1. 16.216 ECE Application Programming
Instructor: Dr. Michael Geiger
Fall 2012
Lecture 19:
Math library
One dimensional arrays

2. ECE Application Programming: Lecture 19
Lecture outline
Announcements/reminders
Program 5 due 10/26
Today’s class
Math library
One dimensional arrays
6/23/2018
ECE Application Programming: Lecture 19

3. ANSI library functions
Example libraries
<stdio.h>: already using frequently
<stdlib.h>: various standard operations—conversion, memory allocation, etc.
<ctype.h> <string.h>: functions for manipulating characters and strings
We’ll cover these in a later lecture
<math.h>: Built-in math functions
6/23/2018
ECE Application Programming: Lecture 19

4. <math.h> functions
Trigonometric functions
All arguments of type double
Single argument in radians:
cos(r), sin(r), tan(r), acos(r), asin(r), atan(r)
Two arguments (x,y coordinates): atan2(x,y)
Remember: # in degrees = (180/pi) * (# in radians)
Exponential functions
exp(x): ex
log(x): natural log of x
log10(x): base 10 log of x
Power functions
pow(x, y): xy
sqrt(x): square root of x
Absolute value of a double: fabs()
6/23/2018
ECE Application Programming: Lecture 19

5. Example: Using math functions
Write a function to:
Given the other two sides, calculate and return the length of the hypotenuse of a right triangle
Remember, c2 = a2 + b2
Given a number in degrees, calculate and return its sine
Remember:
Sine function takes an argument in radians
(# in degrees) = (180 / π) * (# in radians)
π = 4.0 * arctan (1.0)
6/23/2018
ECE Application Programming: Lecture 19

6. ECE Application Programming: Lecture 19
Example solutions
Write a function to: given the other two sides, calculate and return the length of the hypotenuse of a right triangle
double calcHyp(double a, double b) {
return sqrt(a * a + b * b); }
6/23/2018
ECE Application Programming: Lecture 19

7. Example solutions (cont.)
Write a function to: given a number in degrees, calculate and return its sine
double calcSin(double deg) {
double rad;
rad = deg * (4.0 * atan(1.0)) / 180;
return sin(rad); }
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ECE Application Programming: Lecture 19

8. Review of scalar variables
Variables (up to now) have: name type (int, float, double, char) address value
N 28C4 (int)
q 28C8 (float)
r 28CC (float) 35
3.14
8.9
e.g. Name type address value
N integer 28C4 35
q float 28C8 3.14
r float 28CC 8.9
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ECE Application Programming: Lecture 19

9. ECE Application Programming: Lecture 19
Intro to Arrays
Any single element of x may be used like any other scalar variable
x[0] 45
3044
x[1] 55
3048
x[2] 25
304C
x[3] 85
3050
x[4] 75
3054
printf("%d",x[3]); // prints 85
printf("%d",x[7]+x[1]); // prints 115
x[5] 65
3058
x[6]
100
305C
x[7] 60
3060
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ECE Application Programming: Lecture 19

10. ECE Application Programming: Lecture 19
Declaring Arrays
Define an 8 element array:
int x[8];
Elements numbered 0 to 7
Arrays in C always start with location 0 (zero based)
The initial value of each array element is unknown (just like scalar variables)
x[0] ?
3044
x[1] ?
3048
x[2] ?
304C
x[3] ?
3050
x[4] ?
3054
x[5] ?
3058
x[6] ?
305C
x[7] ?
3060
6/23/2018
ECE Application Programming: Lecture 19

11. Declaring/defining Arrays
double A[]={ 1.23, , 2.718, };
A[0]
1.23
4430
You can also define the values to be held in the array and instruct the compiler to figure out how many elements are needed.
Not putting a value within the [] tells the compiler to determine how many locations are needed.
A[1]
4438
A[2]
2.718
4440
A[3]
0.7071
4448
6/23/2018
ECE Application Programming: Lecture 19

12. Working with Arrays (input)
#include <stdio.h>
void main(void) { int x[8]; int i;
// get 8 values into x[]
for (i=0; i<8; i++) { printf("Enter test %d:",i+1); scanf("%d",&x[i]); } }
6/23/2018
ECE Application Programming: Lecture 19

13. Working with Arrays (input)
Sample run (user input underlined):
Enter test 1:80 Enter test 2:75 Enter test 3:90 Enter test 4:100 Enter test 5:65 Enter test 6:88 Enter test 7:40 Enter test 8:90
x[0] 80
x[1] 75
x[2] 90
x[3]
100
x[4] 65
x[5] 88
x[6] 40
x[7] 90
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ECE Application Programming: Lecture 19

14. ECE Application Programming: Lecture 19
Pitfalls
What happens if we change previous code to:
#include <stdio.h>
void main(void) { int x[8]; int i; float sum, avg; // used later
// get 8 values into x[]
for (i=0; i<=8; i++) { printf("Enter test %d:",i+1); scanf("%d",&x[i]); } }
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ECE Application Programming: Lecture 19

15. ECE Application Programming: Lecture 19
Pitfalls (cont.)
Although x has 8 elements, x[8] is not one of those elements!
Compiler will not stop you from accessing elements outside the array
Must make sure you know the size of the array
6/23/2018
ECE Application Programming: Lecture 19

16. ECE Application Programming: Lecture 19
Example
What does the following program print?
int main() {
int arr[10];
int i;
printf("First loop:\n");
for (i = 0; i < 10; i++) {
arr[i] = i * 2;
printf("arr[%d] = %d\n", i, arr[i]); }
printf("\nSecond loop:\n");
for (i = 0; i < 9; i++) {
arr[i] = arr[i] + arr[i + 1];
return 0;
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ECE Application Programming: Lecture 19

17. ECE Application Programming: Lecture 19
Example solution
First loop: arr[0] = 0 arr[1] = 2 arr[2] = 4 arr[3] = 6 arr[4] = 8 arr[5] = 10 arr[6] = 12 arr[7] = 14 arr[8] = 16 arr[9] = 18
Output continued: Second loop: arr[0] = 2 arr[1] = 6 arr[2] = 10 arr[3] = 14 arr[4] = 18 arr[5] = 22 arr[6] = 26 arr[7] = 30 arr[8] = 34
6/23/2018
ECE Application Programming: Lecture 19

18. ECE Application Programming: Lecture 19
Next time
Arrays and functions
Two dimensional arrays
Reminders:
Program 5 due 10/26
6/23/2018
ECE Application Programming: Lecture 19