Arrays

1. Arrays Lecture 11 Winter Quarter

2. What Is an Array? An array is a group of items of the same type under one variable name. It is an ordered list of elements where each is of the same data type. As an example, the days of the week can be represented as a one-dimensional array named daywhere Sunday is the first element, Monday the second, and so on. Winter Quarter

3. Representation Winter Quarter

4. Thinking About Arrays • The days of the month can be thought of as a two-dimensional array. Looking at a calendar, each row represents a week (which can be represented as a one-dimensional array), and the entire month is composed of an array of arrays, that is, a two-dimensional array. Winter Quarter

5. Arrays • Taking the concept further, the days of the year can be represented as a three-dimensional array, that is, an array of arrays of arrays. The first dimension is the day of the week, the second the week of the month, and the third the month of the year. Winter Quarter

6. Declaring Arrays • A one-dimensional array is represented by specifying the array name followed by a number in square brackets [ ] to indicate the array size (or number of elements). • The statement: int day[ 7 ]; would declare a seven element array of integers. • Declaring multi-dimensional arrays is similar:int month[ 4 ][ 7 ];would declare an array of integers with 4 rows and 7 columns. Winter Quarter

7. Assigning Values to Arrays • Once an array is declared, the elements are accessed by specifying the array name and the number of the element. In C, the first element is number 0, so a seven element array would have elements numbered 0 through 6. • For the week of February 6th through the 12th, the array could be assigned as follows: day[0]=6; day[1]=7; day[2]=8; day[3]=9; day[4]=10; day[5]=11; day[6]=12; • The number in the brackets is called an index (or a subscript). Winter Quarter

8. Assigning Values to Arrays • Optionally, the array can have values assigned to its elements when the array is declared. • For example, the statement: int day[ 7 ] = {6, 7, 8, 9, 10, 11, 12} ; declares a seven element array day and initializes (assigns values to) all seven of the elements. • The program on the next slide illustrates how that works. Winter Quarter

9. Program Example for Arrays /*The following program will declare the array day, assign values to the elements, and print the data to the screen. */ #include <stdio.h> int main ( ) {int n, day[ 7 ] = {6, 7, 8, 9, 10, 11, 12} ; for(n = 0 ; n < 7 ; n++ ){ printf("day[ %d ] is February %d, 2005 \n", n, day[n] ) ; } } Winter Quarter

10. Program Output of an Array • When this program is run, it will produce the following output: day[0] is February 6, 2005 day[1] is February 7, 2005 day[2] is February 8, 2005 day[3] is February 9, 2005 day[4] is February 10, 2005 day[5] is February 11, 2005 day[6] is February 12, 2005 Winter Quarter

11. Another Program Example /* Program to generate an array of 10 random numbers */ #include <stdio.h> #include <stdlib.h> #define SIZE 10 /* SIZE defined as symbolic constant */ int main ( ) { int i, numbers [SIZE] ; for(i = 0 ; i < SIZE ; i++ ) { numbers[ i ] = rand ( ); printf ( "The number[ %d ] is %d \n", i, numbers[ i ]); } } Winter Quarter

12. Problem G10 • There is a file in the common area (g10.dat) that you must copy to your directory. • Your program will be reading wristpin diameters from that file. • Your program must compute the average and the standard deviation for the wristpins. • Suggestion: Copy the file and then write a short program in to read and display the data before working on the code for the average and the standard deviation Winter Quarter

13. APPROACH • Read in data from file and sum the values as read. • Compute the average • Use a 2nd loop to compute the sum of the values – average square. • Divide by n-1 • Take square root • You now have s, the standard deviation!!! • SMILEY FACE! Winter Quarter