Max Value in List Array - python-3.x

Question

The following code creates a list with entered values:
def locateLargest():
matrix = []
numberOfRows = int(input("Enter the number of rows: "))
numberOfColumns = 2
for row in range(0, numberOfRows):
matrix.append([])
for column in range(0, numberOfColumns):
value = int(input("Enter a value: "))
matrix[row].append(value)
max_value = None
for value in matrix:
if not max_value:
max_value = value
elif value > max_value:
max_value = value
print(max_value)
locateLargest()
The issue I am running into is that it is asking for each value individual in the row, and is returning the maximum pair of values in the row, not the maximum value's index.
The sample run of what I should be getting is:
Enter the number of rows in the list: 3
Enter a row: 23.5 35 2 10
Enter a row: 4.5 3 45 3.5
Enter a row: 35 44 5.5 11.6
The location of the largest element is at (1,2)
Any ideas?
My current output is:
Enter the number of rows: 2
Enter the number of columns: 6
Enter a value: 2
Enter a value: 2
Enter a value: 2
Enter a value: 2
Enter a value: 2
Enter a value: 2
Enter a value: 7
Enter a value: 6
Enter a value: 4
Enter a value: 3
Enter a value: 6
Enter a value: 2
[7, 6, 4, 3, 6, 2]

Answer 1

This is not very 'pythonic' but will help you achieve your end goal and hopefully understand the process. As Łukasz mentioned, you need to do an iteration for each row, and for each column in each row:
First declare the variable to store your location:
maxPoint = [0,0]
Then enumerate your matrix such that you can get the list from each row, but also retrieve the index of the currently active row:
for idx, row in enumerate(matrix):
Find the max value in the current list of values, ie: [10, 20, 30]
maxRowValue = max(row)
Find which column this maximum value lives in, ie: [0, 1, 2, ...]
maxRowIndex = row.index(maxRowValue)
Determine if max row value is in fact greater than any other previously located points, if it is less discard it:
if maxRowValue <= matrix[maxPoint[0]][maxPoint[1]]:
continue
If the value is greater, save it to the maxPoint variable:
maxPoint = [idx, maxRowIndex]
EDIT
For the sake of completeness, here is the complete code sample with AChampion's performance improvements added:
def locateLargest():
matrix = []
numberOfRows = int(input("Enter the number of rows: "))
numberOfColumns = 2
for row in range(0, numberOfRows):
matrix.append([])
for column in range(0, numberOfColumns):
value = int(input("Enter a value: "))
matrix[row].append(value)
maxPoint = [0,0]
for rIndex, row in enumerate(matrix):
cIndex, maxRowValue = max(enumerate(row), key=lambda x: x[1])
if maxRowValue <= matrix[maxPoint[0]][maxPoint[1]]:
continue
maxPoint = [rIndex, cIndex]
print(maxPoint)
locateLargest()
EDIT 2
Here is the same algorithm without using enumerate:
currentRow = 0
for row in matrix:
maxRowValue = max(row)
maxRowIndex = row.index(maxRowValue)
if maxRowValue > matrix[maxPoint[0]][maxPoint[1]]:
maxPoint = [currentRow, maxRowIndex]
currentRow += 1

Answer 2

Using enumerate() and some generator expressions, you can reduce this code quite a bit:
Generate the rows
Generate the maximum for each row
Find the maximum across all rows
More complex perhaps than some would like:
numberOfRows = int(input("Enter the number of rows: "))
# Generate the rows
rows = (map(int, input("Enter a row: ").split()) for _ in range(numberOfRows))
# Generate the maximum for each row
max_row = (max(enumerate(data), key=lambda x: x[1]) for data in rows)
# Find the maximum across all rows
i, (j, v) = max(enumerate(max_row), key=lambda x: x[1][1])
print("The location of the largest element is at {} [{}]".format((i, j), v))
Input / Output:
Enter the number of rows: 3
Enter a row: 1 2 3
Enter a row: 3 6 3
Enter a row: 1 2 3
'The location of the largest element is at (1, 1) [6]'
If you want to see what is going on change the generators to list comprehensions:
>>> rows = [list(map(int, input("Enter a row: ").split())) for _ in range(numberOfRows)]
Enter a row: 1 2 3
Enter a row: 3 6 3
Enter a row: 1 2 3
>>> rows
[[1, 2, 3], [3, 6, 3], [1, 2, 3]]
>>> max_row = [max(enumerate(data), key=lambda x: x[1]) for data in rows]
>>> max_row
[(2, 3), (1, 6), (2, 3)]
>>> list(enumerate(max_row))
[(0, (2, 3), (1, (1, 6)), (2, (2, 3))]
^^^^^^^^^
i, (j, v)