1. 16.216 ECE Application Programming
Instructor: Dr. Michael Geiger
Spring 2012
Lecture 32: 2-dimensional arrays

2. ECE Application Programming: Lecture 32
Lecture outline
Announcements/reminders
Program 7 due today
Program 8 to be posted ASAP
Grading on previous programs to be completed ASAP
Today
Two-dimensional arrays
6/12/2018
ECE Application Programming: Lecture 32

3. 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]
6/12/2018
ECE Application Programming: Lecture 32

4. ECE Application Programming: Lecture 32
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
6/12/2018
ECE Application Programming: Lecture 32

5. ECE Application Programming: Lecture 32
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!
6/12/2018
ECE Application Programming: Lecture 32

6. 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; }
6/12/2018
ECE Application Programming: Lecture 32

7. ECE Application Programming: Lecture 32
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]++;
6/12/2018
ECE Application Programming: Lecture 32

8. 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);
... }
6/12/2018
ECE Application Programming: Lecture 32

9. 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
6/12/2018
ECE Application Programming: Lecture 32

10. ECE Application Programming: Lecture 32
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;
6/12/2018
ECE Application Programming: Lecture 32

11. 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;
6/12/2018
ECE Application Programming: Lecture 32

12. ECE Application Programming: Lecture 32
Next time
File I/O
6/12/2018
ECE Application Programming: Lecture 32