In <[EMAIL PROTECTED]>, Farraige wrote:
> Let's say we have a table T1:
>
> A B C D E
> ---------------
> 1 4 5 7 7
> 3 4 0 0 0
>
> and we call a method mergeTable(T1, T2, [0,1], [2,4])
>
> It means that we would like to update columns C and E of table T1 with
> data from table T2 but only in case the key columns A and B are equal
> in both tables.... I grant that the given key is unique in both tables
> so if I find a row with the same key in table T2 I do merging, stop and
> go to next row in table T1...
>
> Let's say T2 looks following:
>
> A B C D E
> ---------------
> 2 2 8 8 8
> 1 4 9 9 9
>
> So after execution of our mergeTable method, the table T1 should look
> like :
>
> A B C D E
> 1 4 9 7 9
> 3 4 0 0 0
>
> The 2nd row ['3', '4', '0' ,'0', '0'] didn't change because there was
> no row in table T2 with key = 3 ,4
>
> The main part of my algorithm now looks something like ...
>
> merge(t1, t2, keyColumns, columnsToBeUpdated)
>
> .......
>
> for row_t1 in t1:
> for row_t2 in t2:
> if [row_t1[i] for i in keyColumns] == [row_t2[j] for j
> in keyColumns]:
> # the keys are the same
> for colName in columnsToBeUpdated:
> row_t1[colName] = row_t2[colName]
>
> # go outside the inner loop - we found a row with
> # the same key in the table
> break
>
> In my algorithm I have 2 for loops and I have no idea how to optimise
> it (maybe with map? )
> I call this method for very large data and the performance is a
> critical issue for me :(
Just go through the first table once and build a mapping key->row and then
go through the second table once and look for each row if the key is in
the mapping. If yes: update columns. This runs in O(2*rows) instead if
O(rows**2).
def update_table(table_a, table_b, key_columns, columns_to_be_updated):
def get_key(row):
return tuple(row[x] for x in key_columns)
key2row = dict((get_key(row), row) for row in table_a)
for row in table_b:
row_to_be_updated = key2row.get(get_key(row))
if row_to_be_updated is not None:
for column in columns_to_be_updated:
row_to_be_updated[column] = row[column]
def main():
table_a = [[1, 4, 5, 7, 7],
[3, 4, 0, 0, 0]]
table_b = [[2, 2, 8, 8, 8],
[1, 4, 9, 9, 9]]
update_table(table_a, table_b, (0, 1), (2, 4))
for row in table_a:
print row
Ciao,
Marc 'BlackJack' Rintsch
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