1. EECE.2160 ECE Application Programming
Instructor:
Dr. Michael Geiger
Summer 2018
Lecture 8
Arrays

2. ECE Application Programming: Lecture 8
Lecture outline
Announcements/reminders
Program 4, Ch. 6 activities due Tuesday
Ch. 7 activities due Thursday
Program 5 to be posted; due Friday 6/15
Old spec and starter files (which will become template files) on course schedule page
Exam 2: Monday, 6/18
Will be allowed one 8.5” x 11” note sheet
Today’s lecture
One-dimensional arrays
Two-dimensional arrays
Arrays and functions
Return Exam 1 & go over solution
7/30/2019
ECE Application Programming: Lecture 8

3. Review of scalar variables
Variables (up to now) have: name type (int, float, double, char) address value
N 28C4 (int)
q 28C8 (float)
r 28CC (float) 35
3.14
8.9
e.g. Name type address value
N integer 28C4 35
q float 28C8 3.14
r float 28CC 8.9
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ECE Application Programming: Lecture 8

4. ECE Application Programming: Lecture 8
Intro to Arrays
Any single element of x may be used like any other scalar variable
x[0] 45
3044
x[1] 55
3048
x[2] 25
304C
x[3] 85
3050
x[4] 75
3054
printf("%d",x[3]); // prints 85
printf("%d",x[7]+x[1]); // prints 115
x[5] 65
3058
x[6]
100
305C
x[7] 60
3060
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ECE Application Programming: Lecture 8

5. ECE Application Programming: Lecture 8
Declaring Arrays
Define an 8 element array:
int x[8];
Elements numbered 0 to 7
Arrays in C always start with location 0 (zero based)
The initial value of each array element is unknown (just like scalar variables)
x[0] ?
3044
x[1] ?
3048
x[2] ?
304C
x[3] ?
3050
x[4] ?
3054
x[5] ?
3058
x[6] ?
305C
x[7] ?
3060
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ECE Application Programming: Lecture 8

6. Declaring/defining Arrays
double A[]={ 1.23, , 2.718, };
A[0]
1.23
4430
You can also define the values to be held in the array and instruct the compiler to figure out how many elements are needed.
Not putting a value within the [] tells the compiler to determine how many locations are needed.
A[1]
4438
A[2]
2.718
4440
A[3]
0.7071
4448
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ECE Application Programming: Lecture 8

7. Working with Arrays (input)
#include <stdio.h>
void main(void) { int x[8]; int i;
// get 8 values into x[]
for (i=0; i<8; i++) { printf("Enter test %d:",i+1); scanf("%d",&x[i]); } }
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ECE Application Programming: Lecture 8

8. Working with Arrays (input)
Sample run (user input underlined):
Enter test 1:80 Enter test 2:75 Enter test 3:90 Enter test 4:100 Enter test 5:65 Enter test 6:88 Enter test 7:40 Enter test 8:90
x[0] 80
x[1] 75
x[2] 90
x[3]
100
x[4] 65
x[5] 88
x[6] 40
x[7] 90
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ECE Application Programming: Lecture 8

9. ECE Application Programming: Lecture 8
Pitfalls
What happens if we change previous code to:
#include <stdio.h>
void main(void) { int x[8]; int i; float sum, avg; // used later
// get 8 values into x[]
for (i=0; i<=8; i++) { printf("Enter test %d:",i+1); scanf("%d",&x[i]); } }
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ECE Application Programming: Lecture 8

10. ECE Application Programming: Lecture 8
Pitfalls (cont.)
Although x has 8 elements, x[8] is not one of those elements!
Compiler will not stop you from accessing elements outside the array
Must make sure you know the size of the array
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ECE Application Programming: Lecture 8

11. ECE Application Programming: Lecture 8
Example
What does the following program print?
int main() {
int arr[10];
int i;
printf("First loop:\n");
for (i = 0; i < 10; i++) {
arr[i] = i * 2;
printf("arr[%d] = %d\n", i, arr[i]); }
printf("\nSecond loop:\n");
for (i = 0; i < 9; i++) {
arr[i] = arr[i] + arr[i + 1];
return 0;
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ECE Application Programming: Lecture 8

12. ECE Application Programming: Lecture 8
Example solution
First loop: arr[0] = 0 arr[1] = 2 arr[2] = 4 arr[3] = 6 arr[4] = 8 arr[5] = 10 arr[6] = 12 arr[7] = 14 arr[8] = 16 arr[9] = 18
Output continued: Second loop: arr[0] = 2 arr[1] = 6 arr[2] = 10 arr[3] = 14 arr[4] = 18 arr[5] = 22 arr[6] = 26 arr[7] = 30 arr[8] = 34
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ECE Application Programming: Lecture 8

