1. 16.216 ECE Application Programming
Instructor: Dr. Michael Geiger
Fall 2012
Lecture 21:
2D arrays and functions

2. ECE Application Programming: Lecture 21
Lecture outline
Announcements/reminders
Program 5 due today
Program 6 to be posted, due 11/2
Exam 2: Wednesday, 11/7
Advising: Monday, 10/29 through Tuesday, 11/13
Today’s class
Review
Two dimensional arrays
Arrays and functions
Pointer arithmetic
Two dimensional arrays and functions
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ECE Application Programming: Lecture 21

3. ECE Application Programming: Lecture 21
Review
2D arrays: declared similarly to 1D arrays
Example (see below): int x[3][4];
Index elements similarly to 1-D arrays
Initialize: int y[3][4] =
{ {1, 2, 3, 4},
{5, 6, 7, 8},
{9, 10, 11, 12} };
Typically used with nested for loops
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 21

4. Review: arrays and pointers
Array name is a pointer
Arrays are always passed by address to functions
Can use pointer to access array
Can use pointer arithmetic to move pointer through array
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ECE Application Programming: Lecture 21

5. 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!
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ECE Application Programming: Lecture 21

6. ECE Application Programming: Lecture 21
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 21

7. 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 21

8. 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 21

9. ECE Application Programming: Lecture 21
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 21

10. 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 21

11. ECE Application Programming: Lecture 21
Next time
Character arrays and strings
Reminders:
Program 5 due 10/26
Program 6 to be posted; due 11/2
Exam 2: Wednesday, 11/7
Advising: Monday, 10/29 through Tuesday, 11/13
9/13/2018
ECE Application Programming: Lecture 21