try/except if/else – how to replace 'None' output

Question

I am trying to write a function that will take a target number, and a matrix as its parameter, and will return the ‘co-ordinates’ of the target number. If the target number doesn’t exist, (-1, -1) will be returned.

It is assumed there will only be one occurrence of the target number.

Example matrix:

my_matrix = [
    [1, 4, 7, 12, 15, 1000],
    [2, 5, 19, 31, 32, 1001],
    [3, 8, 24, 33, 35, 1002],
    [40, 41, 42, 44, 45, 1003],
    [99, 100, 103, 106, 128, 1004]
]

example input/output:

target_number = 19
output = (1, 2)

Firstly I wrote the function to our put the indices of the col/row position:

def find_target(target, matrix):
    for i, row in enumerate(matrix):
        for j, col in enumerate(row):
                if col == target:
                    return i, j

This will output ‘None’ is the number doesn’t exists, so I then tried to implement if/else, then tried try/except to outout (-1, -1) if the number doesn’t exists however my attempts at implementing this seem to override the above, and output (-1, -1) even if the number exists.

My attempts:

def find_target(target, matrix):
    for i, row in enumerate(matrix):
        for j, col in enumerate(row):
            try:
                if col == target:
                    return i, j
                else:
                    raise ValueError
            except ValueError:
                return -1, -1

def find_target(target, matrix):
    for i, row in enumerate(matrix):
        for j, col in enumerate(row):
                if col == target:
                    return i, j
                else:
                    return -1, -1

def find_target(target, matrix):
    try:
        for i, row in enumerate(matrix):
            for j, col in enumerate(row):
                    if col == target:
                        return i, j
    except:
        return -1, -1

None of the above seem to give the desired output, they will either output(-1, -1) regardless of input, or will output ‘None’ if the number doesn’t exist. I’m going to keep trying to understand why this is ongoing, but any pointers would be appreciated.

Also, please note I am quite new to this; I appreciate feedback, and I always make a point of trying various methods/ implementations etc before coming here to ask! 🙂

Asked By: techie_

||

Source

Answer 1

Explanations about your failed attempts, in order:

In the 1st one, you force raising the ValueError (and so you defeat the purpose of try…except) as soon as your target value is not found, so it will always return -1, -1 except when your target value is at position (0,0).
The 2nd one is the same, without try…except.
The 3rd one will never return -1,-1 because there’s no error in the code, so the exception is never raised.

The solution is much simpler: you want to return -1,-1 if nothing else is returned, thus after your loops end:

def find_target(target, matrix):
    for i, row in enumerate(matrix):
        for j, col in enumerate(row):
                if col == target:
                    return i, j
    return -1,-1

Answered By: Swifty

Answer 2

Your this piece of code doing correct job but your indentation makes an issue:

def find_target(target, matrix):
    for i, row in enumerate(matrix):
        for j, col in enumerate(row):
                if col == target:
                    return i, j
                else:
                    return -1, -1

Simply place else: block outside the loop.

def find_target(target, matrix):
    for i, row in enumerate(matrix):
        for j, col in enumerate(row):
                if col == target:
                    return i, j
    else:
        return -1, -1

Answered By: Mehmaam

try/except if/else – how to replace 'None' output

Question:

Answers: