Dynamic Programming

Knapsack Problem
(背包問題)

How to be a smart thief? 

Carrying capacity is limited

 

Of course, the thief will try his best to take out the maximum value in his backpack

A thief, An apartment (no one is there)

radio laptop guitar
$30,000 $20,000 $15,000
4kg 3kg 1kg

Capacity: 4 kg

Grokking Algorithms: An illustrated guide for programmers and other curious people. Ch9 p161

A simple way...

Put 📻  into 📦

or 
Don't put 📻  into 📦

Put 💻  into  📦

or 
Don't put 💻  into 📦

Put 🎸 into  📦

or 
Don't put 🎸 into 📦

Grokking Algorithms: An illustrated guide for programmers and other curious people. Ch9 p161

A simple way...

Put 📻  into 📦

or 
Don't put 📻  into 📦

Put 💻  into  📦

or 
Don't put 💻  into 📦

Put 🎸 into  📦

or 
Don't put 🎸 into 📦

2   x   2   x   2   = 8

Your bag  empty = $0  📻 = $30,000
 💻 = $20,000  🎸 = $15,000  📻 💻 = $50,000
 💻 🎸 = $35,000  📻 🎸 = $45,000  💻 📻 🎸 = $65,000

If there were one more extra item ...

the thief need to calculate           of possiblities

2^n

If there were 10 items, the thief   would  calculate        of possibilities, which is equal to 1,024

2^{10}

If there were one more extra item ...

Besides maximum value, the thief hasn't considered the weight yet, coz the bag has a capacity limit. 

Your bag  empty = $0  📻 = $30,000
 💻 = $20,000  🎸 = $15,000  📻 💻 = $50,000
 💻 🎸 = $35,000  📻 🎸 = $45,000  💻 📻 🎸 = $65,000

Besides maximum value, the thief hasn't considered the weight yet, coz the bag has a capacity limit. 

x

x

x

Your bag
capacity : 4 kg
 empty = $0  
(v) waste to check
 📻 = $30,000
4kg (v)
 💻 = $20,000
3kg (v)
 🎸 = $15,000
1kg (v)
 📻 💻 = $50,000
7kg (x)
 💻 🎸 = $35,000
4kg (v)
 📻 🎸 = $45,000
5kg (x)
 💻 📻 🎸 = $65,000
8kg(x)

How to be smart?

Let's take out your notebook

Step 1

1. Divide the backpack into a smaller backpack.

If we can solve the max value of 1 kg and 3 kg bag, then we can solve the max value of 4 kg bag.

Capacity: 4 kg

Capacity: 1 kg

Capacity: 3 kg

Step 2

2. Take a note of the max value on every small bags

Then we can store it for future usage, that is to say, we can make a comparison of the max value on each item.

📦capacity: 1 kg 📦capacity: 2 kg 📦capacity: 3 kg 📦capacity: 4 kg

 

 

 

Your notebook

1. Divide the backpack into a smaller backpack.

2. Calculate the maximum value of each backpack.

3. Record the maximum value on the table

📦capacity: 1 kg 📦capacity: 2 kg 📦capacity: 3 kg 📦capacity: 4 kg
 🎸
1kg 

 

 

Your notebook

$15000
🎸

$15000
🎸

$15000
🎸

$15000
🎸

📻  💻  🎸 
$30,000 $20,000 $15,000
4kg 3kg 1kg
📦capacity: 1 kg 📦capacity: 2 kg 📦capacity: 3 kg 📦capacity: 4 kg
 🎸
1kg
​$15000
🎸
​$15000
🎸
​$15000
🎸
​$15000
🎸
📻
4kg

 

Your notebook

📻  💻  🎸 
$30,000 $20,000 $15,000
4kg 3kg 1kg

​$15000
🎸

​$15000
🎸

​$15000
🎸

​$30000
📻 

📻 or 👆

📦capacity: 1 kg 📦capacity: 2 kg 📦capacity: 3 kg 📦capacity: 4 kg
 🎸
1kg
​$15000
🎸
​$15000
🎸
​$15000
🎸
​$15000
🎸
📻
4kg
​$15000
🎸
​$15000
🎸
​$15000
🎸
​$30000
📻 
💻
3kg

Your notebook

📻  💻  🎸 
$30,000 $20,000 $15,000
4kg 3kg 1kg

​$15000
🎸

​$15000
🎸

💻  or 👆

​$20000
💻 

💻 + 1 or 👆

​$35000
💻 🎸

What if the thief saw an iPhone before he left?

