1. EECE.2160 ECE Application Programming
Instructor:
Dr. Michael Geiger
Spring 2017
Lecture 21
2-D arrays; arrays and functions

2. ECE Application Programming: Lecture 21
Lecture outline
Announcements/reminders
Program 6 due 3/22
Exam 2 in class 3/29
Will be allowed one double-sided 8.5” x 11” note sheet
Review
One-dimensional arrays
Today’s lecture
Two-dimensional arrays
Arrays and functions
4/6/2019
ECE Application Programming: Lecture 21

3. ECE Application Programming: Lecture 21
Review: arrays
Arrays: groups of data with same type
int x[10] has 10 elements, x[0] through x[9]
Can also define with initial values
e.g. double list[] = {1.2, 0.75, };
Compiler will determine size of array from list
If initialization list has fewer values than size given, remaining values = 0
i.e. int list[5] = {1, 2, 3} same as
int list[5] = {1, 2, 3, 0, 0}
Must be sure to access inside bounds
You can access x[12] or x[-1], for example
Will access whatever’s at those locations
4/6/2019
ECE Application Programming: Lecture 21

4. 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]
4/6/2019
ECE Application Programming: Lecture 20

5. ECE Application Programming: Lecture 20
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 20

6. ECE Application Programming: Lecture 20
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!
4/6/2019
ECE Application Programming: Lecture 20

7. 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 21

8. ECE Application Programming: Lecture 21
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]++;
4/6/2019
ECE Application Programming: Lecture 21

9. 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 21

10. ECE Application Programming: Lecture 21
Example
Write a function for each of the following
findAvg(): Given an array of doubles (arr) and the # of elements in the array (n), find the average of all array elements
findMax(): Given an array of ints (arr) and the # of elements (n), find the largest (i.e., most positive) element in the array
sclAry(): 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
4/6/2019
ECE Application Programming: Lecture 21

11. 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; }
4/6/2019
ECE Application Programming: Lecture 21

12. Passing Arrays to functions (findBig)
//******************************************* // 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; }
4/6/2019
ECE Application Programming: Lecture 21

13. 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; }
4/6/2019
ECE Application Programming: Lecture 21

14. 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 21

15. 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 ???
4/6/2019
ECE Application Programming: Lecture 21

16. 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 21

17. ECE Application Programming: Lecture 21
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!
Array arguments to functions are always passed by address
4/6/2019
ECE Application Programming: Lecture 21

18. ECE Application Programming: Lecture 21
Final notes
Next time
Spring Break!
Monday, 3/20: Character arrays and strings
Reminders:
Program 6 due 3/22
Exam 2 in class 3/29
Will be allowed one double-sided 8.5” x 11” note sheet
4/6/2019
ECE Application Programming: Lecture 21