**Reading**

Section 7.3 of *Problem Solving with C++*.

**Lecture**

Often with arrays it's more powerful to return the *index* of an element
with a certain property, rather than the *value*
of the element. For example, if `string *name`

is an array of names of contest participants, and `int`

` *score`

is an array of
scores, such that participant `name[i]`

has score `score[i]`

, then knowing the *value* of the largest
element in `score`

won't tell me who
the winner is, but knowing the *index*
(i) of the largest element in `score`

will.

So, with this in mind, let's consider writing a function `maxIndex`

that will return
the index of the element with the maximum value in array arrayIn of `sizeIn`

objects of type `int`

.

int maxIndex(int *arrayIn, int sizeIn)

`{`

` int iMax = 0;`

` for(int i = 1; i < sizeIn; i++)`

` if (arrayIn[iMax] < arrayIn[i])`

` iMax = i;`

` return iMax;`

`}`

Very simple function! Now, using our above example, the winner
of our contest is `NAME[`

`maxindex`

`(SCORE,N)]`

.

Consider the following function that uses our `maxIndex`

function:

void mystery(int *arrayIn, int sizeIn)

`{`

` for(int size = sizeIn; size > 1; size--)`

` {`

` int k = maxIndex(arrayIn,size);`

` int temp = arrayIn[size-1];`

` arrayIn[size-1] = arrayIn[k];`

` arrayIn[k] = temp;`

` }`

`}`

` `

What does the `mystery`

function do? Well, it starts by swapping the largest element in
the array with the last element of the array, and then pretends the last array
slot isn't there any more. Hopefully, you see that
this puts the largest at the back of the array, the next largest in the second
to last spot, etc. until the array is in **sorted order**! This sorting
algorithm is known as *Selection
Sort*, because it *selects*
the largest of the remaining elements and puts it in its proper spot in the
sorted array. If you define the `swap`

function (as in Class 18), you can write a particularly succinct version of this
function:

void selectionSort(int *arrayIn, int sizeIn)

`{`

` for(int size = sizeIn; size > 1; size--)`

` swap(arrayIn[maxindex(arrayIn,size)],arrayIn[size-1]);`

`}`

This is actually a subtle and important function for the way it
uses pass-by-reference on array elements. **Make sure you understand how it
works!**

Searching for values in arrays is another fundamental operation.
The basic format is `search(`

`A,N,x`

`)`

, where we search for value `x`

in the array `A`

of `N`

elements, and return the index of an
element of `A`

that matches the
value `x`

. If no such element
is found, an index of `N`

may be returned
and, since it is not a valid index, the caller of the function can determine
that no match was found. For example, to search in an array of objects of type `string`

, I'd define the
following function:

int search(string *arrayIn, int sizeIn, string target)

`{`

` int i = 0;`

` while(i < sizeIn && arrayIn[i] != target)`

` i++;`

` return i;`

`}`

` `

1. Write a program that reads in a list of 10 names (first name followed by last name) and prints them out in the usual order - i.e. alphabetically by last name, using first names to break ties.

Last modified by LT M. Johnson 10/23/2007 08:59:49 AM