#include <iostream>
#include <cstring>
#include <cstdlib>

using namespace std;

/** Gets input from keyboard
 *
 *  @return - char *expr
 */
char *input_expression()
{
    cout << "Enter expression: ";
    char *expr = new char[256];
    cin.getline(expr, 256);
    return expr;
}

/** Deletes element from array
 *
 * @param arr - array of any type
 * @param size - size of array arr
 * @param pos - index of element to delete
 */
template <class T>
void delete_element(T *arr, int &size, int pos)
{
    for (int i = pos; i < size; i++)
        arr[i]=arr[i+1];
    --size;
}

/** Gets intermediate result and makes appropriate changes in expression
 * which represented by array of numbers and array of operations.
 *
 * Example: 4 + 2 * 3  => numbers=(4, 2, 3), operations=(+, *)
 * After applaying mid result: numbers=(4, 6), operations=(+,)
 *
 * @param operations - array of operations in expression
 * @param numbers - array of numbers in expression
 * @param op_counter - size of array operations
 * @param num_counter - size of array num_counter
 * @param i - index of changed elements
 * @param mid_result - result of calculation of current operation
 */
void apply_mid_result(char *operations, double *numbers,
                        int op_counter, int num_counter,
                        int i, double mid_result)
{
    delete_element(operations, op_counter, i);
    delete_element(numbers, num_counter, i);
    numbers[i] = mid_result;
}

/** Parses given expression as string to arrays of numbers and operations.
 *
 * @param expr - arithmetic expression, stored in array of char
 * @param numbers - array of double, used as storage of all operands
 * @param num_counter - size of array numbers
 * @param operations - array of char, used as storage of all operations
 * @param op_counter - size of array operations
 */
void parse_expression(char *expr, double *numbers, int &num_counter,
                                  char *operations, int &op_counter)
{
    char oper_signs[] = "*/+-";
    double number;
    char *pch = expr;
    while (pch != NULL)
    {
       number = strtod(pch, &pch);
       numbers[num_counter] = number;
       ++num_counter;
       pch = strpbrk(pch, oper_signs);
       if (pch != NULL)
       {
           operations[op_counter] = *pch;
           ++op_counter;
           ++pch;
       }
    }
}

/** Calculates according to priority of operations, returns result of calculation.
 *
 * NOTE: can calculate expressions without brackets and with standard
 * arithmetic operations - addition, subtraction, multiplication, division.
 *
 * @param expr - arithmetic expression, stored in array of char
 * @return double result
 */
double calculate(char *expr)
{
    char operations[127];
    int op_counter = 0;

    double numbers[128];
    int num_counter = 0;

    parse_expression(expr, numbers, num_counter, operations, op_counter);

    double result = 0, mid_result;
    int i = 0;

    // first - high priority operations
    while (i < op_counter)
    {
        if (operations[i] == '*')
        {
            mid_result = numbers[i] * numbers[i+1];
            apply_mid_result(operations, numbers, op_counter, num_counter, i, mid_result);
        }
        else if (operations[i] == '/')
        {
            mid_result = numbers[i] / numbers[i+1];
            apply_mid_result(operations, numbers, op_counter, num_counter, i, mid_result);
        }
        else ++i;
    }

    // last - low priority operations
    i = 0;
    while (i < op_counter)
    {
        if (operations[i] == '+')
        {
            mid_result = numbers[i] + numbers[i+1];
            apply_mid_result(operations, numbers, op_counter, num_counter, i, mid_result);
        }
        else if (operations[i] == '-')
        {
            mid_result = numbers[i] - numbers[i+1];
            apply_mid_result(operations, numbers, op_counter, num_counter, i, mid_result);
        }
        else ++i;
    }

    return mid_result;
}


int main()
{
    char *expr = input_expression();
    cout << "\nExpression: " << expr;
    double result = calculate(expr);
    cout << "\nResult: " << result << "\n";
    return 0;
}