Lập trình C++ | Một số kĩ thuật cơ bản.

[rox_rook]

* Câu hỏi : làm sao để set và reset trạng thái của kiểu ouput thông qua hàm flags ?

answer :

#include <iostream> int main(){  int integer_value = 1000;  double double_value = 0.334343;    //display flags value in original format  std::cout << "-- Original format --\n";  std::cout << "The value of flags variable is : " << std::cout.flags() << "\n";  std::cout << "integer value " << integer_value << "\n";  std::cout << "double value " << double_value << "\n";      //use cout flags function to save original format  std::ios_base::fmtflags original_format = std::cout.flags();  //change format  std::cout << std::showbase << std::oct << std::scientific << "\n";     std::cout << "-- New format --\n";  std::cout << "The value of flags variable is : " << std::cout.flags() << "\n";  std::cout << "integer value " << integer_value << "\n";  std::cout << "double value " << double_value << "\n";      //now, restore format  std::cout.flags(original_format);  std::cout << "-- Restore --\n";  std::cout << "The value of flags variable is : " << std::cout.flags() << "\n";  std::cout << "integer value " << integer_value << "\n";  std::cout << "double value " << double_value << "\n";      return 0;}

Output :

-- Original format --
The value of flags variable is : 4098
integer value 1000
double value 0.334343

-- New format --
The value of flags variable is : 011500
integer value 01750
double value 3.343430e-01
-- Restore --
The value of flags variable is : 4098
integer value 1000
double value 0.334343

* Câu hỏi : làm sao in ra kết quả của việc cấp phát lỗi ?
answer :

     #include <iostream> int main(){  double* alloc_arry[5];  for(int x = 0; x < 5; ++x){    alloc_arry[x] = new double[5000000];    if(alloc_arry[x] == 0){      std::cout << "Fail allocating memory...\n" << x << "\n";      break;    }    else{      std::cout << "Successfully allocating new memory for alloc_arry"                   << "at " << x << "\n";    }    delete[] alloc_arry[x];  }    return 0;}

Output :

Successfully allocating new memory for alloc_arryat 0
Successfully allocating new memory for alloc_arryat 1
Successfully allocating new memory for alloc_arryat 2
Successfully allocating new memory for alloc_arryat 3
Successfully allocating new memory for alloc_arryat 4

* Câu hỏi : làm sao catch fail allocate memory ?
answer :

#include <iostream>#include <new> int main(){  double* alloc_arry[5];  try{    for(int x = 0; x < 5; ++x){      alloc_arry[x] = new double[99999999];      std::cout << "Allocating memory...\n" << x << "\n";    }  }  //handle exception  catch(std::bad_alloc &memory_allocation_exception){      std::cout << "Exception occurs !!"                   << memory_allocation_exception.what() << "\n";  }    return 0;}

Output

Allocating memory...
0
Allocating memory...
1
Allocating memory...
2
Exception occurs !!std::bad_alloc

* Câu hỏi : làm sao để set new handler cho fail allocating memory.
answer :

#include <iostream>#include <new> void custom_new_handler(){  std::cerr << "handle, abort...!";  abort();} int main(){  double* alloc_arry[5];  std::set_new_handler(custom_new_handler);  for(int x = 0; x < 5; ++x){      alloc_arry[x] = new double[99999999];      std::cout << "Allocating memory...\n" << x << "\n";  }    return 0;}

Output :

Allocating memory...
0
Allocating memory...
1
Allocating memory...
2
handle, abort...!

