1. EECE.2160 ECE Application Programming
Instructor:
Dr. Michael Geiger
Fall 2018
Lecture 22
Arrays and functions

2. ECE Application Programming: Lecture 22
Lecture outline
Announcements/reminders
Program 5 due 11/2  11/7
Exam 2 in class Monday, 11/5
Will cover lectures (except lecture 16)
Lec. 25: Exam 2 Preview (Fri. 11/2)
Today’s lecture
Review: one- and two-dimensional arrays
Arrays and functions
5/14/2019
ECE Application Programming: Lecture 22

3. ECE Application Programming: Lecture 22
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
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ECE Application Programming: Lecture 22

4. Review: 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
Typically use nested for loops to access
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 22

5. Review: 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 22

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!
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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 20

8. ECE Application Programming: Lecture 20
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 20

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 22

10. ECE Application Programming: Lecture 22
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
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ECE Application Programming: Lecture 22

11. Passing Arrays to functions (findAvg)
double findAvg(double arr[], int n) { int i; double sum = 0; for (i=0; i < n; i++) sum += arr[i]; return sum / n; }
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ECE Application Programming: Lecture 22

12. Passing Arrays to functions (findMax)
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 22

13. ECE Application Programming: Lecture 22
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?
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ECE Application Programming: Lecture 22

14. 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 22

15. Passing Arrays to functions (SclAry)
#include <stdio.h> void SclAry(int test [], int n, int s); void main(void) { int i; int x[]={ 51,62,73,84,95,100,66,57,48,79 }; SclAry(x,10,10); for (i=0; i<N; i++) printf("%d ",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 22

16. ECE Application Programming: Lecture 22
Passing Arrays to functions (SclAry)
Output of program:
For reference:
int x[]={ 51,62,73,84,95,100,66,57,48,79 };
Function call changed array—why?
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ECE Application Programming: Lecture 22

17. 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 22

18. ECE Application Programming: Lecture 22
Final notes
Next time
Character arrays and strings
Reminders:
Program 5 due 11/2  11/7
Exam 2 in class Monday, 11/5
Will cover lectures (except lecture 16)
Lec. 25: Exam 2 Preview (Fri. 11/2)
5/14/2019
ECE Application Programming: Lecture 22