Stanford splash spring 2016 basic programming

Stanford Splash Spring 2016
Basic Programming
Yu-Sheng (Yosen) Chen
chen.yosen@gmail.com

Self Introduction
Yosen Chen
● 1st year Master’s student in Stanford ICME (Institute for Computational &
Math Engineering). Got an EE Master’s degree in Taiwan in 2010.
● Will be a Facebook software intern this summer
● 5 years of industry working experience in Taiwan (software/algorithm
engineer)
● 2 years of research experience in Taiwan (computer vision)
● Publications: 3 camera patents (pending), 1 computer vision patent, 1 best
paper in IEEE ITC.
● Programming experience
○ C/C++ (7+ years), Python, Matlab, Perl, Java, ...

Why Programming?
● High salary?
● Change the way people communicate/work/build relationships?
● Redefine how a device can play a role in people’s lives?
● The world is flat: Everyone can learn programming once they have access to
the internet!
Google
services
FacebookFrom glassdoor.com

What Can You Learn from This Lecture?
● Basic programming concept (control flows, functions, iterative, recursive, ...)
● Basic data structure concept: what tools can we use to deal with problems
● Basic complexity concept: how much work does it take? how many times of
basic operations, ex:
○ 10*100=1000, less than 10 operations
○ 10+10+10+...+10=1000, at least 100 operations
● Be able to deal with basic programming problems
● How to learn on your own (Programming learning never ends!!)
Note that in this lecture, I will only use C++, but the concepts are generally the
same for all languages.

Control Flows
● if/else:
○ if (condition) statement1 else statement2
○
● While:
○ while (condition) statement
○
○
Output: 10, 9, 8, 7, 6, 5, 4, 3, 2, 1,
What if we write like this: while(true) { /*do something*/ }
→ it will never end unless we add some “break;” inside the loop
body.
Reference: http://www.cplusplus.com/doc/tutorial/control/

Control Flows (cont’)
● for:
○ for (initialization①; condition②; increase③) {statement④;}
○ How it works
■ A. initialization is executed. This is executed a single time, at the beginning of the loop.
■ B. condition is checked. If it is true, the loop continues; otherwise, the loop ends.
■ C. statement(s) is executed.
■ D. Increase (or decrease) is executed, then the loop gets back to step B.
○ It goes like:
■ Start → ①②{④③②}{④③②}{④③②}...{④③②} → End
■ Start → ①② → End
○ for (;;) is identical to while(1)
Reference http://www.cplusplus.com/doc/tutorial/control/

Control Flows (cont’)
● switch:
○ to check for a value among a number of possible constant expressions.
Reference http://www.cplusplus.com/doc/tutorial/control/

Functions
● A function is a group of statements that together perform a task. Every C++
program has at least one function, which is main(), and all the most trivial
programs can define additional functions. You can divide up your code into
separate functions.
● A function can have its input(s) and output(s)
● Functions can call other functions, including itself (recursive calls)
● We can create variables inside the functions, but they will be released when
we exit from the function.
Return_type Function_name (input1, input2, …, inputN)
{
Statement1; Statement2; ...
return Output;
}
Reference http://www.tutorialspoint.com/cplusplus/cpp_functions.htm

Recursive vs. Iterative Method
● Recursive function: a function who calls itself.
○ It will recursively call itself until it reaches some stopping condition (usually its base case)
● Iterative method uses a loop structure (while, for), it terminates when loop
condition fails.
● Let’s see how to solve a problem in both way. Problem: 1*2*3*...*n = ?
Execution result:
3628800
3628800

Complexity ( the smaller, the better)
● Complexity: What is the order of the number of basic operations?
● Given an input with size N, how many basic operations will be executed in this
function/algorithm? A measure of algorithm efficiency.
● Let’s see some examples
○ Calculate 1+2+3+...+N = ?
■ Naive method: 1+2+3+...+N, Complexity: (N-1) additions.
■ Better method: (1+N)*N/2, Complexity: 1 add, 1 div, and 1 mult.
○ Is N a prime number?
■ Naive method: check if all the remainders of N/k, where k = 2, 3, 4, ... N-1, are nonzero,
then it’s a prime number. Complexity: N-2 divisions
■ Better method: check if all the remainders of N/k, where k=2, 3, 4, … sqrt(N), are nonzero,
then it’s a prime number. Complexity: sqrt(N) -1 divisions
Note: A composite number N is a positive integer which is not prime (i.e., which has
factors other than 1 and itself, at least one factor <= sqrt(N))

Array vs. Linked-List
Array
Linked-List
Remove element
Insert element
Picture credit: java67.blogspot.com https://en.wikipedia.org/wiki/Linked_list
Array Linked-List
Adv. random access,
no extra memory
Distributed memory;
fast insert, remove, resize
Disadv
.
Slow insert, remove, resize iterating access;
need memory to store next
address