[rox_rook]

* Câu hỏi : làm sao dùng auto_ptr để allocate memory ?
answer :

#include <iostream>#include <memory> class Integer{public :  Integer(int x = 0):value(x){    std::cout << "construct an integer(..) " << value << "\n";  }  ~Integer(){    std::cout << "Destructor an integer..." << value << "\n";  }  void set(int x){     value = x;   }  int get() const{     return value;  }private :  int value;}; int main(){  std::auto_ptr<Integer> super_pointer(new Integer(99));  super_pointer->set(777);  std::cout << "Integer now : " << (*super_pointer).get() << '\n';  return 0;}

Ouput :

construct an integer(..) 99
Integer now : 777
Destructor an integer...777

* Câu hỏi : làm sao convert to C-style string ?
answer :

#include <iostream>#include <string> int main(){  std::string first("STRINGS");  const char* fir_ptr = 0;  int m_length = first.length();  char* sec_ptr = new char[m_length + 1];    first.copy(sec_ptr, m_length, 0);  sec_ptr[m_length] = '\0';    std::cout << "string first is = " << first              << "\n -> convert to C-style " << first.c_str();  std::cout << "\n";  fir_ptr = first.data();    for(int x = 0; x < m_length; ++x){    std::cout << *(fir_ptr + x);  }    std::cout << "second pointer = " << sec_ptr << "\n";  delete[] sec_ptr;    return 0;}

Output

string first is = STRINGS
 -> convert to C-style STRINGS
STRINGSsecond pointer = STRINGS

* Câu hỏi, làm sao dùng các algorithm đơn giản của STL : binary_seach, find, find_if ?
answer :

#include <iostream>#include <algorithm>#include <vector> template <typename C>inline void print_container(C& container, const char* con_name = ""){  typename C::const_iterator iter;  std::cout << con_name << "\n";  for(iter = container.begin(); iter != container.end(); ++iter){    std::cout << *iter << " - ";  }  std::cout << "\n";} bool greater_than_10(int value){  return value > 10;} int main(){  const int SIZE = 10;  int arr[SIZE] = {10,2,17,5,16,8,13,11,20,7};    std::vector<int> vect(arr, arr + SIZE);  std::cout << "vector vect contains : ";  print_container(vect, "vector");    std::vector<int>::iterator location;  location = std::find(vect.begin(), vect.end(), 16);    if(location != vect.end())    std::cout << "\n Found 16 at location " << (location - vect.begin()) << "\n";  else    std::cout << "\n 16 not found.\n";      location = std::find(vect.begin(), vect.end(), 100);  if(location != vect.end())    std::cout << "\nFound 100 at location " << (location - vect.begin()) << "\n";  else    std::cout << "\n 100 not found ";      location = std::find_if(vect.begin(), vect.end(), greater_than_10);  if(location != vect.end()){    std::cout << "\nThe first value greater than 10 is : ";    std::cout << *location << " found at location " << (location - vect.begin());  }  else{    std::cout << "No values greater than 10 were found.";  }    std::sort(vect.begin(), vect.end());  std::cout << "\nAfter sort :\n";  print_container(vect, "vector");     if(std::binary_search(vect.begin(), vect.end(), 13))    std::cout << "\n 13 was found in vect.\n";  else    std::cout << "\n 13 was not found in vect.\n";       if(std::binary_search(vect.begin(), vect.end(), 100))    std::cout << "\n 100 was found in vect.\n";  else    std::cout << "\n 100 was not found in vect.\n";      return 0;}

Output :

vector vect contains : vector
10 - 2 - 17 - 5 - 16 - 8 - 13 - 11 - 20 - 7 - 

 Found 16 at location 4

 100 not found 
The first value greater than 10 is : 17 found at location 2After sort :
vector
2 - 5 - 7 - 8 - 10 - 11 - 13 - 16 - 17 - 20 - 

 13 was found in vect.

 100 was not found in vect.