13. Two-dimensional arrays
Two-dimensional arrays: can be used to represent tabular data
Declaration: <type> <name>[<rows>][<cols>]
Example (see below): int x[3][4];
Index elements similarly to 1-D arrays
Col. 0
Col. 1
Col. 2
Col. 3
Row 0
x[0][0]
x[0][1]
x[0][2]
x[0][3]
Row 1
x[1][0]
x[1][1]
x[1][2]
x[1][3]
Row 2
x[2][0]
x[2][1]
x[2][2]
x[2][3]
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ECE Application Programming: Lecture 8

14. ECE Application Programming: Lecture 8
Initializing 2D arrays
Can initialize similarly to 1D arrays, but must specify dimensions
Each row treated like a 1D array; rows separated by commas:
int y[3][4] = { {1, 2, 3, 4},
{5, 6, 7, 8},
{9, 10, 11, 12} }; 1 2 3 4 5 6 7 8 9 10 11 12
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ECE Application Programming: Lecture 8

15. ECE Application Programming: Lecture 8
2D arrays and loops
Typically use nested loops to work with 2-D arrays
One loop inside another:
for (i = 0; i < 3; i++) {
for (j = 0; j < 4; j++) {
x[i][j] = y[i][j] * 2; }
Be careful in loop body—switching your loop indices will cause trouble
Using x[j][i] would take you outside of the array!
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ECE Application Programming: Lecture 8

16. Example: Working with 2-D arrays
Complete this program, which counts the # of negative values in each row of a 2-D array (assume the necessary #includes are done): #define NRows 3 // # of rows #define NCols 4 // # of columns int main() { double x[NRows][NCols] = // 2-D array { { 10, 2.5, 0, 1.5}, {-2.3, -1.1, -0.2, 0}, {10.5, -6.1, 23.4, -9.2} }; int negCnt[NRows] = {0}; // Initialize entire row count array to 0 int i, j; // Row and column indices /* INSERT CODE HERE--Visit every element in array x and count the number of negative values in each row */ // Now print the row counts for (i = 0; i < NRows; i++) printf(“Row %d has %d negative values.\n”, i, negCnt[i]); return 0; }
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ECE Application Programming: Lecture 8

17. ECE Application Programming: Lecture 8
Example solution
/* Code to be added to visit every element in array x and count the number of negative values in each row */ for (i = 0; i < NRows; i++) for (j = 0; j < NCols; j++) if (x[i][j] < 0) negCnt[i]++;
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ECE Application Programming: Lecture 8

18. Passing arrays to functions
Do not need to specify array size (for reasons I’ll explain shortly)
Compiler will actually ignore 1-D array size, even if you put it in prototype
Therefore cannot check array size inside function
Prototype typically has array name and brackets to indicate you’re dealing with array
e.g. int findAvg(int arr[ ], int n);
n = # elements in array
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ECE Application Programming: Lecture 8

19. ECE Application Programming: Lecture 8
Example
Write a function for each of the following
Given an array of doubles (arr) and the # of elements in the array (n), find the average of all array elements
Given an array of ints (arr) and the # of elements (n), find the largest element in the array
Given an array of test scores (tests), the # of elements in the array (n), and an amount to scale those scores by (s), add s to every element in tests
Do not print scores in function; we’ll print in main program
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ECE Application Programming: Lecture 8

20. Passing Arrays to functions (findAvg)
//******************************************* // function findAvg // On Entry: // arr[] - array with values to avg // n number of values to avg // On Exit: // returns avg of first n elements of test[] double findAvg(double arr[], int n) { int i;
double sum=0; double avg; for (i=0; i<n; i++) sum+=arr[i]; avg = sum / n; return avg; }
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ECE Application Programming: Lecture 8

21. Passing Arrays to functions (findMax)
//******************************************* // function findMax // On Entry: // arr[] - array with values // n number of elements to examine // On Exit: // returns biggest (most positive value in // the first n elements of test[] int findMax(int arr[], int n) { int i, big; big = arr[0]; for (i=1; i<n; i++) if (arr[i]>big) big = arr[i]; return big; }
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ECE Application Programming: Lecture 21

22. ECE Application Programming: Lecture 21
SclAry() function
Consider function that takes as arguments
An array
The array size
A scaling factor to add to each element
Function can’t “return” array … so is there any point to it?
7/30/2019
ECE Application Programming: Lecture 21

23. Passing Arrays to functions (SclAry)
//******************************************* // function SclAry // On Entry: // tests[] - array with values to scale // n number of values to scale // s number of points to scale // On Exit: // The first n values of tests[] are // scaled by s points void SclAry(int test[], int n, int s) { int i; for (i=0; i<n; i++) test[i]=test[i]+s; // or use test[i]+=s; }
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ECE Application Programming: Lecture 21

