C++ – Tech Blog of Edward

std::vector

admin / 7 November, 2018

Declaration

vector<int> v;

// Size of 100 and initialize all to 0
vector<int> v(100, 0)

// Reserve size of 3, to prevent extra complexity for copying
v.reserve(3);

vector<int> v;

// Size of 100 and initialize all to 0

vector<int> v(100, 0)

// Reserve size of 3, to prevent extra complexity for copying

v.reserve(3);

Simple operation

int n;
v.push_back(n);

1 2	int n; v.push_back(n);

Fill with something

fill(v.begin(), v.end(), 101);

1	fill(v.begin(), v.end(), 101);

Combine two vectors

vector<int> head, tail;

head.insert(head.end(), tail.begin(), tail.end());

vector<int> head, tail;

head.insert(head.end(), tail.begin(), tail.end());

In-place solution for creating object

class Coord {
private:
	int x, y;
public:
	Coord(int x, int y)
		: x(x), y(y) {}
};

class Coord {

private:

int x, y;

public:

Coord(int x, int y)

: x(x), y(y) {}

};

Originally, object gets created first in the main stack and then copied to the vector’s memory address

vector<Coord>v;
Coord tmp(1, 2);
v.push_back(tmp);

vector<Coord>v;

Coord tmp(1, 2);

v.push_back(tmp);

In-place object creation can be achieved and save the computing time for copying the object

vector<Coord>v;
v.emplace_back(1, 2); // Pass the parameters of the constructor directly

1 2	vector<Coord>v; v.emplace_back(1, 2); // Pass the parameters of the constructor directly

Algorithms, C++

My dynamic programming journey [2] – Coin change problem

admin / 28 October, 2018

As suggested in both Quora and StackOverflow, the coin change problem should be the second problems for beginner to solve. There are actually multiple variations of this problem, however, my approach will be finishing the basic version first, which is: Given a set of coins with values, find the possible way to form N (order is not considered). Example: Given…

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Algorithms, C++

My dynamic programming journey [1] – Fib with memorization

admin / 28 October, 2018

It is rather complicated to consider the advance level of dynamic programming at the beginning, so I have decided to start with a basic example: finding Fibonacci numbers. Since the concepts of recursion and memorization are often required in dynamic programming, therefore, I have implemented a version of memorization. Set base case of 0 and 1 equal to 1 Check…

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Algorithms, C++

Breadth first search

admin / 27 May, 2018

Steps: Add root to queue If left child not null, add it to queue If right child not null, add it to queue Repeat 2 & 3 until the queue is empty

void print_tree_bfs() {
    queue<Node *> q;
    Node *cur = this->root;
    q.push(cur);
    while (!q.empty()) {
        cur = q.front();
        cout << cur->data << " ";
        if (cur->left)
            q.push(cur->left);
        if (cur->right)
            q.push(cur->right);
        q.pop();
    }
}

void print_tree_bfs() {

queue<Node *> q;

Node *cur = this->root;

q.push(cur);

while (!q.empty()) {

cur = q.front();

cout << cur->data << " ";

if (cur->left)

q.push(cur->left);

if (cur->right)

q.push(cur->right);

q.pop();

}

C, C++, Language

CMake command

admin / 26 May, 2018

Set variable Basic usage of SET

SET(GCC_PATH    /usr/bin/gcc)
# SET variable "GCC_PATH" to "/usr/bin/gcc"

1 2	SET(GCC_PATH /usr/bin/gcc) # SET variable "GCC_PATH" to "/usr/bin/gcc"

To include existing variables in the SET

SET(GCC_VERSION    ${GCC_PATH}_7_2)
# Use ${...}
# GCC_VERSION is now SET to "/usr/bin/gcc_7_2")

SET(GCC_VERSION ${GCC_PATH}_7_2)

# Use ${...}

# GCC_VERSION is now SET to "/usr/bin/gcc_7_2")

To include multiple items in SET

SET(CUSTOM_FLAGS
    -Wall
    -Wextra
    -std=c11
)

# OR 

SET(CUSTOM_FLAGS    -Wall -Wextra -std=c11)

SET(CUSTOM_FLAGS

-Wall

-Wextra

-std=c11

)

# OR

SET(CUSTOM_FLAGS -Wall -Wextra -std=c11)

Include include path (header) Suppose we have the header files like this

/openssl/include/foo.h
/openssl/include/bar.h

/googletest/include/abc.h
/googletest/include/def.h

/openssl/include/foo.h

/openssl/include/bar.h

/googletest/include/abc.h

/googletest/include/def.h

To globally add the include path, use include_directories

include_directories(
    /openssl/include
    /googletest/include
)

include_directories(

/openssl/include

/googletest/include

)

To specially add the include path for target, use target_include_directories

target_include_directories(
    EXAMPLE_TARGET_NAME
    /openssl/include
    /googletest/include
)

target_include_directories(

EXAMPLE_TARGET_NAME

/openssl/include

/googletest/include

)

…

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C++

Google C++ style guide

admin / 10 May, 2018

Filenames should be all lowercase with underscores (_) or dashes (-) hello_world.cc All types like classes, structs, type aliases, enums, and type should be capitalized. class Person Variables should be all lowercase with underscores string table_name Data member in the class (not struct) should contain a trailing underscores string var_in_class_ Use a struct only for passive objects that carry…

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C++

C++ 11

admin / 10 May, 2018

Auto type

// BEFORE
for(vector<Person>::iterator it = personVec.begin(); it != personVec.end(); it++)
{...}

