Lecture 7: Array & String (2)

std::vector

std::vector 是 C++ 提供的標準樣板函式庫 (Standard Template Library, STL)。 可以把這個想成更為方便使用的 array 。 支援任何型態、動態調整大小及使用變數宣告大小。

Ref: Vector in C++ STL - GeeksforGeeks

// C++ program to illustrate the
// iterators in vector
#include <iostream>
#include <vector>
#include <iterator>
  
int main()
{
    std::vector<int> g1;
  
    for (int i = 1; i <= 5; i++)
        g1.push_back(i);
  
    std::cout << "Output of begin and end: ";
    for (auto i = g1.begin(); i != g1.end(); ++i)
        std::cout << *i << " ";
  
    std::cout << "\nOutput of cbegin and cend: ";
    for (auto i = g1.cbegin(); i != g1.cend(); ++i)
        std::cout << *i << " ";
  
    std::cout << "\nOutput of rbegin and rend: ";
    for (auto ir = g1.rbegin(); ir != g1.rend(); ++ir)
        std::cout << *ir << " ";
  
    std::cout << "\nOutput of crbegin and crend : ";
    for (auto ir = g1.crbegin(); ir != g1.crend(); ++ir)
        std::cout << *ir << " ";
  
    return 0;
}

define

#include <vector>
  
int main()
{
    std::vector<int> g1;
}

#include <vector> 是使用 std::vector 要 include 的函式庫。 宣告變數時需要使用 < > 來指定 std::vector array 需要存放的型態，這邊以 int 為例，也可以使用其他型態，例如：double、float、long 等。 但是不建議用 char ，因為有 std::string 支援字串處理，使用起來比 std::vector<char> 方便。

set/get element's value, push() & pop()

// C++ program to illustrate the
// Modifiers in vector
#include <vector>
#include <iostream>
using namespace std; // shortcut of std::

int main()
{
    // Assign vector
    vector<int> v;

    // fill the array with 10 five times
    v.assign(5, 10);

    cout << "The vector elements are: ";
    for (int i = 0; i < v.size(); i++)
        cout << v[i] << " ";

    // inserts 15 to the last position
    v.push_back(15);
    int n = v.size();
    cout << "\nThe last element is: " << v[n - 1];

    // prints the vector
    cout << "\nThe vector elements are: ";
    for (int i = 0; i < v.size(); i++)
        cout << v[i] << " ";

    // removes last element
    v.pop_back();

    // inserts 5 at the beginning
    v.insert(v.begin(), 5);

    cout << "\nThe first element is: " << v[0];

    // prints the vector
    cout << "\nThe vector elements are: ";
    for (int i = 0; i < v.size(); i++)
        cout << v[i] << " ";

    // removes the first element
    v.erase(v.begin());

    cout << "\nThe first element is: " << v[0];

    // prints the vector
    cout << "\nThe vector elements are: ";
    for (int i = 0; i < v.size(); i++)
        cout << v[i] << " ";

    // inserts at the beginning
    v.emplace(v.begin(), 5);
    cout << "\nThe first element is: " << v[0];

    // prints the vector
    cout << "\nThe vector elements are: ";
    for (int i = 0; i < v.size(); i++)
        cout << v[i] << " ";

    // Inserts 20 at the end
    v.emplace_back(20);
    n = v.size();
    cout << "\nThe last element is: " << v[n - 1];

    // prints the vector
    cout << "\nThe vector elements are: ";
    for (int i = 0; i < v.size(); i++)
        cout << v[i] << " ";

    // change v[3] = 30
    v[3] = 30;

    // prints the vector
    cout << "\nThe vector elements are: ";
    for (int i = 0; i < v.size(); i++)
        cout << v[i] << " ";

    // erases the vector
    v.clear();
    cout << "\nVector size after erase(): " << v.size();

    // prints the vector
    cout << "\nThe vector elements are: ";
    for (int i = 0; i < v.size(); i++)
        cout << v[i] << " ";

    // two vector to perform swap
    vector<int> v1, v2;
    v1.push_back(1);
    v1.push_back(2);
    v2.push_back(3);
    v2.push_back(4);

    cout << "\n\nVector 1: ";
    for (int i = 0; i < v1.size(); i++)
        cout << v1[i] << " ";

    cout << "\nVector 2: ";
    for (int i = 0; i < v2.size(); i++)
        cout << v2[i] << " ";

    // Swaps v1 and v2
    v1.swap(v2);

    cout << "\nAfter Swap \nVector 1: ";
    for (int i = 0; i < v1.size(); i++)
        cout << v1[i] << " ";

    cout << "\nVector 2: ";
    for (int i = 0; i < v2.size(); i++)
        cout << v2[i] << " ";
}

2-d std::vector

#include <iostream>
#include <iomanip>
#include <vector>

using namespace std;

int main()
{
    // define 2-d vector
    vector< vector<int> > table;