Queue vs. Stack
Queue: First in first out (operations: enqueue, dequeue, see back/front)
Stack: First in last out, like crowded caltrain or elevators (operations: push/pop, see top)
Picture credit: En.wikipedia.org www.creativebloq.com quincypublicschools.com

More!
● Trees
● Hash table
Picture credit: Collegelabs.co http://www.tutorialspoint.com/data_structures_algorithms/tree_data_structure.htm
class TreeNode
{
int data;
TreeNode* leftChild;
TreeNote* rightChild;
};
By Jorge Stolfi - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=6471238
A small phone book as a
hash table
The beauty of hash table is
search/insert/delete in constant time,
regardless of the size of the table

Let’s solve some coding
problems!

Let’s See What Problems We Can Solve Now!
● Surprisingly, we now are able to solve some real interview questions!!
● Problem#1: Number of the trailing zeros in a factorial
● Problem#2: valid parenthesis expression → symmetricity problem
● Problem#3: Fibonacci sequence
● All the above have been asked in real job interviews. (software engineer, data
scientist, trader, …)

Problem#1: Num of Trailing Zeros
● Given an integer n, return the number of trailing zeroes in n!
● Analysis:
○ Don’t try to calculate the n factorial (it could be very large, overflow!)
○ Idea A: number of trailing zeros = min{p, q} where p and q are the exponent numbers of
2p
and 5q
in n!’s prime factor decomposition.
○ Ex: 210
*56
= 24
x(2x5)6
= 24
x(10)6
→ 6 trailing zeros
○ So we need to count the 2’s and 5’s exponent numbers for all numbers between 1 and
n?
○ Actually, we just have to count 5’s exponents, since num of 5’s is less than 2’s
EX: 10!
1x2x3x4x5x6x7x8x9x10, we have 1+2+1+3+1=8 2’s, but only 2 5’s
min{8, 2} is 2.
2’s multiples are way more than 5’s multiples!

Problem#1: Num of Trailing Zeros (cont’)
● So, how to count the number of 5’s?
● Idea B: for i=1 to n, we accumulate the exponents of 5 in i’s prime factor
decomposition
Convert this into code:
EX: 28!
1x2x3x4x5x6x7x8x9x10x...x15x...x20x...x25x…x28
Count: 1 1 1 1 2
Sum: 1 2 3 4 6 → 6 trailing zeros

In fact, we have a better way to do it (no need to iterate all number from 1 to n)
Idea C: EX: 28!
1x2x3x4x5x6x7x8x9x10x...x15x...x20x...x25x…x28
Count: 1 1 1 1 2 Sum = 6 → 6 trailing zeros
28/5 = 5 … 3, which means among 1 to 28, 5 of them are 5’s multiples (exponent of 5 >=1)
28/25 = 1 … 3, which means among this 5 multiples, 1 of them have exponent of 5 >=2
So the sum = 5 + 1 = 6.
One more example, EX: 152!
1x...x5x...x10x...x15x...x20x...x25x…x50x...x100x...x125x...x150x151x152
1 1 1 1 2 2 2 3 2
152/125 = 1...27, which means among this 6 multiples, 1 of them has exponent of 5 >=3
So the sum = 30+6+1 = 37.

Idea C:
Let’s convert it into code (iterative) i=5 cnt = 0 + 30
i=25 cnt = 30 + 6
i=125 cnt = 36 + 1
i=625 Condition fails
Return cnt=37
One more example, EX: 152!
1x...x5x...x10x...x15x...x20x...x25x…x50x...x100x...x125x...x150x151x152
1 1 1 1 2 2 2 3 2
152/125 = 1...27, which means among this 6 multiples, 1 of them has exponent of 5 >=3
So the sum = 30+6+1 = 37.

Interpret idea C into a recursive method (only 1 line!)
Complexity comparison between Idea A, idea B, Idea C
Ex: n = 152
trailingZeros(152) = 152/5(=30) + trailingZeros(30);
trailingZeros(0) = 0; so finally we have 30+6+1+0+0=37
Idea A Idea B Idea C
Details Count both exp of 2 and
5, find min of them
Only count exp of 5 Count exp of 5 in a
clever way
Complexity Double of Idea B’s Iterate every number
from 1 to n
Iterate only 5, 25, 125,
…, n

Problem#2: Valid Parenthesis Expression
● A good example to make use of stack
● Given a string, tell if it’s a valid parenthesis expression (all are paired properly)
○ () → OK
○ (()) → OK
○ (()()(())) → OK
○ ()( → X
○ ))(( → X
○ (()()()()()()()()()()()()()()()()()()()()()()())) → X
● Solution: use a stack to deal with all paired parenthesis, if finally the stack is
not empty, then it’s not a valid expression

