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Searching through arrays efficiently

Sorting arrays

Using character arrays as "STRINGS"

Applying What You Learnstrings … get it? hahahaha

Lists

- Defn => A collection of homogeneous components
- linear collection
- variable length collection
- Length <=> the actual number of values stored in the list
- Example -- a file of time card information

Joe, 40, Clyde, 38.5, Sniudly, 42.75 ...

scores : 85 79 92 57 68 80 . . .

0 1 2 3 4 5 98 99

Lists- Arrays can be used to implement a list
- declare the array large
- keep track of how many elements used
- We often do operations on the lists
- create a list, add an item, delete an item
- print the list, search the list for a value
- sort the list
- A list of numbers

scores : 85 79 92 57 68 80 . . .

0 1 2 3 4 5 98 99

Sequential Search- Consider the list unordered (not sorted)
- For a function to search for a targert we must specify
- name of the array to be searched
- length of the list (number of array elements)
- a flag parameter which tells whether or not the search was successful
- an index value to be returned which tells where in the list the item was found

scores

5

boolean & found

int & location

scores : 85 79 92 57 68 80 . . .

0 1 2 3 4 5 98 99

Sequential Search- Algorithm example
- Note use of reference parameters for the found flag and the location

void search (int list[ ], int length, int target, boolean & found, int &location) { location = 0; while ((location < length) && (target != list[location])) location++; found = (index < length); } // if found == TRUE, location OK . . . search (scores, 5, 92, found_it, where_its_at);

Sorted List -- Faster Search

- Sorted list => components arranged in order
- alphabetical
- numerically ascending or descending
- Advantage of a sorted list
- need to search only until the value found is larger than target value

Sorting

- Means arranging the list elements into some order (for instance, strings into alphabetical order, or numbers into ascending or descending order).

Dale Nell

Weems Chip

Headington Mark

Cooper Sonia

Huang Jeff

Cooper Sonia

Dale Nell

Headington Mark

Huang Jeff

Weems Chip

sorting

list 1 : 85 79 92 57 68 80 . . .

list 2 :

Sorting Algorithm- Make a pass through the list, look for smallest number

list 1 : 85 79 92 57 68 80 . . .

list 2 : 57

Sorting Algorithm- Make a pass through the list, look for smallest number
- Write that number in another list, cross it off first list

list 1 : 85 79 92 57 68 80 . . .

list 2 : 57 68

Sorting Algorithm- Make a pass through the list, look for smallest number
- Write that number in another list, cross it off first list
- Repeat process, always look for smallest number remaining

Sorting Algorithm

- Make a pass through the list, look for smallest number
- Write that number in another column, cross it off first list
- Repeat process, always look for smallest number remaining
- Stop when all numbers have been crossed off

Sequential Search in a Sorted List

- Note difference from previous search

void search_ord ( int list[ ], int target, int length, int & index, boolean & found)

{ index = 0; list [length] = target; // store an item beyond end while (target > list [index]) index++; found = (index < length && ltem = = list[index]; )

Explain how the last statement works

Inserting into an Ordered List

- We wish to insert a new number into the list in the right position
- find where it goes -- look until you find a number bigger than the new number

59

length : 5

list 2 : 14 22 45 61 87

Inserting into an Ordered List- We wish to insert a new number into the list in the right position
- find where it goes -- look until you find a number bigger than the new number
- shift that number all the rest of the elements down

59

length : 5

list 2 : 14 22 45 59 61 87

Inserting into an Ordered List- We wish to insert a new number into the list in the right position
- find where it goes -- look until you find a number bigger than the new number
- shift that number all the rest of the elements down
- insert the new number in the vacated spot

59

length : 5

Inserting into an Ordered List

- We wish to insert a new number into the list in the right position
- find where it goes -- look until you find a number bigger than the new number
- shift that number all the rest of the elements down
- insert the new number in the vacated spot
- be sure to increment the length

length : 5

length : 6

Binary Search in an Ordered List

- Examines the element in the middle of the array.
- Is it the sought item?
- If so, stop searching.
- Is the middle element too small?
- Then start looking in second half of array.
- Is the middle element too large?
- Then begin looking in first half of the array.
- Repeat the process in the half of the list that should be examined next.
- Stop when
- item is found, or when
- there is nowhere else to look and it has not been located.