24. Passing Arrays to functions (SclAry)
#include <stdio.h> void SclAry(int tests[], int n, int s); void main(void) { int i; int x[]={ 51,62,73,84,95,100,66,57,48,79 }; int N=sizeof(x)/sizeof(int); SclAry(x,N,10); for (i=0; i<N; i++) printf("%4d",x[i]); printf("\n"); } void SclAry(int test[], int n, int s) { int i; for (i=0; i<n; i++) test[i]=test[i]+s; // or use test[i]+=s; }
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ECE Application Programming: Lecture 8

25. Passing Arrays to functions (SclAry)
Output of program:
For reference:
int x[]={ 51,62,73,84,95,100,66,57,48,79 };
??? What's wrong with this picture ???
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ECE Application Programming: Lecture 8

26. Passing Arrays to functions (SclAry)
Output of program:
For reference:
int x[]={ 51,62,73,84,95,100,66,57,48,79 };
??? What's wrong with this picture ???
The array in the main program was UPDATED ... (say "Hmmmm") Does this seem contrary to all we know about functions? (say "Yes") Is this how it really works? (Yep, it is) Is your head getting ready to explode? (say "Almost")
??? SO WHAT IS GOING ON ???
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ECE Application Programming: Lecture 8

27. Passing Arrays to functions
Before call to SclAry After call to SclAry
test[0] 51
3044
test[0] 61
3044
test[1] 62
3048
test[1] 72
3048
test[2] 73
304C
test[2] 83
304C
test[3] 84
3050
test[3] 94
3050
test[4] 95
3054
test[4]
105
3054
test[5]
100
3058
test[5]
110
3058
test[6] 66
305C
test[6] 76
305C
test[7] 57
3060
test[7] 67
3060
test[8] 48
3064
test[8] 58
3064
test[9] 79
3068
test[9] 89
3068
Passing the name only (i.e. test vs. test[4]) passes the ADDRESS of element zero of the array.
Put another way: myfunc(ary) same as myfunc (&ary[0])
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ECE Application Programming: Lecture 8

28. ECE Application Programming: Lecture 8
Arrays and pointers
Array name is a pointer to first array element
Can use pointers and arrays interchangeably
You can use [ ] to “index” a pointer
Example:
int myArr[] = {1, 3, 5, 7, 9};
int *aPtr;
aPtr = myArr;
for(int i =0; i < 5; i++)
printf(“%d”, aPtr[i]);
What does this print?
 contents of array!
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ECE Application Programming: Lecture 8

29. 2-D arrays and functions
When passing 2-D array to function, can omit first dimension (rows) but must list columns
Example:
// Assume n = # of rows
int f(int arr[][4], int n);
int main() {
int x[3][4];
f(x, 3);
... }
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ECE Application Programming: Lecture 8

30. Exam 1 stats Average: 85.5 Median: 93 Std. deviation: 17.2 Max: 100
Doesn’t include XC points
Grade distribution: mostly 90s & 80s
If below these ranges, please see me
Question-by-question averages:
Q1: 40.2 / 46 (87%)
Q2: 28.5 / 34 (84%)
Q3: 16.7 / 20 (84%)
XC: 7.3 / 10 (9 responses)
7/30/2019
EECE.2160: Exam 1 Review

31. FAQ Are the exam grades curved?
No—if curve necessary, applied to final average
Typically not needed unless overall avg < 80%
In summer, tend to use spring curve as guide
Minimal improvements (<1%) to most final grades
What percentage of my overall grade is this exam?
For most, 15% (programs 50%, textbook activities 10%, Exam 2 10%, Exam 3 15%)
Minimum of Exams 1 & 2 = 10%, maximum = 15%
7/30/2019
EECE.2160: Exam 1 Review

32. FAQ (continued) Why are the extra credit points listed separately?
Final curve based on scores without extra credit
Extra credit added later to help those who did problems, rather than hurt those who didn’t
Sum of final score + extra credit used for final grade
What about the pace of the class? (Question 3d)
Several chose “too fast” but also noted that it’s a summer class, so “too fast” is kind of unavoidable
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EECE.2160: Exam 1 Review

33. ECE Application Programming: Lecture 8
Final notes
Next time
Character arrays and strings
Reminders:
Program 4, Ch. 6 activities due Tuesday
Ch. 7 activities due Thursday
Program 5 to be posted; due Friday 6/15
Old spec and starter files (which will become template files) on course schedule page
Exam 2: Monday, 6/18
Will be allowed one 8.5” x 11” note sheet
7/30/2019
ECE Application Programming: Lecture 8