The `for`

loop is a very useful tool for doing things over and over again for a certain number of times already known *in advance*. There are two possibilities that we would like to consider:

- What if we do not know in advance how many iterations we will need?
- What if we would like to stop a loop before it is due to end?

An example for the first kind would be a Newton iteration that should run until the value of \(f(x)\) is "small" enough, for example \(10^{-12}\). Before actually performing the iterations we do not know how many steps it will take, so a `for`

loop is not exactly the right type of loop. We could get around this limitation if we introduce a maximum number of allowed iterations and then use the (as-of-yet unknown) mechanism for terminating a loop prematurely once we find a good enough approximate root.

A `while`

loop tells MATLAB® to continue iterating as long as a certain condition (which you specify) is satisfied. The syntax is:

`while`

`<condition> <statements>`

`end`

MATLAB evaluates the `<condition>`

and if it is true (or a non-zero number) it performs the `<statements>`

, if not, it continues after the `end`

. After each time it evaluates `<statements>`

MATLAB goes back and evaluates `<condition>`

again, etc. Note that `<condition>`

does *not* get evaluated in the middle of evaluating `<statements>`

but, rather, only before evaluating them. Here's a simple way of adding two positive integers (very silly):

`x=5; y=6; while y>0 x=x+1; y=y-1;`

`end`

Of course, this fails miserably if `y`

is not a positive integer (doesn't do anything, do you understand why?)

**Exercise 16 . **

*Solve the following problems using a*

`while`

*loop:*

*Show the numbers from 1 to 10**Show the numbers from 10 to -10*-
*Find out how many divisors 28 has (*`mod`

*or*`rem`

*will be useful here)* *Find out if a number is prime*-
*Use an external*`while`

*and an internal*`for`

*loop to find the first 100 prime numbers.* -
*A*perfect number*is a number*\(n\)*whose divisors (including 1 but excluding itself) add up to*\(n\)*itself. For example, 6 is a perfect number. Check if a number is perfect.* -
*Use two nested*`while`

*loops to find the first 3 perfect numbers.*

**Homework 5.** *Consider the following sequence defined completely by the first element* \(S_1\)^{¶}:

\begin{equation} S_{n+1}= \begin{cases} S_n/2 & \text{ if } S_n \text{ is even}\\ 3 S_n+1 & \text{ if } S_n \text{ is odd} \end{cases} \end{equation}

*A still*^{||}*open question in mathematics is whether all such sequences always arrive at 1 for large enough* \(n\) *(the alternatives being that some sequences may rise indefinitely, or that there may be a closed orbit that does not include 1). Compute the number of iterations it takes to arrive at* \(1\) *given a starting value* \(s\) *using a while loop. Since we do not know how long it will take to arrive at 1 (though you can assume that it will happen eventually) we might want to construct this sequence using a while-loop. What starting number smaller than 10,000 has the longest trajectory? What's the largest number on that trajectory?*

^{§}This is the subject of the Collatz Conjecture.

^{||}Despite a recent "near" solution.