Loops - all done. Programs due this week. Exam Tuesday, 
Nov. 24. Sections 5.1 - 5.5 and 5.7

Arrays - Chapter 8

An array is a data structure - a composite of related data 
items stored under the same name.

Why do we need arrays?  Used to store a collection of 
similar items - for example, the hours worked by all 
employees. 

without arrays you can store just one at a time or store them 
in separate variables:

double hours_1, hours_2, hours_3, ...

With arrays, you can store them all in a single variable. 
One variable declaration

double hours[10];

Allocates a group of 10 type double storage cells.


Storage model for double hours[10]

hours[0] hours[1]  hours[2] hours[3]  hours[4] ... hours[9]
    ?      ?          ?       ?         ?            ?

Each storage location name consists of 2 parts: the variable 
name (hours) and the subscript (0 through 9) in brackets.

hours[0] is the name of the first storage location, not 
hours[1]. The last storage location is hours[9], not 
hours[10].


We use the names shown above to reference the individual 
storage locations.

hours[0] = 30;
hours[1] = 20;
hours[2] = 15.5;
hours[3] = 40.0;
hours[4] = hours[3] + 10;
hours[9] = (hours[0] + hours[3]) / 2;


hours[0] hours[1]  hours[2] hours[3]  hours[4] ... hours[9]
30.0      20.0      15.5     40.0       50.0         35.0
   



printf("You worked %f hours in week %d\n", 
           hours[0], 1);
printf("You worked %f hours in week %d\n", 
           hours[1], 2);

You worked 30.00000 hours in week 1
You worked 10.00000 hours in week 2

More flexible way to declare arrays:

#define  hours_size  10

int main()
{
     double hours[hours_size];

Also creates an array with 10 storage locations for array 
elements. Can change this to 20 elements easily - just 
change the constant definition:

#define hours_size  20

We use for loops often to process arrays. Enables us to 
access all array elements in sequence and to process all the 
elements in a consistent and uniform manner.

#define hours_size 10

int main()
{
     double hours[hours_size];
     int I;        /* should be i but stupid word-processor
                         capitalizes it automatically!   */
 
     /* Set all array elements to 0 */
     for (I = 0; I < hours_size; I++)
	hours[I] = 0;

     /* What does this one do? */
     for (I = 0; I < hours_size; I++)
	hours[I] = 10 * i;
     /*  answer - sets array elements to 0, 10, 20, ... */

     /* This one is more complicated - see if you can figure
          it out. */
    hours[0] = 20;
    for (i = 1; i < hours_size; i++)
       hours[i] = hours[i-1] + 5;
    /* answer - sets array elements to 20, 25, 30, ... */
   

    /* print the array subscripts and element values */
    printf("%s    %s\n", "I", "hours[i]");
    for (i = 0; i < hours_size; i++)
       printf("%d    %f\n", i, hours[i]);

    return 0;
}

i     hours[i]
0    20.000000
1    25.000000
2    30.000000
3    35.000000
4    40.000000
5    45.000000
6    50.000000
7    55.000000
8    60.000000
9    65.000000

  

Reading data into an array:  Read the values one at a time.

for (int I = 0; I < hours_size; I++)
{
     printf("Enter value for hours[%d]: ", i);
     scanf("%lf", &hours[I]);
}

Enter value for hours[0]: 10
Enter value for hours[1]: 8
Enter value for hours[2]: 16
Enter value for hours[3]: 5
Enter value for hours[4]: 50
Enter value for hours[5]: 50
Enter value for hours[6]: 60
Enter value for hours[7]: 70
Enter value for hours[8]: 80
Enter value for hours[9]: 90

Class exercise:  Write a loop that finds the sum of all values 
in array hours. Also write a statement to find the average of 
these values. Hint: You want to be able to find 

   hours[0] + hours[1] + ... hours[hours_size - 1]

without writing all the array elements as we do above.


Class exercise:  Write a loop that finds the sum of all values 
in array hours. Also write a statement to find the average of 
these values. Hint: You want to be able to find 

   hours[0] + hours[1] + ... hours[hours_size - 1]

without writing all the array elements as we do above.

double sum, average;
sum = 0;
for (I = 0; I < hours_size; I++)
    sum += hours[I];      /* sum = sum + hours[I];  */

average = sum / hours_size;