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Transcript of 7 methodology-of-problem-solving-120712073958-phpapp02
Methodology ofProblem Solving
Efficiently Solving Computer Programming Problems
Doncho Minkov
Telerik Software Academyacademy.telerik.com
Technical Trainer
http://minkov.it/
Table of Contents
1. Problem Solving
2. Understanding the requirements
3. Analyzing the problem
4. Brainstorming
5. Using Pen and Paper
6. Checking your ideas
2
How to Solve Problems?
1. Read and analyze the problems
2. Use a sheet of paper and a pen for sketching
3. Think up, invent and try ideas
4. Break the problem into subproblems
5. Check up your ideas
6. Choose appropriate data structures
7. Think about the efficiency
8. Implement your algorithm step-by-step
9. Thoroughly test your solution
4
Read and Analyze the Problems
Consider you are at traditional computer programming exam or contest
You have 5 problems to solve in 6 hours
First read carefully all problems and try to estimate how complex each of them is
Read the requirements, don't invent them!
Start solving the most easy problem first
Leave the most complex problem last
Approach the next problem when the previous is completely solved and well tested
6
Analyzing the Problems
Example: we are given 3 problems:
1. Set of Numbers
Count the occurrences of a number in set of numbers
2. Students
Read a set of students and print the name of the student with the highest mark
3. Binary Representation
Find the 'ones' and 'zeros' of a set of numbers in their binary representation
7
Analyzing the Problems (2)
Read carefully the problems and think a bit
Order the problems from the easiest to the most complex:
1. Set of Numbers
Whenever find the number increase a count variable with one
2. Students
Temporary student with current highest mark
3. Binary representation
If we know the 'ones', we can easily find the zeros8
Use a Sheet of Paper and a Pen
Never start solving a problem without a sheet of paper and a pen
You need to sketch your ideas
Paper and pen is the best visualization tool
Allows your brain to think efficiently
Paper works fasterthan keyboard / screen
Other visualization toolcould also work well
10
Paper and Pen
Consider the "Binary representation" problem
We can sketch it to start thinking
Some ideas immediately come, e.g.
Divide by 2 and check for the remainder
Bitwise shift right and check for the rightmost bit
Bitwise shift left and check for the leftmost bit
Count only the "zeros"
Count only the "ones"
Count both "ones" and "zeros"11
Think up, Invent and Try Ideas
First take an example of the problem
Sketch it on the sheet of paper
Next try to invent some idea that works for your example
Check if your idea will work for other examples
Try to find a case that breaks your idea
Try challenging examples and unusual cases
If you find your idea incorrect, try to fix it or just invent a new idea
13
Invent and Try Ideas – Example
Consider the "Binary Representation" problem
Idea #1: divide by 2 and check the remainder
How many times to do this?
Where to check?
Idea #2: Bitwise shift and check the left/rightmost bit
Left shift or right shift?
How many times to repeat this?
Is this fast enough?
14
Check-up Your Ideas
Check-up your ideas with examples
It is better to find a problem before the idea is implemented
When the code is written, changing radically your ideas costs a lot of time and effort
Carefully select examples for check-up
Examples should be simple enough to be checked by hand in a minute
Examples should be complex enough to cover the most general case, not just an isolated case
16
Invent New Idea If Needed
What to do when you find your idea is not working in all cases?
Try to fix your idea
Sometime a small change could fix the problem
Invent new idea and carefully check it
Iterate
It is usual that your first idea is not the best
Invent ideas, check them, try various cases, find problems, fix them, invent better idea, etc.
17
Think About the Efficiency
Think about efficiency before writing the first line of code
Estimate the expected running time(e.g. using asymptotic complexity)
Check the requirements
Will your algorithm be fast enough to conform with them
You don't want to implement your algorithm and find that it is slow when testing
You will lose your time19
Efficiency is not Always Required
Best solution is sometimes just not needed
Read carefully your problem statement
Sometimes ugly solution could work for your requirements and it will cost you less time
Example: if you need to sort n numbers, any algorithm will work when n ∈ [0..500]
Implement complex algorithms only when the problem really needs them!
20
Start Coding: Check List
Never start coding before you find correct idea that will meet the requirements
What you will write before you invent a correct idea to solve the problem?
Checklist to follow before start of coding:
Ensure you understand well the requirements
Ensure you have invented a good idea
Ensure your idea is correct
Ensure you know what data structures to use
Ensure the performance will be sufficient22
Coding Check List – Example
Checklist before start of coding:
Ensure you understand well the requirements
Yes, counts values of bits
Ensure you have invented a correct idea
Yes, the idea seems correct and is tested
Ensure the performance will be sufficient
Linear running time good performance
23
Implement yourAlgorithm Step-by-Step
"Step-by-step" approach is always better than "build all, then test"
Implement a piece of your program and test it
Then implement another piece of the program and test it
Finally put together all pieces and test it
Small increments (steps) reveal errors early
"Big-boom" integration takes more time
24
Step #1 –Check The Value Of The First Bit
What happens to the number?
Do we need the first bit anymore?
Remove it by right shift
25
int number=10;int firstBit = number & 1;
Now we have the value of the first bit
int number=10;int firstBit = number & 1;number >>= 1;
Step #1 –Test
Testing if the correct bit is extracted and checked
Get feedback as early as possible:
The result is as expected:
26
int number = 10;int firstBit = number & 1;number >>= 1;Console.WriteLine(firstBit);Console.WriteLine(number);
0 // the first bit5 // the number without the first bit
Step #2 –Get All The Bits Of a Number
How many bits we want to check?
