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algorithms-python/dynamic_programming/knapsack.py at master · zinating/algorithms-python · GitHub
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/
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dynamic_programming
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knapsack.py
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dynamic_programming
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knapsack.py
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"""
Given weights and values of n items, put these items in a knapsack of
capacity W to get the maximum total value in the knapsack.
Note that only the integer weights 0-1 knapsack problem is solvable
using dynamic programming.
"""
def
mf_knapsack
(
i
,
wt
,
val
,
j
):
"""
This code involves the concept of memory functions. Here we solve the subproblems
which are needed unlike the below example
F is a 2D array with -1s filled up
"""
global
f
# a global dp table for knapsack
if
f
[
i
][
j
]
<
0
:
if
j
<
wt
[
i
-
1
]:
val
=
mf_knapsack
(
i
-
1
,
wt
,
val
,
j
)
else
:
val
=
max
(
mf_knapsack
(
i
-
1
,
wt
,
val
,
j
),
mf_knapsack
(
i
-
1
,
wt
,
val
,
j
-
wt
[
i
-
1
])
+
val
[
i
-
1
],
)
f
[
i
][
j
]
=
val
return
f
[
i
][
j
]
def
knapsack
(
w
,
wt
,
val
,
n
):
dp
=
[[
0
]
*
(
w
+
1
)
for
_
in
range
(
n
+
1
)]
for
i
in
range
(
1
,
n
+
1
):
for
w_
in
range
(
1
,
w
+
1
):
if
wt
[
i
-
1
]
<=
w_
:
dp
[
i
][
w_
]
=
max
(
val
[
i
-
1
]
+
dp
[
i
-
1
][
w_
-
wt
[
i
-
1
]],
dp
[
i
-
1
][
w_
])
else
:
dp
[
i
][
w_
]
=
dp
[
i
-
1
][
w_
]
return
dp
[
n
][
w_
],
dp
def
knapsack_with_example_solution
(
w
:
int
,
wt
:
list
,
val
:
list
):
"""
Solves the integer weights knapsack problem returns one of
the several possible optimal subsets.
Parameters
---------
W: int, the total maximum weight for the given knapsack problem.
wt: list, the vector of weights for all items where wt[i] is the weight
of the i-th item.
val: list, the vector of values for all items where val[i] is the value
of the i-th item
Returns
-------
optimal_val: float, the optimal value for the given knapsack problem
example_optional_set: set, the indices of one of the optimal subsets
which gave rise to the optimal value.
Examples
-------
>>> knapsack_with_example_solution(10, [1, 3, 5, 2], [10, 20, 100, 22])
(142, {2, 3, 4})
>>> knapsack_with_example_solution(6, [4, 3, 2, 3], [3, 2, 4, 4])
(8, {3, 4})
>>> knapsack_with_example_solution(6, [4, 3, 2, 3], [3, 2, 4])
Traceback (most recent call last):
...
ValueError: The number of weights must be the same as the number of values.
But got 4 weights and 3 values
"""
if
not
(
isinstance
(
wt
, (
list
,
tuple
))
and
isinstance
(
val
, (
list
,
tuple
))):
raise
ValueError
(
"Both the weights and values vectors must be either lists or tuples"
)
num_items
=
len
(
wt
)
if
num_items
!=
len
(
val
):
raise
ValueError
(
"The number of weights must be the "
"same as the number of values.
\n
But "
f"got
{
num_items
}
weights and
{
len
(
val
)
}
values"
)
for
i
in
range
(
num_items
):
if
not
isinstance
(
wt
[
i
],
int
):
raise
TypeError
(
"All weights must be integers but "
f"got weight of type
{
type
(
wt
[
i
])
}
at index
{
i
}
"
)
optimal_val
,
dp_table
=
knapsack
(
w
,
wt
,
val
,
num_items
)
example_optional_set
:
set
=
set
()
_construct_solution
(
dp_table
,
wt
,
num_items
,
w
,
example_optional_set
)
return
optimal_val
,
example_optional_set
def
_construct_solution
(
dp
:
list
,
wt
:
list
,
i
:
int
,
j
:
int
,
optimal_set
:
set
):
"""
Recursively reconstructs one of the optimal subsets given
a filled DP table and the vector of weights
Parameters
---------
dp: list of list, the table of a solved integer weight dynamic programming problem
wt: list or tuple, the vector of weights of the items
i: int, the index of the item under consideration
j: int, the current possible maximum weight
optimal_set: set, the optimal subset so far. This gets modified by the function.
Returns
-------
None
"""
# for the current item i at a maximum weight j to be part of an optimal subset,
# the optimal value at (i, j) must be greater than the optimal value at (i-1, j).
# where i - 1 means considering only the previous items at the given maximum weight
if
i
>
0
and
j
>
0
:
if
dp
[
i
-
1
][
j
]
==
dp
[
i
][
j
]:
_construct_solution
(
dp
,
wt
,
i
-
1
,
j
,
optimal_set
)
else
:
optimal_set
.
add
(
i
)
_construct_solution
(
dp
,
wt
,
i
-
1
,
j
-
wt
[
i
-
1
],
optimal_set
)
if
__name__
==
"__main__"
:
"""
Adding test case for knapsack
"""
val
=
[
3
,
2
,
4
,
4
]
wt
=
[
4
,
3
,
2
,
3
]
n
=
4
w
=
6
f
=
[[
0
]
*
(
w
+
1
)]
+
[[
0
]
+
[
-
1
]
*
(
w
+
1
)
for
_
in
range
(
n
+
1
)]
optimal_solution
,
_
=
knapsack
(
w
,
wt
,
val
,
n
)
print
(
optimal_solution
)
print
(
mf_knapsack
(
n
,
wt
,
val
,
w
))
# switched the n and w
# testing the dynamic programming problem with example
# the optimal subset for the above example are items 3 and 4
optimal_solution
,
optimal_subset
=
knapsack_with_example_solution
(
w
,
wt
,
val
)
assert
optimal_solution
==
8
assert
optimal_subset
==
{
3
,
4
}
print
(
"optimal_value = "
,
optimal_solution
)
print
(
"An optimal subset corresponding to the optimal value"
,
optimal_subset
)
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