[rox_rook]

* Câu hỏi : Làm sao dùng function object in ra các phần tử trong 1 container của STL ?
answer :

#include <iostream>#include <vector>#include <algorithm> class print_single_element{public :  void operator()(int elem) const{    std::cout << elem << " ";  }}; int main(){  std::vector<int> vect;  for(int x = 0; x < 10; ++x){    vect.push_back(x);  }    for_each(vect.begin(), vect.end(), print_single_element());    return 0;}

Output :

0 1 2 3 4 5 6 7 8 9

*Câu hỏi : Làm sao dùng function object để tìm trong 1 dãy xem coi số nào là số nguyên tố ?
answer :

#include <iostream>#include <list>#include <algorithm>#include <cstdlib>#include <cmath> bool is_prime(int num){    if(num <= 1) return false;    if (num == 2 || num == 3) return true;    if (num % 2 == 0 || num % 3 == 0) return 0;     int variable = 5,        factor = 2,        _limitFactor = static_cast<int>(sqrt(static_cast<double>(num)));    while(variable <= _limitFactor){        if(num % variable == 0)            return false;        variable += factor;        factor = 6 - factor;    }    return true;} int main(){  std::list<int> numbers;    for(int x = 13; x < 49; ++x){    numbers.push_back(x);  }    std::list<int>::iterator position;  position = find_if(numbers.begin(), numbers.end(), is_prime);  if(position != numbers.end()){    std::cout << *position << " is prine number.!!\n";  }  else{    std::cout << "No prime number found.\n";  }    return 0;}

Output :

13 is prine number.!!

* Câu hỏi : Làm sao shift 1 chuỗi lên n units, ví dụ abc->bcd ?


answer :

#include <iostream>#include <string> int main() {  std::string text;  std::cout << "Please enter a string : \n";  std::cin >> text;   int units;  std::cout << "\nHow many units would you like to shift: ";  std::cin >> units;   for(std::size_t x = 0; x < text.size(); ++x){    char c = text[x] + units;    std::cout << c;  }  std::cout << "\n";  return 0;}

Ouput :

Please enter a string : 
abcdef

How many units would you like to shift: 2
cdefgh

*Câu hỏi : làm sao cắt chuỗi bằng 1 chuỗi cho trước ?
answer :

#include <iostream>#include <cstring>#include <string>#include <vector> std::vector<std::string> tokenize_string_by_delims(std::string line, std::string delims) {  std::vector<std::string> word;  char *str = new char[line.size()];  std::memcpy(str, line.c_str(), line.size());    for(char *token = std::strtok(str, delims.c_str()); token != 0;         token = std::strtok(0, delims.c_str())){    word.push_back(token);  }    delete[] str;  return word;} int main(){  std::string sentence = " first  second  third  fourth ";  std::string delims(" ");  std::vector<std::string> tokens = tokenize_string_by_delims(sentence, delims);   for(unsigned i = 0; i < tokens.size(); ++i)    std::cout << tokens[i] << "\n";      return 0;}

Output :

first
second
third
fourth

* Câu hỏi : làm sao dùng va_list, va_start, va_end, va_arg trong C bên C++ ?
answer :

#include <iostream>#include <iomanip>#include <cstdarg> double average(int count, ...){  double total = 0;  va_list list;  va_start(list, count);    for(int x = 1; x <= count; ++x){    total += va_arg(list, double);  }    va_end(list);    return total/count;} int main(){  double first = 35.7;  double second = 123.55;  double third = 13.33;  double fourth = 10.2;    std::cout << "\nAverage of first and second :";  std::cout << average(2, first, second);  std::cout << "\nAverage of first, second and third : \n";  std::cout << average(3, first, second, third);  std::cout << "\nAverage of first, second, third and fourth: \n";  std::cout << average(4, first, second, third, fourth);    return 0;}

Output :

Average of first and second :79.625
Average of first, second and third : 
57.5267
Average of first, second, third and fourth:

* Câu hỏi : làm sao dùng atexit và exit để kết thúc chương trình ?
answer :

#include <iostream>#include <cstdlib> void print_message(){  std::cout << "Terminating by atexit...\n";} int main(){  atexit(print_message);    std::cout << "\t 1 to terminate with atexit.\n";  std::cout << "\t 2 to terminate normally.\n";    int user_choice;  std::cin >> user_choice;    if(user_choice == 1){    exit(EXIT_SUCCESS);  }    std::cout << "Normal termination...";    return 0;}

Output

         1 to terminate with atexit.
         2 to terminate normally.
1
Terminating by atexit...