About Class #18

Swap First and Second

Write a function called swappy:

• it has one parameter, a list
• it'll create an entirely new list (how?) that's copied from the list passed in
• it'll swap the first and second element of the new list
• if the list is less than two elements, it'll just give back the original list
• finally, its return value is the new list with the first and second element swapped

print(swappy(['a', 'b', 'c']))
# prints ['b', 'c', 'a']

Swap First and Second

Here's one way to do it…

def swappy(my_list):
	# create a copy of the original list
	new_list = my_list[:]
	if len(new_list) >= 2:
		new_list[0], new_list[1] = new_list[1], new_list[0]
    return new_list

An Average Point

Write a function called average_point:

• it has one parameter, a list of lists
• each sub list represents a point on the coordinate plain, so each will be composed of two elements
• for example: [[0, 0], [5, 10]] … is a list of two points (0, 0) and (5, 10)
• it'll create a new single point where the x value is the average of all of the x's and the y value is average of all of the y's
• finally, it'll return this new single point

print(average_point([[2, 2], [3, 1], [4, 0]]))
# prints out [3.0, 1.0]

An Average Point Solution

Nested lists can be tricky sometimes, but here's a solution that could work:

def average_point(points):
	x_total = 0
	y_total = 0
	for p in points:
		x_total += p[0]
		y_total += p[1]
	return [x_total / len(points), y_total / len(points)]
	

All Y

Using the same list of points (a list of lists) as the previous example, write a function called all_y:

• it should take a list of points as one parameter
• it should return a new list with only points where the y value is greater than the x value

print(all_y([[2, 200], [3, 1], [4, 9]])) # --> [[2, 200], [4, 99]]

def all_y(points):
	new_points = []
	for point in points:
		if point[1] > point[0]:
			new_points.append(point)
	return new_points

Sorting an List Without Sort!

1. go through each position in the list and look at that element
2. compare the value of the current element with the value of the next element
3. if the value of the current element goes after the value of th next element, swap elements
4. remember, the idiomatic way of swapping in python is a, b = b, a
5. of course, once you're at the last element, there's no next element to check
6. once you've gone through all of the elements… REPEAT the whole process if you've swapped at least once