Problem#2: Valid Parenthesis Expression (cont’)
● Solution:
○ Iterate through the input string, from the first characters to the last one
■ If we see a ‘(’, then push it into the stack (waiting for being paired)
■ If we see a ‘)’, then check the top of the stack,
● if it’s a ‘(’, good, then pop it, which means the incoming ‘)’ is paired with a ‘(’
● Otherwise (i.e., stack is empty), the incoming ‘)’ cannot be paired, return false
○ In the end
■ If the stack is not empty, then it means some are not paired, return false
■ If the stack is empty, then all the parentheses are paired, return true
Let’s see an example: ( ( ) ( ( ) ) ) )
iter# 1: ( 2: ( 3: ) 4: ( 5: ( 6: ) 7: ) 8: ) 9: )
Push ( Push ( Pop ( Push ( Push ( Pop ( Pop ( Pop ( Empty, can’t pop
return false!( (( ( (( ((( (( (

Problem#2: Valid Parenthesis Expression (cont’)
Convert it into code
● Iterate through the input string, from the first
characters to the last one
○ If we see a ‘(’, then push it into the
stack (waiting for being paired)
○ If we see a ‘)’, then check the top of
the stack,
■ if it’s a ‘(’, good, then pop it,
which means the incoming ‘)’ is
paired with a ‘(’
■ Otherwise (i.e., stack is empty),
the incoming ‘)’ cannot be
paired, return false
● In the end
○ If the stack is not empty, then it means
some are not paired, return false
○ If the stack is empty, then all the
parentheses are paired, return true

Problem#3: Fibonacci Sequence
● Fibonacci sequence, F(1)=F(2)=1; F(n) = F(n-1) + F(n-2)
● 1,1,2,3,5,8,13,21,34,55,89,144,.., how to write a function to find F(n) = ?
● Idea A: Intuitively, we can convert it into recursive functions
In fact, it’s very inefficient!
Lots of redundancy!
EX:
Why should we calculate F(3) 3
times and F(4) 2 times, and many
F(1)’s and F(2)’s.
In fact, F(n) has about 1.6n
recursive calls!

Problem#3: Fibonacci Sequence (cont’)
● Another approach to Fibonacci Sequence
● Idea B: for each iteration, we sum prev and prevprev as curr, then before next
iteration, we copy prev’s value to prevprev, and curr to prev
● Convert into iterative code (iterate n-2 times)
1 1 2 3 5 8 13 ...
pv=1, pvpv=1
curr = 1+1 = 2
Copy pv to pvpv (1)
Copy curr to pv (2)
pv=2, pvpv=1
curr = 2+1 = 3
Copy pv to pvpv (2)
Copy curr to pv (3)
pv=3, pvpv=2
curr = 3+2 = 5
Copy pv to pvpv (3)
Copy curr to pv (5)

Problem#3: Fibonacci Sequence (cont’)
● There are some even more advanced ideas!!
● F(n) can be solved in nearly const time! (regardless of how large n is)
● Involves Matrix computation and diagonalization! (topics in Colleges)

Learning Programming is Free!!
● Some useful online resource
○ https://www.codecademy.com/ (Learn to code interactively, recommended for beginners)
○ https://codefights.com/home (Test your skills agains some company robots)
○ https://leetcode.com/ (LeetCode Online judges, and most important, find the best rated
solutions in its “discuss” tab!)
○ http://www.cplusplus.com/ (A good website to consult with for any C++ standard
function/library)
○ https://checkio.org/ (A good website to learn Python, like a RPG game)
○ https://docs.python.org (A good website to consult with for any Python standard libs)
● It’s not hard to find resources to learn coding, the problem is how to find
answers when you have problems.
○ Google it! It always works, except for using wrong keywords.

Any Advice for New Coders?
● Be expert in one single language (strong, complete), then know basics in other 2-3
languages
○ EX: like me, C++, Python, Matlab, Perl, Java, …
○ At least be familiar with 1 compiled language and a script one.
● Be patient for debugging
○ printing out all the values stage by stage is the dumbest, but most useful way.
● Learning never ends
○ new standard, new libs, new problems, new fields, new languages…
● If you can tell a computer how to solve a problem, that means you really understand the
solution. (because computers know nothing but only digits!)
● Programming is just an approach to solving a problem.
○ It is the knowledge of the solutions that really make you irreplaceable! Not the
code itself!!!

How to find this slides
Google: slideshare YosenChen, the first result!

Stanford splash spring 2016 basic programming

More Related Content

What's hot

Viewers also liked

Similar to Stanford splash spring 2016 basic programming

Recently uploaded

Stanford splash spring 2016 basic programming