1. 16.216 ECE Application Programming
Instructor: Dr. Michael Geiger
Summer 2012
Lecture 8
1D and 2D arrays
Arrays, pointers, and functions
Exam 2 Review

2. ECE Application Programming: Lecture 8
Lecture outline
Announcements/reminders
Program 6 due today
Lecture this Friday (8/10)
Program 7 due Friday, 8/10
Program 8 due Tuesday, 8/14
Exam 3 Thursday, 8/16
Today’s lecture: arrays
One-dimensional arrays
Arrays, pointers, and functions
Two-dimensional arrays
5/30/2018
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. Passing arrays to functions
Do not need to specify array size (for reasons I’ll explain shortly)
Compiler will actually ignore 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[ ]);
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ECE Application Programming: Lecture 8

14. 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

15. 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

16. Working with Arrays (find biggest)
// WAY 1-initialize biggest one so // far to zero big=0; for (i=0; i<8; i++) { // if x[i] > biggest one so far if (x[i]>big) { // make that the "new" biggest big=x[i]; } } printf("Biggest is %d\n",big);
x[0] 80
x[1] 75
x[2] 90
x[3]
100
x[4] 65
x[5] 88
x[6] 40
x[7] 90
??? What happens if there are only negative values in the array ???
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ECE Application Programming: Lecture 8

17. Working with Arrays (find biggest)
// WAY 2-initialize biggest one so // far to smallest possible value big= ; for (i=0; i<8; i++) { // if x[i] > biggest one so far if (x[i]>big) { // make that the "new" biggest big=x[i]; } } printf("Biggest is %d\n",big);
x[0] 80
x[1] 75
x[2] 90
x[3]
100
x[4] 65
x[5] 88
x[6] 40
x[7] 90
???What happens if program is ported to another system??? ??? (where integers are stored in 2 or 8 bytes)???
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ECE Application Programming: Lecture 8

18. Working with Arrays (find biggest)
// WAY 3-initialize biggest one so // far to smallest possible value big= INT_MIN; // BUT use symbol for (i=0; i<8; i++) { // if x[i] > biggest one so far if (x[i]>big) { // make that the "new" biggest big=x[i]; } } printf("Biggest is %d\n",big);
x[0] 80
x[1] 75
x[2] 90
x[3]
100
x[4] 65
x[5] 88
x[6] 40
x[7] 90
The above works, but there is one more (slightly better) way
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ECE Application Programming: Lecture 8

19. Working with Arrays (find biggest)
// WAY 4-initialize biggest one so // far to the first array element big= x[0]; for (i=1; i<8; i++) // start at 1 { // if x[i] > biggest one so far if (x[i]>big) { // make that the "new" biggest big=x[i]; } } printf("Biggest is %d\n",big);
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

20. Passing Arrays to functions (findBig)
//******************************************* // function findBig // On Entry: // arr[] - array with values // n number of elements to examine // On Exit: // returns biggest value in the first n // elements of test[] int findBig(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 8

21. 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 tests[], 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

22. 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 tests[], 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

23. 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

24. 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

25. 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

26. 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

27. ECE Application Programming: Lecture 8
Pointer arithmetic
When using pointers/arrays interchangeably, can make use of pointer arithmetic
Can’t change where array name points, but you can change pointer
If p is a pointer, p++ means “point to next element”
“Next element” determined by base type
Can compare pointers
p == NULL  pointer points nowhere
p == q  p and q point to same location
Example
int num[4] = {1,2,3,4}, *p;
p = num; //same as p = &num[0];
printf(“%d\n”, *p);
++p;
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ECE Application Programming: Lecture 8

28. 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

29. 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

30. 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

31. 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

32. 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

33. 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

34. Example: 2-D arrays and functions
Say we have a program that stores student exam scores in a 2-D array:
Each row represents an individual student
Each column represents one of the 3 exams
Write functions to:
Calculate the exam average for each student and store it in a 1-D array that is accessible in the main program
Assume all exams have equal weight
Calculate the average for each exam and store it in a 1-D array that is accessible in the main program
Each function takes the same arguments:
The 2-D array
The # of students in the class
The 1-D array that will be used to hold the averages
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ECE Application Programming: Lecture 8

35. ECE Application Programming: Lecture 8
Example solution
void studentAvg(double grades[][3], int nStudents, double averages[]) { int i, j; // Row/column # /* Go through each row, sum all columns, and divide by 3 to get each student’s avg */ for (i = 0; i < nStudents; i++) { averages[i] = 0; // Initialize sum for (j = 0; j < 3; j++) { averages[i] += grades[i][j]; } averages[i] /= 3;
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ECE Application Programming: Lecture 8

36. Example solution (cont.)
void examAvg(double grades[][3], int nStudents, double averages[]) { int i, j; // Row/column # /* Go through each column, sum all rows, and divide by nStudents to get each exam avg */ for (j = 0; j < 3; j++) { averages[j] = 0; // Initialize sum for (i = 0; i < nStudents; i++) { averages[j] += grades[i][j]; } averages[j] /= nStudents;
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ECE Application Programming: Lecture 8

37. Exam 2 stats & grade distribution
Question-by-question averages:
Q1: 14.2 / 20 (71%)
Q2: 26.3 / 40 (66%)
Q3: 23.7 / 40 (59%)
Extra credit
6 students completed
6.1 average
Average: 64.2
Median: 69
Std. deviation: 22.8
Max: 93
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ECE Application Programming: Lecture 8

38. ECE Application Programming: Lecture 8
Next time
Character arrays and strings
PE5: Arrays and strings
Reminders
Program 6 due today
Lecture this Friday (8/10)
Program 7 due Friday, 8/10
Program 8 due Tuesday, 8/14
Exam 3 Thursday, 8/16
5/30/2018
ECE Application Programming: Lecture 8