How do you add value to only y coordinates in a list?-CodePudding

I'm currently doing a school project where I need to code a program in python that subtracts a value from a list of coordinates. I'm not allowed to use import to make things easier.

I have the list: [[1,2],[2,3],[3,4]] and I need to subtract a value from the second value in each list inside this list.

So for an example: [[1,2],[2,3],[3,4]]
becomes: [[1,1],[2,2],[3,3]]

This is the code I have thus far:

def coordinates(y):
    for i in (y):
        (i[1] - 5)
        return (y)

I just can't get it to work though. Does anyone know a good way to do this?

CodePudding user response：

Given you're attempting to always subtract from the y coordinate, indexing into the list's set of lists would provide the easiest solution to your problem.

The following subtracts 1 from the lists second coordinate and returns the list.

def coordinates(l):
    for i in range(len(l)):
        l[i][1] = l[i][1] - 1
    return l

CodePudding user response：

Let's assume that you don't want to modify the original list. In that case:

def coordinates(l):
    return [[x, y-1] for x, y in l]

print(coordinates([[1,2],[2,3],[3,4]]))

Output:

[[1, 1], [2, 2], [3, 3]]

CodePudding user response：

I think you should make your function generic by passing everything it needs to know to is as arguments instead of hardcoding everythings into it — which will allow it to be reused with different lists and/or amounts.

This following illustrates what I mean:

def add_y(coords, y):
    for coord in coords:
        coord[1]  = y
    return coords

data = [[1,2],[2,3],[3,4]]

print(data)  # -> [[1, 2], [2, 3], [3, 4]]
data = add_y(data, -1)
print(data)  # -> [[1, 1], [2, 2], [3, 3]]