Welcome! Today, we'll explore Data Projection Techniques. Data projection is akin to using a special light to make diamonds shine brighter amidst other gems, aiding their identification.
This lesson will shed light on the concept of data projection, its implementation with C++'s std::transform function, and how to integrate it with filtering. Let's forge ahead!
Data projection involves applying a function to elements of a data stream, resulting in a reshaped view. A common data projection instance is selecting specific fields from databases.
Data projection in C++ uses the std::transform function. Here's an illustration of finding each number's square in a vector of numbers:
C++1#include <iostream> 2#include <vector> 3#include <algorithm> 4 5// Function to get a number's square 6int square(int n) { 7 return n * n; 8} 9 10int main() { 11 std::vector<int> numbers = {1, 2, 3, 4, 5}; // our data stream 12 std::vector<int> squared_numbers(numbers.size()); 13 14 // std::transform applies the square function to each number in the vector 15 std::transform(numbers.begin(), numbers.end(), squared_numbers.begin(), square); 16 17 // Print squared numbers 18 for (int n : squared_numbers) { 19 std::cout << n << " "; 20 } 21 // Output: 1 4 9 16 25 22 23 return 0; 24}
For complex operations on data streams, C++ employs lambda functions (anonymous functions). Let's convert a vector of sentences to lowercase:
C++1#include <iostream> 2#include <vector> 3#include <string> 4#include <algorithm> 5#include <cctype> 6 7// Function to convert a string to lowercase 8std::string to_lowercase(std::string s) { 9 std::transform(s.begin(), s.end(), s.begin(), ::tolower); 10 return s; 11} 12 13int main() { 14 std::vector<std::string> sentences = {"HELLO WORLD", "C++ IS FUN", "I LIKE PROGRAMMING"}; // our data stream 15 std::vector<std::string> lower_sentences(sentences.size()); 16 17 // std::transform applies the lambda function to each sentence in the vector 18 std::transform(sentences.begin(), sentences.end(), lower_sentences.begin(), 19 [](std::string s) { return to_lowercase(s); }); 20 21 // Print lowercased sentences 22 for (const std::string& sentence : lower_sentences) { 23 std::cout << sentence << "\n"; 24 } 25 // Output: hello world 26 // c++ is fun 27 // i like programming 28 29 return 0; 30}
C++ seamlessly combines projection and filtering. Now, let's lowercase sentences containing "C++" while dismissing others:
C++1#include <iostream> 2#include <vector> 3#include <string> 4#include <algorithm> 5#include <cctype> 6 7// Function to check if a string contains "C++" 8bool contains_cpp(const std::string& s) { 9 return s.find("C++") != std::string::npos; 10} 11 12// Function to convert a string to lowercase 13std::string to_lowercase(std::string s) { 14 std::transform(s.begin(), s.end(), s.begin(), ::tolower); 15 return s; 16} 17 18int main() { 19 std::vector<std::string> sentences = {"HELLO WORLD", "C++ IS FUN", "I LIKE PROGRAMMING"}; // our data stream 20 std::vector<std::string> filtered_sentences; 21 22 // std::copy_if selects sentences containing 'C++' 23 std::copy_if(sentences.begin(), sentences.end(), std::back_inserter(filtered_sentences), contains_cpp); 24 25 // std::transform applies the to_lowercase function to each filtered sentence 26 std::transform(filtered_sentences.begin(), filtered_sentences.end(), filtered_sentences.begin(), to_lowercase); 27 28 // Print filtered and lowercased sentences 29 for (const std::string& sentence : filtered_sentences) { 30 std::cout << sentence << "\n"; 31 } 32 // Output: c++ is fun 33 34 return 0; 35}
By creating a DataProjector class, we'll encapsulate our projections for reusable, cleaner code:
C++1#include <iostream> 2#include <vector> 3#include <algorithm> 4#include <iterator> 5#include <functional> 6 7class DataProjector { 8public: 9 DataProjector(const std::vector<std::string>& data) : data(data) {} 10 11 // Method to apply a function to each element 12 std::vector<std::string> project(std::function<std::string(const std::string&)> func) { 13 std::vector<std::string> result(data.size()); 14 std::transform(data.begin(), data.end(), result.begin(), func); 15 return result; 16 } 17 18 // Method to filter data and apply a function to each filtered element 19 std::vector<std::string> filter_and_project(std::function<bool(const std::string&)> filter_func, 20 std::function<std::string(const std::string&)> project_func) { 21 std::vector<std::string> filtered_data; 22 std::copy_if(data.begin(), data.end(), std::back_inserter(filtered_data), filter_func); 23 return DataProjector(filtered_data).project(project_func); 24 } 25 26private: 27 std::vector<std::string> data; 28}; 29 30// Function to check if a string contains "C++" 31bool contains_cpp(const std::string& s) { 32 return s.find("C++") != std::string::npos; 33} 34 35// Function to convert a string to lowercase 36std::string to_lowercase(std::string s) { 37 std::transform(s.begin(), s.end(), s.begin(), ::tolower); 38 return s; 39} 40 41int main() { 42 std::vector<std::string> sentences = {"HELLO WORLD", "C++ IS FUN", "I LIKE PROGRAMMING"}; // our data stream 43 44 // Creating a DataProjector object with our sentences 45 DataProjector projector(sentences); 46 47 // Applying filter_and_project to filter sentences containing 'C++' and convert them to lowercase 48 std::vector<std::string> lower_filtered_sentences = projector.filter_and_project(contains_cpp, to_lowercase); 49 50 // Print filtered and lowercased sentences 51 for (const std::string& sentence : lower_filtered_sentences) { 52 std::cout << sentence << "\n"; 53 } 54 // Output: c++ is fun 55 56 return 0; 57}
Awesome! You've conquered Data Projection Techniques! You've understood data projection, used std::transform, and combined projection with filtering.
This knowledge is your treasure box key, unlocking data manipulation aspects like raw data cleaning or data transformations for various applications. Now, revisit these concepts with practice exercises for mastery. Happy coding!