    // setup 2-d vector
    for (int i = 0; i < 9; i++) // index start from 0
    {   
        table.push_back(vector<int>()); // push a vector<int> without varaible name
        for (int j = 0; j < 9; j++) // index start from 0
        {
            table[i].push_back((i + 1) * (j + 1));
        }
    }

    // print 2-d vector
    for (int i = 0; i < table.size(); i++) // index start from 0
    {   
        for (int j = 0; j < table[i].size(); j++) // index start from 0
        {
            std::cout << (i+1) << '*' << (j+1) << " = "
                      << std::setw(2) << table[i][j] << ' '
                      << std::flush;
        }
        std::cout << std::endl;
    }
    return 0;
}

atoi, atol

Ref: Converting Strings to Numbers in C/C++ - GeeksforGeeks

// For C++11 above
#include <cstdlib>
#include <iostream>
using namespace std;

int main()
{
    const char *str1 = "42";
    const char *str2 = "3.14159";
    const char *str3 = "31337 geek";

    int num1 = atoi(str1);
    int num2 = atoi(str2);
    int num3 = atoi(str3);

    cout << "atoi(\"" << str1 << "\") is " << num1 << '\n';
    cout << "atoi(\"" << str2 << "\") is " << num2 << '\n';
    cout << "atoi(\"" << str3 << "\") is " << num3 << '\n';

    return 0;
}

Ref: Write your own atoi() - GeeksforGeeks

// Initialize result
int res = 0;

// Initialize sign as positive
int sign = 1;

// Initialize index of first digit
int i = 0;

// If number is negative,
// then update sign
if (str[0] == '-')
{
    sign = -1;

    // Also update index of first digit
    i++;
}

// Iterate through all digits
// and update the result
for (; str[i] != '\0'; ++i)
    res = res * 10 + str[i] - '0';

// Return result with sign
return sign * res;

use a std::vector to save a number

#include <iostream>
#include <cctype>    // isdigit()
#include <algorithm> // reverse()
#include <vector>    // vector<int>()

using namespace std;

int main()
{
    char input_buffer[10000 + 1]; // + 1 for the last '\0'
    vector<int> a;

    cout << "Input a non-negative integer number (a): ";
    cin >> input_buffer;

    for (int i = 0; input_buffer[i] != '\0'; i++)
    {
        if (!isdigit(input_buffer[i]))
        {
            cout << "Invalid input: " << input_buffer << endl;
            return 0;
        }
        else
        {
            int digit = input_buffer[i] - '0';
            a.push_back(digit);
        }
    }

    reverse(a.begin(), a.end());

    // clean up input buffer
    for (int i = 0; i < (10000 + 1); i++)
    {
        input_buffer[i] = '\0';
    }
    // remove leading zeros
    for (int i = a.size() - 1; i >= 1; i--)
    {
        if (a[i] == 0)
        {
            a.pop_back();
        }
        else
        {
            break;
        }
    }

    cout << "(a) = ";
    for (int i = a.size() - 1; i >= 0; i--)
    {
        cout << a[i];
    }
    cout << endl;
}

Output:

$ ./a.out
Input a non-negative integer number (a): 000001234500000⏎
(a) = 1234500000

check isdigit

Ref: std::isdigit - cpprefrence.com

    for (int i = 0; input_buffer[i] != '\0'; i++)
    {
        if (!isdigit(input_buffer[i]))
        {
            cout << "Invalid input: " << input_buffer << endl;
            return 0;
        }

push digits & reverse

    for (int i = 0; input_buffer[i] != '\0'; i++)
    {
        if (!isdigit(input_buffer[i]))
        {
            cout << "Invalid input: " << input_buffer << endl;
            return 0;
        }
        else
        {
            int digit = input_buffer[i] - '0';
            a.push_back(digit);
        }
    }
    // check gdb
    reverse(a.begin(), a.end());
    // check gdb

std::reverse

Reference: std::reverse - cppreference.com

#include <vector>
#include <iostream>
#include <algorithm>

int main()
{
    std::vector<int> v{1, 2, 3};
    std::reverse(v.begin(), v.end());
    for (int i = 0; i < v.size(); i++)
    {
        std::cout << v[i];
    }
    std::cout << '\n';
}

clean up input buffer & remove leading zeros

    // clean up input buffer
    for (int i = 0; i < (10000 + 1); i++)
    {
        input_buffer[i] = '\0';
    }
    // remove leading zeros
    for (int i = a.size() - 1; i >= 1; i--) // start from the most significant digit
    {
        if (a[i] == 0)
        {
            a.pop_back(); // remove the last element in vector,
                          // which is the most significant digit
        }
        else
        {
            break; // stop if process the most significant non-zero digit
        }
    }

print the number

    cout << "(a) = ";
    for (int i = a.size() - 1; i >= 0; i--)
    {
        cout << a[i];
    }
    cout << endl;