c_cpp CPP - 教程005 - 字符串，数学函数

Posted 2021-05-14

// ----- STRING TUTORIAL -----

#include <cstdlib>
#include <iostream>
#include <string>
#include <vector>
#include <numeric>
#include <sstream>


int main() {

    // A C character string is an array of characters
    // with a null character at the end \0
    char cString[] = {'A', ' ', 'S', 't', 'r', 'i', 'n', 'g', '\0'};
    std::cout << cString << "\n";
    
    // Get array size (null is included)
    std::cout << "Array Size " << sizeof(cString) << "\n";
    
    // C strings are troublesome because if you forget \0, or
    // add to much information it can lead your program to crash,
    // or for your system to crash
    
    // You can create a vector of strings
    std::vector<std::string> strVec(10);
    
    // C++ std::string can grow in size and is much safer
    std::string str("I'm a string");
    strVec[0] = str;
    
    // You can access characters with an index
    std::cout << str[0] << "\n";
    
    // You can also use at()
    std::cout << str.at(0) << "\n";
    
    // Front returns first char and back returns last
    std::cout << str.front() << " " << str.back() << "\n";
    
    // Get the string length
    std::cout << "Length : " << str.length() << "\n";
    
    // You can copy a string to another
    std::string str2(str);
    strVec[1] = str2;
    
    // You can copy after the 1st 4 characters
    std::string str3(str, 4);
    strVec[2] = str3;
    
    // Repeat a value to make a string
    std::string str4(5, 'x');
    strVec[3] = str4;
    
    // Combine strings with append or +
    strVec[4] = str.append(" and your not");
    str += " and your not";
    
    // Append part of a string
    str.append(str, 34, 37);
    strVec[5] = str;
    
    // Erase characters from a string from an index to another
    // or the last
    str.erase(13, str.length() - 1);
    strVec[6] = str;
    
    for(auto y: strVec)
        std::cout << y << "\n";
    
    // find() returns index where pattern is found
    // or npos
    if(str.find("string") != std::string::npos)
        std::cout << "1st not " << str.find("string") << "\n";
    
    // substr(x, y) returns a substring starting at
    // index x with a length of y
    std::cout << "Substr " << str.substr(6,6) << "\n";
    
    // Reverse a string by passing the beginning and end
    // of a string
    reverse (str.begin(), str.end());
    std::cout << "Reverse " << str << "\n";
    
    // Case conversion
    transform(str2.begin(), str2.end(), str2.begin(), ::toupper);
    std::cout << "Upper " << str2 << "\n";
    transform(str2.begin(), str2.end(), str2.begin(), ::tolower);
    std::cout << "Lower " << str2 << "\n";
    
    // You can get the ascii code for a char by saving
    // the char as an int or with (int)
    // a - z : 97 - 122
    // A - Z : 65 - 90
    char aChar = 'Z';
    int aInt = aChar;
    std::cout << "A Code " << (int)'a' << "\n";
    
    // Convert int to string
    std::string strNum = std::to_string(1+2);
    std::cout << "String " << strNum << "\n";
    
    // ----- PROBLEM : SECRET STRING -----
    // Receive an uppercase string and hide its meaning
    // by turning it into ascii codes
    // Translate it back to the original letters
    
    std::string normalStr, secretStr = "";
    std::cout << "Enter your string in uppercase : ";
    std::cin >> normalStr;
    
    // Cycle through each character converting 
    // them into ascii codes which are stored in
    // a string
    for(char c: normalStr)
        secretStr += std::to_string((int)c);
        // secretStr += std::to_string((int)c - 23);
    
    std::cout << "Secret : " << secretStr << "\n";
    
    normalStr = "";
    
    // Cycle through numbers in string 2 at a time
    for(int i = 0; i < secretStr.length(); i += 2){
        
        // Get the 2 digit ascii code
        std::string sCharCode = "";
        sCharCode += secretStr[i];
        sCharCode += secretStr[i+1];
        
        // Convert the string into int
        int nCharCode = std::stoi(sCharCode);
        
        // Convert the int into a char
        char chCharCode = nCharCode;
        // char chCharCode = nCharCode + 23;
        
        // Store the char in normalStr
        normalStr += chCharCode;
    }
    
    std::cout << "Original : " << normalStr << "\n";
    
    // ----- END OF PROBLEM : SECRET STRING -----
    
    // ----- BONUS PROBLEM -----
    // Allow the user to enter upper and lowercase
    // letters by subtracting and adding 1 value
    // ----- END OF BONUS PROBLEM -----
    
    return 0;
    
}

// ----- END OF STRING TUTORIAL ——

// ----- MATH FUNCTIONS -----
// C++ has numerous math functions
// http://en.cppreference.com/w/cpp/numeric/math

// Needed for math functions
#include <cmath>

int main() {
    
    std::cout << "abs(-10) = " << std::abs(-10) << "\n";
    
    std::cout << "max(5,4) = " << std::max(5,4) << "\n";
    
    std::cout << "min(5,4) = " << std::min(5,4) << "\n";
    
    std::cout << "fmax(5.3,4.3) = " << std::fmax(5.3,4.3) << "\n";
    
    std::cout << "fmin(5.3,4.3) = " << std::fmin(5.3,4.3) << "\n";
    
    // e ^ x
    std::cout << "exp(1) = " << std::exp(1) << "\n";
    
    // 2 ^ x
    std::cout << "exp2(1) = " << std::exp2(1) << "\n";
    
    // e * e * e ~= 20 so log(20.079) ~= 3
    std::cout << "log(20.079) = " << std::log(20.079) << "\n";
    
    // 10 * 10 * 10 = 1000, so log10(1000) = 3
    std::cout << "log10(1000) = " << std::log10(1000) 
            << "\n";
    
    // 2 * 2 * 2 = 8
    std::cout << "log2(8) = " << std::log2(8) 
            << "\n";
    
    // 2 ^ 3
    std::cout << "pow(2,3) = " << std::pow(2,3) 
            << "\n";
    
    // Returns what times itself equals the provided value
    std::cout << "sqrt(100) = " << std::sqrt(100) 
            << "\n";
    
    // What cubed equals the provided
    std::cout << "cbrt(1000) = " << std::cbrt(1000) 
            << "\n";
    
    // Hypotenuse : SQRT(A^2 + B^2)
    std::cout << "hypot(2,3) = " << std::hypot(2,3) 
            << "\n";
    
    std::cout << "ceil(10.45) = " << std::ceil(10.45) 
            << "\n";
    
    std::cout << "floor(10.45) = " << std::floor(10.45) 
            << "\n";
    
    std::cout << "round(10.45) = " << std::round(10.45) 
            << "\n";
    
    // Also sin, cos, tan, asin, acos, atan, atan2,
    // sinh, cosh, tanh, asinh, acosh, atanh
    
    return 0;
}

// ----- END OF MATH FUNCTIONS -----