String Library Routines

- String assignment

String comparison:returns -1 if s1 < s2returns 0 if they are equalreturns +1 if s1 > s2

Returns length of the string

Using typedef with Arrays

- Specify an array type
- this can be used throughout program
- helps program self document
- Example :

typedef char default_string [80]; . . .

defalt_string fname, descrip;void reverse (default_string s);

Two dimensional Arrays

- A collection of components
- all of the same type
- structured in TWO dimensions
- each component accessed by a PAIR of indices representing the component’s position in each dimension

0 1 2 3 4

0

1

2

3

Which cell isLocation (2,3) ?

0

1

2

3

Declaring Two Dimensional Arrays- Syntax: data_type array_name [row_dim][col_dim];
- Example:
- First element isint_table[0][0]
- Last element isint_table[4][3]

int int_table [5][4];

Processing Two-D Arrays

- Arrays processed in some pattern
- random
- along rows
- along columns
- whole array
- We will use the declaration shown below:

int int_table [5][4];int row, col;

Processing Two-D Arrays

- What does the routine below do with the array? What should we name the function?

total_a_row

0

Processing Two-D Arrays

- What does this routine below do with the array? What should we name the function?

total_column

0

Processing Two-D Arrays

- This function initializes an array.
- Fill in the blanks with the correct identifiers

3

col

table

value

row

row

col

col

col

Printing a Table- We must process each row, item by item
- Which will be the inner loop?Which will be the outer loop?
- The LCV of the inner loop must be the one which changes the most often

What goeshere?

endl

Passing Arrays as Parameters

- Recall declaration for 1-D array
- we didn’t specify the size in the brackets
- we sent the size as a separate parameter
- Recall name of the array is a pointer constant
- tells where the array starts in memory
- this is what is passed to the function

void whatever ( float num_list [ ], int size);

Passing 2-D Arrays as Parameters

- For a 2-D array, declare
- We are sending the starting address
- also how many elements it takes to jump us to the next row
- that is -- the number of columns
- Note that this could be for an array for ANY number of rows, but exactly 4 columns
- As with 1-D, we also send another parameter for the function as a limiting value

void whatever ( float num_table [ ][4], int num_rows);

Alternate 2-D Array Definition

- Think of the 2-D array as an array of arrays
- Example
- Each element of renters is an array of rent_pmts

typedef float rent_pmts [12];rent_pmts renters [6];

0 1 … 10 11

0

1

…

5

Multidimensional Arrays

- C++ arrays not limited to two dimensions
- limitation is amount of memory

const int NUM_BLDGS = 10;const int APTS_PER_BLDG = 12;float apt_pmts [NUM_BLDGS][6][APTS_PER_BLDG];

Multidimensional Arrays

- C++ arrays not limited to two dimensions
- limitation is amount of memory

const int NUM_BLDGS = 10;const int APTS_PER_BLDG = 12;float apt_pmts [NUM_BLDGS][6][APTS_PER_BLDG];

This gives a 3-D array with 720 itemsapt_pmt [bldg][tennant][apt]

Testing and Debugging

- Initialize all components of an array
- no guarantee of what is there to start with
- Use the same number of indeces as the declaration of array
- Make sure indeces are in order
- don’t reverse row and column references

Testing and Debugging

- Use meaningful identifiers for the array name and indeces
- Double check upper and lower bounds on indeces
- don’t walk off the edge of the array
- When declaring multidimensional array as a formal parameter
- must state sizes of all but first dimension

Testing and Debugging

- When calling function with array as parameter
- sizes of multi-dim actual parameter must match exactly sizes of formal
- Use typedef statement to define multi-dimensional array type
- use this for actual and formal parameter declaration

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