// IN C++ 11
for(auto it = personVec.begin(); it != personVec.end(); it++)
{...}

// BEFORE

for(vector<Person>::iterator it = personVec.begin(); it != personVec.end(); it++)

{...}

// IN C++ 11

for(auto it = personVec.begin(); it != personVec.end(); it++)

{...}

Auto type + Ranged-based for loop

// BEFORE
int i = 0;
for(vector<Person>::iterator it = personVec.begin(); it != personVec.end(); it++, i++){
    personVec[i].printName();
}

// IN C++ 11 *Work only for container with begin() and end()
//     Read only access
for(auto person : personVec){
     person.printName();
}

//     To write
for(auto& person : personVec){
     person.name = "Overwritten";
}

// BEFORE

int i = 0;

for(vector<Person>::iterator it = personVec.begin(); it != personVec.end(); it++, i++){

personVec[i].printName();

}

// IN C++ 11 *Work only for container with begin() and end()

// Read only access

for(auto person : personVec){

person.printName();

}

// To write

for(auto& person : personVec){

person.name = "Overwritten";

}

Initializer list

// BEFORE
personVec.push_back(person1);
personVec.push_back(person2);
personVec.push_back(person3);

// IN C++ 11
personVec = { person1, person2, person3 }

// BEFORE

personVec.push_back(person1);

personVec.push_back(person2);

personVec.push_back(person3);

// IN C++ 11

personVec = { person1, person2, person3 }

Strongly-typed enum

// BEFORE
enum Color{ cRed, cGreen };
Color mcolor = cRed;

// IN C++ 11
enum struct Color{ Red, Green };
Color mcolor = Color::Red;

// BEFORE

enum Color{ cRed, cGreen };

Color mcolor = cRed;

// IN C++ 11

enum struct Color{ Red, Green };

Color mcolor = Color::Red;

Compile time assert

// Before C++ 11, it provides only the run time assert
assert( a == 0 ); // Terminate program if a == 0

// IN C++ 11
static_assert( a == 0 ); // Compile time assert

// Before C++ 11, it provides only the run time assert

assert( a == 0 ); // Terminate program if a == 0

// IN C++ 11

static_assert( a == 0 ); // Compile time assert

C++

fscanf problems with double

admin / 28 November, 2017

Trying to get input from the FILE* stream with image processing filter. Example One: Sample input (.txt)

2
0.7 0
0 0

0.7 0

0 0

Code snippet for scanning the double

fscanf(filterFile, "%lf", &filter.m[i][j]);

1	fscanf(filterFile, "%lf", &filter.m[i][j]);

Setting breakpoint in visual studio 2017, the filter.m returns set of values as the following

m[0][0] = 0.69999999999999996
m[0][1] = 0.00000000000000000
m[1][0] = 0.00000000000000000
m[1][1] = 0.00000000000000000

m[0][0] = 0.69999999999999996

m[0][1] = 0.00000000000000000

m[1][0] = 0.00000000000000000

m[1][1] = 0.00000000000000000

Which the first value, 0.7, wrongly being scanned as 0.69999999999999996. Example Two: Sample input (.txt)

5
0.04 0.04 0.04 0.04 0.04 
0.04 0.04 0.04 0.04 0.04 
0.04 0.04 0.04 0.04 0.04 
0.04 0.04 0.04 0.04 0.04 
0.04 0.04 0.04 0.04 0.04

0.04 0.04 0.04 0.04 0.04

…

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Algorithms, C++, std

std::map

admin / 29 July, 2017

Simple insert

map<string, int> dataMap;

// Two methods to insert
// 1. Insert with pair
dataMap.insert(pair<string, int>("TESTING", 1));

// 2. Insert with array
dataMap["TESTING"] = 1;

map<string, int> dataMap;

// Two methods to insert

// 1. Insert with pair

dataMap.insert(pair<string, int>("TESTING", 1));

// 2. Insert with array

dataMap["TESTING"] = 1;

Access to second element using key (C++ 11)

cout << dataMap.at("TESTING");

// Console output = 1

cout << dataMap.at("TESTING");

// Console output = 1

Iterate

map<string, int>::iterator iter;

for (iter = dataMap.begin(); iter != dataMap.end(); iter++) {
	string first = iter->first;
	int second = iter->second;
}

map<string, int>::iterator iter;

for (iter = dataMap.begin(); iter != dataMap.end(); iter++) {

string first = iter->first;

int second = iter->second;

}

Find element

if (dataMap.find("TESTING") != dataMap.end()) 
	{...} // Found
else
	{...} // NOT Found

if (dataMap.find("TESTING") != dataMap.end())

{...} // Found

else

{...} // NOT Found

std

std::set

admin / 29 July, 2017

Simple insert

set<int> s;
s.insert(52);

1 2	set<int> s; s.insert(52);

Quick match

const bool found = s.find(123) != s.end();

1	const bool found = s.find(123) != s.end();

Store search result

set<int>::iterator it;

it = s.find(123)

set<int>::iterator it;

it = s.find(123)

Check if duplicate insert

s.insert(1);

auto a = s.insert(1);
auto b = s.insert(4);

// Fail insertion
// a.second == 0
cout << a.second;

// Success insertion
// b.second == 1
cout << b.second;

s.insert(1);

auto a = s.insert(1);

auto b = s.insert(4);

// Fail insertion

// a.second == 0

cout << a.second;

// Success insertion

// b.second == 1

cout << b.second;