📱 $20,000 /
1kg

Grokking Algorithms: An illustrated guide for programmers and other curious people. Ch9 p171

📦capacity: 1 kg 📦capacity: 2 kg 📦capacity: 3 kg 📦capacity: 4 kg
 🎸
1kg
​$15000
🎸
​$15000
🎸
​$15000
🎸
​$15000
🎸
📻
4kg
​$15000
🎸
​$15000
🎸
​$15000
🎸
​$30000
📻 
💻
3kg
​$15000
🎸
​$15000
🎸
​$20000
💻 
​$35000
💻 🎸
📱
1kg

Your notebook

📻  💻  🎸  📱
$30,000 $20,000 $15,000 $20,000
4kg 3kg 1kg 1kg

📱  or 👆

$20000

📱

📱+ 1  or 👆

📱+ 2  or 👆

📱+ 3  or 👆

$35000

📱🎸

$35000

📱🎸

$40000

📱 💻

We made it!

📦capacity: 1 kg 📦capacity: 2 kg 📦capacity: 3 kg 📦capacity: 4 kg
 🎸
1kg
​$15000
🎸
​$15000
🎸
​$15000
🎸
​$15000
🎸
📻
4kg
​$15000
🎸
​$15000
🎸
​$15000
🎸
​$30000
📻 
💻
3kg
​$15000
🎸
​$15000
🎸
​$20000
💻 
​$35000
💻 🎸
📱
1kg
​$20000
📱
$35000
📱🎸
$35000
📱​🎸
$40000
📱💻

We made it!

small bag1
capacity
small bag2
capacity
... bag W
capacity
item 1
weight
item 2
weight
...
 
item n
weight

We made it!

Our calculation time was       , but what about now?

is the amount of items

2^n
n

The calculation time is                      now.

n*W

is the amount of all kinds of backpack

W

Complexity?

Time Space
Exhaustive method
Dynamic programming
O(2^n)
O(1)
O(nW)
O(nW)

Complexity?

 

  • Divide a problem into sub-problems.
     

  • Solve each sub-problems and store it for future usage

Dynamic Programming

Lovely Rabbits

5

0

1

4

2

3

month pair
0 1
1 1
2 2
3 3
4 5
5 8

Find out the regular pattern

  1. The number of rabbits will be the same as the previous month at least.
  2. It takes 2 months to give birth since the rabbits are born.
  1. The life of rabbits is eternal. 
  2. Every rabbit can only give birth to male and female twins every time.

Assume 

Clue

Fibonacci Problem

f(0)=1
f(1)=1
f(n)=f(n-1)+f(n-2)

for n > 1,

function fibonacci(num) {
  if (num <= 1) return 1;
  return fibonacci(num - 1) + fibonacci(num - 2);
}
  1. The number of rabbits will be the same as the previous month at least.
  2. It takes 2 months to give birth since the rabbits are born.

Clue

f(n-1)
f(n-2)

Fibonacci Problem

Our calculation time is almost about       .

2^n

How to be smart?

Let's take out your notebook

How to be smart?

1. Can we divide the program into sub-programs?

2. Can we store something first and use it later?

month pair
0
1
2
3
4
5

Wrote down the result in your notebook

f(5)
f(4)
f(3)
f(2)
f(1)
f(0)

1

1

2

3

5

8

month pair
0 1
1 1
2 2
3 3
4 5
5 8

Wrote down the result in your notebook

f(5)
f(4)
f(3)
f(2)
f(1)
f(0)
month pair
0 1
1 1
2 2
3 3
4 5
5 8
6
7

Can I know              ?

f(7)
f(6)
f(5)
f(4)
f(7)

13

21

f(5)

Complexity?

Time Space
Exhaustive method
Dynamic programming
O(2^n)
O(1)
O(n)
O(n)

almost

Complexity?

Exercise

What if there were a watch?

 
$17,000 /
0.5 kg

Grokking Algorithms: An illustrated guide for programmers and other curious people. Ch9 p171

📦:
1
📦:
2
📦:
3
📦:
4
 🎸
📻
💻
📱
📦:
0.5
📦:
1
📦: 1.5 📦: 2 📦: 2.5 📦: 3 📦: 3.5 📦: 4
 🎸
📻
💻
📱

Can he steal some of rice if there were a bag of rice?

Grokking Algorithms: An illustrated guide for programmers and other curious people. Ch9 p171

 
$40,000 /
5 kg

4 kg

Fibonacci Problem

function fibonacci(num) {
  if (num <= 1) return 1;
  return fibonacci(num - 1) + fibonacci(num - 2);
}

Implement in Dynamic Programming way

 

  • Divide a problem into sub-problems.
     

  • Each table cell is your sub-problems.
     

  • Each value in a cell is what you try to optimize.
     

  • Think about how to divide your problem into sub-problems

Recap

Reference

Illustration & Material

Dynamic programming in illustrated way

By 陳Ken

Dynamic programming in illustrated way

Knapsack Problem and Fibonacci as examples

  • 644