16.216 ECE Application Programming

1. 16.216ECE Application Programming Instructor: Dr. Michael Geiger Spring 2013 Lecture 20: 2D arrays and functions

2. Lecture outline • Announcements/reminders • Program 6 due today • Program 4, 5 regrades due 3/25 • Exam 2 Wednesday, 3/27 • Program 7 posted; due 4/1 • Today’s class • Review • Two dimensional arrays • Arrays and functions • Pointer arithmetic • Two dimensional arrays and functions ECE Application Programming: Lecture 21

3. 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 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 ECE Application Programming: Lecture 21

5. 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? • 1 3 5 7 9  contents of array! ECE Application Programming: Lecture 21

6. 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; printf(“%d\n”, *p); 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(intarr[][4], int n); int main() { int x[3][4]; f(x, 3); ... } 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 ECE Application Programming: Lecture 21

9. Example solution void studentAvg(double grades[][3], intnStudents, double averages[]) { inti, 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] = grades[i][0]; // Initialize sum for (j = 1; j < 3; j++) { averages[i] += grades[i][j]; } averages[i] /= 3; } } ECE Application Programming: Lecture 21

10. Example solution (cont.) void examAvg(double grades[][3], intnStudents, double averages[]) { inti, 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] = grades[0][j]; // Initialize sum for (i = 1; i < nStudents; i++) { averages[j] += grades[i][j]; } averages[j] /= nStudents; } } ECE Application Programming: Lecture 21

11. Next time • Exam 1 Preview • Reminders: • Program 6 due today • Program 4, 5 regrades due 3/25 • Exam 2 Wednesday, 3/27 • Program 7 posted; due 4/1 ECE Application Programming: Lecture 21