1. Lecture 14: Problems with Lots of Similar Data

2. What is an Array?
An array is a data structure consisting of related data items of the same type.
Stored in a group of memory locations
Defining arrays
int arrayName[ 100 ];
Examples
int a[5];
float b[120], x[24]; …
a[0] 5
memory 10 15 3 23
a[1]
a[2]
a[3]
a[4]
Name of the array
Specifying the type of each element
The number of the elements

3. What is an Array? Defining arrays (cont.)
Using a symbolic constant for the size of an array.
Making programs more scalable.
#define SIZE 100
int a[ SIZE ];
float b[ SIZE ];
Using only uppercase letters for symbolic constant names.
Not a variable, cannot assign it a value in an executable statement.

4. Referring to Array Elements
Formally called a subscript or an index
Format
arrayName[ position number ]
First element at position 0
The i-th element of array a is referred to as a[i-1]
A subscript must be an integer or an integer expression. Ex.
(k = 2, j = 1)
a[k+j] += 12;
The expression is evaluated to
determine the subscript.
Name of array (all elements of this array have the same name a) …
a[0] 5
memory 10 15 3 23
a[1]
a[2]
a[3]
a[4]
subscript of the element within array a

5. Referring to Array Elements
An array a of size of n has the following elements:
a[0], a[1], a[2], …, a[n-1]
Array elements are like normal variables
printf(“%d”, a[0]*a[3]);
x = (a[2]+a[3])/2;
For a defined array without initialization, the values of the elements are undefined.
Avoid to referring to an element outside the array bounds. …
a[0] 5
memory 10 15 3 23
a[1]
a[2]
a[3]
a[4]
C has NO array bounds checking to prevent the computer from referring to an element that does not exist.

6. Initializing the Array
Using a loop to initialize the array’s elements
Initialized to a uniform value
Initialized to different values - using scanf
Initializing an array in a definition with an initializer list
Defining an array followed by an equals sign and braces, { }, containing a comma-separated list of initializers.
int n[5] = {1, 2, 3, 4, 5};
If not enough initializers, rightmost elements become 0.
int n[5] = {1, 2}
int n[5] = {0} all elements are 0.
If too many initializers, a syntax error occurs
If size omitted, # of initializers determine the size
int n[ ] = {1, 2, 3, 4, 5, 6};
6 initializers, therefore 6 element array.

7. Practice Question Q. What is the output of the following program?
#include<stdio.h>
int aFunc( int );
int x = 1; /* global variable */
int main( void ) {
int y = 0;
x = 10;
y = aFunc( x );
printf(”%d %d\n", x, y);
return 0; }
int aFunc( int x)
x *= 10;
return x;
10 10
Solution: B

8. Practice Question
Q. What is the output of the following code fragment?
#define SIZE 5
int aray[SIZE] = {2, 3, 4};
printf(“%d\n”, aray[1] + aray[2] + aray[3]); 2 5 9 7
Solution: D

9. Practice Question Q. What is the value of aray[5]? 4 Not sure 5
#define SIZE 5
int aray[SIZE] = {2, 3, 4, 5};
Q. What is the value of aray[5]? 4
Not sure 5
Solution: C
Q. What is the value of aray[1]? 2
Not sure 3
Solution: D

10. Practice Question
Q. To initialize an array to be {1, 2, 3, 4, 0}, which of the following is WRONG?
int ary[ ] = {1, 2, 3, 4, 0};
int ary[5] = {0};
int k;
for (k = 0; k < 4; k++)
ary[k] = k+1;
int ary[5];
ary = {1, 2, 3, 4, 0};
int ary[5] = {1, 2, 3, 4};
Solution: C