#APS105 Slides Lecture

Example 1: Use pointers to get average of an array of integers.

#include <stdio.h>

double average(int numbers[], int numbersLength) {
	int sum = 0;
	for (int i = 0; i < numbersLength; ++i) {
		// sum += numbers[i];
		sum += *(numbers + i); // using pointers to add the sum
	}
	return (double) sum / numbersLength;
	// type casting so we get a double in return instead of an int
}
int main(void) {
	int numbers[] = {1, 2, 3};
	int numbersLength = 3;
	printf("%lf", average(numbers, numbersLength));
	return 0;
}

Difference between ++i and i++:

• ++i and i++ both add 1 to the variable i
• i++ doesn't return the output with 1 added to the variable
  • e.g. if you do printf("%d", i++);, it adds 1 to i, but in the print statement it doesn't print out the updated i.
• "Unless you have a good reason not to, always prefer to use ++i" - Eyolfson

Example 2: Use arrays and pointers to get intersection of lines.

#include <stdio.h>
#include <stdbool.h>
bool linesIntersect(double m1, double b1, double m2, double b2, double *x, double *y) {
	if (m1 == m2) {
		return false;
	}
	*x = (b2 - b1) / (m1 - m2);
	*y = m1 * *x + b1;
	return true;
}

bool allLinesIntersect(double m[], double b[], int length, double *x, double *y) {
	for (int i = 0; i < length - 1; ++i) {
		if (!linesIntersect(m[i], b[i], m[i], b[i], *x, *y)) {
			return false;
		}
	}
	return true;
}

int main(void) {
	double x = 0;
	double y = 0;
	double m1 = 1.0;
	double b1 = 3.0;
	double m2 = 2.0;
	double b2 = 1.0;
	if (linesIntersect(m1, b1, m2, b2, &x, &y)) {
		printf("two lines intersect at %lf %lf", x, y);
	}
	return 0;
}

2020 Question 7: Check bit size of a computer

#include <stdio.h>
int main(void) {
    int test = 1;
    int count = 1;
    while (test > 0) {
        test *= 2; // this works because if you multiply by 2 enough times the result will have the sign flip
        ++count;
    }
    printf("\nsize: %d\n", count);
    return 0;
}

"This was a cute question that almost no one got [...] it was kind of silly." - Eyolfson

• How nice of the teaching team! Very silly, very quirky.
• Eyolfson said this is not out of scope, just not nice to ask—awesome.
  • "2020 was a bloodbath—yours is not going to be a bloodbath"