) { loopBody }
1. Execute initStatement first.
2. Execute loopBody repeatedly until loopCondition is false.
3. After each execution of loopBody, execute incrStmt.

With each of these iteration structures, the loopBody code gets repeated as long as loopCondition evaluates to boolean true. The do loop differs from the other two in that its loopCondition gets checked at the bottom of the loop, so its loopBody is always executed at least once.

A common programming error is to place a semicolon after the while.

while (notFinished()); doSomething();

The first semicolon terminates the while statement entirely and produces a loop with an empty loopBody. Even though doSomething() is indented, it does not get executed inside the loop. The loopBody is responsible for changing the loopCondition. If notFinished() is initially true, then the empty loopBody will cause an infinite loop. If notFinished() is initially false, then the loop terminates immediately and doSomething() gets executed exactly once.

C++ provides break and continue for finer control over code executed inside loops.

while ( moreWorkToDo ) { statement1; if ( specialCase ) continue; statement2; if ( noMoreInput ) break; statement3; // continue jumps here } // break jumps here

break jumps out of the current control structure, whether it is a switch, for, while, or do..while.

continue only operates inside loops, and it skips the rest of the current iteration to the check if there is moreWorkToDo.

Exercises: Iteration

1.	Write the function `isPrime(int n)` that returns true if `n` is prime and false otherwise. Supply an interactive `main()` to test your function.
2.	Write the function `primesBetween(int min, int max)` that displays on the screen all the prime numbers that are between `min` and `max`. Supply an interactive `main()` to test your function.
3.	Write the function `power2(int n)` that computes and returns the value of `2` raised to the power `n`. Supply an interactive `main()` to test your function.
4.	Write a binary logarithm function `binLog(int n)` that computes and returns an `int` equal to the integral part of `log2(n)`, where `n` is positive. This is equivalent to finding the exponent of the largest power of 2 that is less than or equal to `n`. For example, `binLog(25)` is `4`. There are at least two simple, iterative ways to do this computation. Supply an interactive `main()` to test your function.

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