All for the number
But how many are they
Example: the number 10
10(10) = 8 + 2 = 1010(2)– 4 bits
How the find out when to stop the checking?
When right shifting the zeros in the front get more and more
At some point the number will become zero27
Step #2 –Get All The Bits Of a Number
Until the number becomes equal to 0
1. Check the value of the bit
2. Right shift the number
28
while (number != 0){
int firstBit = number & 1;number >>= 1;Console.Write("{0} ", firstBit);
}
Step #2 –Test
Testing with 10
29
0 1 0 1
Testing with 1111
1 1 1 0 1 0 1 0 0 0 1
Seems correct
1111 = 1024 + 64 + 16 + 4 + 2 + 1 = 210 + 26 + 24 + 22 + 21 + 20
Step #3 –Check For The Occurrences Of Bit Value
So far:
We can get the values of all the bits of a number
Lets count the number of ones and zeros
30
while (number != 0){
int firstBit = number & 1;number >>= 1;if (firstBit == 1){
oneCount++;}else{
zeroCount++;}
}
Step #3 –Test
Test with number 111 (1101111(2))
The result is correct
31
ones:6 zeros:1
Test with number 1234 (10011010010(2))
The result is correct
ones:5 zeros:6
Step #4 –Count The Bits Of The Whole Set
Pass through all the numbers
Count their bits
32
int zeroCount = 0, oneCount = 0, n = 5;for (int i = 0; i < n; i++){
int number = int.Parse(Console.ReadLine());while (number != 0){
int firstBit = number & 1;number >>= 1;if (firstBit == 1) { oneCount++; }else { zeroCount++; }
}Console.WriteLine("ones:{0};zeros:{1}",oneCount,zeroCount);
}
Step #4 –Count The Bits Of The Whole Set (2)
The result is surprisingly incorrect:
The count of ones and zeros appears as to stack for all the numbers of the set!
We defined the counters outside the iteration of the set loop
Put them inside33
Input: 12345
Output: ones:1; zeros:0ones:2; zeros:1ones:4; zeros:1ones:5; zeros:3ones:7; zeros:4
The result is surprisingly incorrect:
Step #4 – Fixing the Bug
Now it is OK34
int n = 5;
for (int i = 0; i < n; i++)
{
int zeroCount = 0, oneCount = 0;
int number = int.Parse(Console.ReadLine());
while (number != 0)
{
int firstBit = number & 1;
number >>= 1;
if (firstBit == 1) { oneCount++; }
else { zeroCount++; }
}
Console.WriteLine("ones:{0}; zeros:{1}",
oneCount,zeroCount);
}
Thoroughly Test your Solution
Wise software engineers say that:
Inventing a good idea and implementing it is half of the solution
Testing is the second half of the solution
Always test thoroughly your solution
Invest in testing
One 100% solved problem is better than 2 or 3 partially solved
Testing existing problem takes less time than solving another problem from scratch
36
How to Test?
Testing could not certify absence of defects
It just reduces the defects rate
Well tested solutions are more likely to be correct
Start testing with a good representative of the general case
Not a small isolated case
Large and complex test, but
Small enough to be easily checkable
37
How to Test? (2)
Test the border cases
E.g. if n ∈ [0..500] try n=0 , n=1, n=2, n=499, n=500
If a bug is found, repeat all tests after fixing it to avoid regressions
Run a load test
How to be sure that your algorithm is fast enough to meet the requirements?
Use copy-pasting to generate large test data
38
Read the Problem Statement
Read carefully the problem statement
Does your solution print exactly what is expected?
Does your output follow the requested format?
Did you remove your debug printouts?
Be sure to solve the requested problem, not the problem you think is requested!
Example: "Write a program to print the number of permutations on n elements" means to print a single number, not a set of permutations!
39
Testing – Example
Test with a set of 10 numbers
Serious error found change the algorithm
Change the algorithm
The counters never reset to zero
Test whether the new algorithm works
Test with 1 number
Test with 2 numbers
Test with 0 numbers
Load test with 52 000 numbers
40
Test With 10000 Numbers –Example
41
int n = 10000, startNumber=111;
for (int i = startNumber; i < startNumber + n; i++){
int zeroCount = 0;int oneCount = 0;//int number = int.Parse(Console.ReadLine());int number = i;int originalNumber = number;while (number != 0){
int firstBit = number & 1;number >>= 1;if (firstBit == 1){ oneCount++; }else { zeroCount++; }
}}
Replace the reading from the console with a easier type of check
Test With 10000 Numbers –Example (2)
The result is perfect:
42
111(10) = 1101111(2) -> ones:6;zeros:1
…
10110(10) = 10011101111110(2) -> ones:10;zeros:4
Testing Tips
You are given the task of the binary representation
The program must read from the console
Integer numbers N and B
N integer numbers
While writing and testing the program
Don't read from the console
First hardcode the numbers
Later make it
When you are sure it works43
Hard-Coded Input – Example
44
// Hardcoded input data – for testing purposesint n = 5;int num1 = 11;int num2 = 127;int num3 = 0;int num4 = 255;int num5 = 24;
// Read the input from the console//int n = int.Parse(Console.ReadLine());//for (int i=0; i<n; i++)//{// int num = int.Parse(Console.ReadLine());// // Process the number …// // …//}
// Now count the bits for these n numbers …
SummaryProblems solving needs methodology:
Understanding and analyzing problems
Using a sheet of paper and a pen for sketching
Thinking up, inventing and trying ideas
Decomposing problems into subproblems
Selecting appropriate data structures
Thinking about the efficiency and performance
Implementing step-by-step
Testing the nominal case, border cases and efficiency
45
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