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Arrays

2. Input, Process, Overwrite
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Input, Process, Overwrite
// input 20 temps and calculate an avg
double sum = 0, temp, avg;
for (int i=0; i<20; i++) {
cout << "Enter a temp: ";
cin >> temp; // overwrites the last
sum += temp; // temp value entered }
avg = sum / 20;
There is only one temperature in memory at a time!

3. Input, Process, Overwrite
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Input, Process, Overwrite
Question: How can we quickly create 20 temperature variables that are easy to access? (And so keep ALL temps in memory once they are entered)? Answer: Arrays

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Array
Definition:
A sequence of values in memory (called Elements) where:
each Element is the same Data Type
- the number of Elements (size) is constant

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Array
Declaration Syntax: type identifier [ size ] ; where size is a Literal or Constant Integer. Examples: int score[15]; const int NUM_TEMPS = 20; float temp[NUM_TEMPS];

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Array
Declaration Syntax: type identifier [ size ] ; size can NOT be a variable! This is a Syntax Error int numTemps = 20; float temp[numTemps];

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Array
Declaration Semantics: type identifier [ num ] ; allocates a sequence of num locations of type type, indexed 0 to (num – 1)

8. Declaration Semantics: Example int a[5];
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Array
Declaration Semantics: Example int a[5];

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Array
Accessing an element: identifier [ index ] where index is an integer between 0 and (numOfEle – 1) This can be used the same as a Variable of the declared Type.

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Array
Accessing an element: identifier [ index ] The index is thought of as a subscript, so: list[3] is often translated into English as: "list sub 3"

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Array
Example 1: int a[5]; a[1] = 12; a[4] = -3; // array index out of // bounds error! a[5] = 0;

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Array
Example 2: double sum = 0, temp[20], avg; for (int i=0; i<20; i++) { cout << "Enter a temp: "; cin >> temp[i]; sum += temp[i]; } avg = sum / 20; And: all 20 temps entered are still in memory!

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Array
Style (Best Practice): Use a constant for num. of elements const int NUM_TEMPS = 20; double sum=0, temp[NUM_TEMPS], avg; for (int i=0; i<NUM_TEMPS; i++) { cout << "Enter a temp: "; cin >> temp[i]; sum += temp[i]; } avg = sum / NUM_TEMPS;

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Array
Issue: At the time a program is written, the number of elements: - may not be known, or - may need to vary (a different number each time the program is used)

15. Solution: a Partial Array consists of
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Partial Array
Solution: a Partial Array consists of
an array where only the first n elements is in use, where n is 0 to (NumElements – 1)
an integer variable that is the number of elements in use (n).
[usually] an integer constant that is the MAX number of elements.

16. Partial Array Example: // partial array of temperatures, max 20
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Partial Array
Example:
// partial array of temperatures, max 20
const int MAX_TEMPS = 20;
int numTemps = 0;// start ”empty”
double temp[MAX_TEMPS];
The program must ensure
0 <= numTemps < MAX_TEMPS

17. Partial Array Maximum Number of Elements:
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Partial Array
Maximum Number of Elements:
When the Maximum Number of Elements is not given in the problem, the programmer must make an ”intelligent guess”.
Example: a list of test scores.
What is the maximum number of tests any user would need for this program?
Guess too small: your program is unusable.
Guess too large: your program wastes memory.

18. Partial Array Example: const int MAX_TEMPS = 20;
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Partial Array
Example:
const int MAX_TEMPS = 20;
int numTemps = 0; // start ”empty”
double temp[MAX_TEMPS];
cout << ”How many temps? (0-20)”;
cin >> numTemps; //should validate 0-20
for (int i = 0; i < numTemps; i++) {
...temp[i]...// process the temps }

19. General Algorithm for Arrays
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General Algorithm for Arrays
For most problems involving an array,
use a for loop
const int MAX_ELE = whatever;
int numEle;
type a[MAX_ELE];
for (int i = 0; i < numEle; i++) {
... // do something with a[i] }

20. Common Algorithms Initialize const int MAX = 100; double a[MAX];
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Common Algorithms
Initialize
const int MAX = 100;
double a[MAX];
int num = 0; // for Partial Array
// set ALL elements to "unused"/"empty"
for (int i=0; i < MAX; i++) {
a[i] = 0.0; }

21. Common Algorithms Populate (User Input)
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Common Algorithms
Populate (User Input)
do { // validate number of values used
cout << "How many values? ";
cin >> num;
} while (num < 0 || num > MAX);
for (int i=0; i < num; i++) {
cout << "Enter a value: ";
cin >> a[i]; }

22. Common Algorithms Sum // sum of all values in the array
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Common Algorithms
Sum
// sum of all values in the array
double sum = 0.0;
for (int i=0; i < num; i++) {
sum += a[i]; }

23. Common Algorithms Max // find the highest value
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Common Algorithms
Max
// find the highest value
double m = a[0]; // start with first ele
for (int i=1; i < num; i++) {
if (a[i] > m)
m = a[i]; }

24. Common Algorithms Min // find the lowest value
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Common Algorithms
Min
// find the lowest value
double m = a[0]; // start with first ele
for (int i=1; i < num; i++) {
if (a[i] < m)
m = a[i]; }

25. Common Algorithms Min // find the lowest value
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Common Algorithms
Min
// find the lowest value
double m = a[0]; // start with first ele
for (int i=1; i < num; i++) {
if (a[i] < m)
m = a[i];
else // common mistake!
m = something else; }

26. Common Algorithms Print // print all values in an array
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Common Algorithms
Print
// print all values in an array
for (int i=0; i < num; i++) {
cout << a[i] << ” ”; }

27. Append to a Partial Array
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Common Algorithms
Append to a Partial Array
// add a new value to the end of the list
double newValue;
...
if (num < MAX) {// check if room available
a[num] = newValue;
num ++; }
// else array is full, can not append

28. Remove from Partial Array
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Common Algorithms
Remove from Partial Array

29. Remove from a Partial Array
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Common Algorithms
Remove from a Partial Array
// remove r’th ele from the list & "save" it
int r; // index of the element to remove
double rem; // copy of removed value
...
if (r>=0 && r<num) { // ensure r is "in range"
rem = a[r]; // save a copy (optional)
num--; // one less item in list
for (int i = r; i < num; i++)
a[i] = a[i+1];
a[num] = EMPTY_VALUE; // optional }
// else r is out of range, can't remove

30. Common Algorithms Linear Search
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Common Algorithms
Linear Search
// find INDEX where value found; -1=not found
double searchValue;
...
int found = -1; // assume not found
for (int i=0; i<num; i++) {
if (a[i] == searchValue)
found = i; // found in a[i] }
// after loop: found = -1 means "not found"
// found != -1 means "found in this element"

31. Common Algorithms Linear Search
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Common Algorithms
Linear Search
// find INDEX where value found; -1=not found
double searchValue;
...
int found = -1; // assume not found
for (int i=0; i<num; i++) {
if (a[i] == searchValue)
found = i;
else // common mistake!
found = -1; }

32. (Easy to Memorize) Bubble Sort
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Common Algorithms
(Easy to Memorize) Bubble Sort
for (int i=0; i<num-1; i++) {
for (int j=i+1; j<num; j++)
if (a[i] > a[j]) {
// swap a[i] with a[j]
double t = a[i];
a[i] = a[j];
a[j] = t; }

33. Bubble Sort: Decreasing Order (Hi to Low)
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Common Algorithms
Bubble Sort: Decreasing Order (Hi to Low)
for (int i=0; i<num-1; i++) {
for (int j=i+1; j<num; j++)
if (a[i] < a[j]) { // change > to <
// swap a[i] with a[j]
double t = a[i];
a[i] = a[j];
a[j] = t; }

34. Vocabulary Array Element Index Partial Array Append Populate Sort
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Vocabulary
Term
Definition
Array
An indexed sequence of values in memory where each value is the same data type and the number of values is constant
Element
One of the items (values) in an array.
Index
An integer that references an element of an array. Its value must be between 0 and the array’s size minus one.
Partial Array
An array where only the first n elements is in use, with 0 <= n < size of the array.
Append
To add a new item to the end of a list of items.
Populate
To enter data into a variable, array, or other data structure.
Sort
To order a list by values. Ex: sorted is ; increasing or decreasing