查看“相对分子质量计算器”的源代码

```python
# A dictionary of some common elements and their relative atomic masses
elements = {'H': 1.008, 'He': 4.003, 'Li': 6.941, 'Be': 9.012, 
            'B': 10.81, 'C': 12.01, 'N': 14.01, 'O': 16.00, 
            'F': 19.00, 'Ne': 20.18, 'Na': 22.99, 'Mg': 24.31, 
            'Al': 26.98, 'Si': 28.09, 'P': 30.97, 'S': 32.07, 
            'Cl': 35.45, 'K': 39.10, 'Ar': 39.95, 'Ca': 40.08, 
            'Sc': 44.96, 'Ti': 47.87, 'V': 50.94, 'Cr': 52.00, 
            'Mn': 54.94, 'Fe': 55.85, 'Ni': 58.69, 'Co': 58.93, 
            'Cu': 63.55, 'Zn': 65.38, 'Ga': 69.72, 'Ge': 72.63, 
            'As': 74.92, 'Se': 78.96, 'Br': 79.90, 'Kr': 83.80, 
            'Rb': 85.47, 'Sr': 87.62, 'Y': 88.91, 'Zr': 91.22, 
            'Nb': 92.91, 'Mo': 95.94, 'Tc': 98.00, 'Ru': 101.1, 
            'Rh': 102.9, 'Pd': 106.4, 'Ag': 107.9, 'Cd': 112.4, 
            'In': 114.8, 'Sn': 118.7, 'Sb': 121.8, 'I': 126.9, 
            'Te': 127.6, 'Xe': 131.3, 'Cs': 132.9, 'Ba': 137.3, 
            'La': 138.9, 'Ce': 140.1, 'Pr': 140.9, 'Nd': 144.2, 
            'Pm': 145.0, 'Sm': 150.4, 'Eu': 152.0, 'Gd': 157.3, 
            'Tb': 158.9, 'Dy': 162.5, 'Ho': 164.9, 'Er': 167.3, 
            'Tm': 168.9, 'Yb': 173.0, 'Lu': 175.0, 'Hf': 178.5, 
            'Ta': 180.9, 'W': 183.8, 'Re': 186.2, 'Os': 190.2, 
            'Ir': 192.2, 'Pt': 195.1, 'Au': 197.0, 'Hg': 200.6, 
            'Tl': 204.4, 'Pb': 207.2, 'Bi': 208.9, 'Th': 232.0, 
            'Pa': 231.0, 'U': 238.0, 'Np': 237.0, 'Pu': 244.0, 
            'Am': 243.0, 'Cm': 247.0, 'Bk': 247.0, 'Cf': 251.0, 
            'Es': 252.0, 'Fm': 257.0, 'Md': 258.0, 'No': 259.0, 
            'Lr': 262.0, 'Rf': 267.0, 'Db': 270.0, 'Sg': 271.0, 
            'Bh': 270.0, 'Hs': 277.0, 'Mt': 276.0, 'Ds': 281.0, 
            'Rg': 280.0, 'Cn': 285.0, 'Nh': 284.0, 'Fl': 289.0, 
            'Mc': 288.0, 'Lv': 293.0}

# A function to calculate the relative molecular mass of a chemical formula
def calculate_rmm(formula):
    # Initialize the result to zero
    result = 0
    # Initialize an empty string to store the current element symbol
    element = ""
    # Initialize an empty string to store the current number of atoms
    number = ""
    # Initialize an empty list to store the stack of parentheses
    stack = []
    # Loop through each character in the formula
    for char in formula:
        # If the character is an uppercase letter
        if char.isupper():
            # If there is a previous element symbol
            if element:
                # If there is a previous number of atoms
                if number:
                    # Convert the number to an integer and multiply it by the relative atomic mass of the element
                    value = int(number) * elements[element]
                else:
                    # Otherwise assume the number is one and multiply it by the relative atomic mass of the element
                    value = elements[element]
                # If there is a stack of parentheses 
                if stack: 
                    # Append the value to the stack 
                    stack.append(value) 
                else: 
                    # Otherwise add it to the result 
                    result += value 
                # Reset the number to an empty string 
                number = "" 
            # Set the current element symbol to the character 
            element = char 
        # If the character is a lowercase letter 
        elif char.islower(): 
            # Append it to the current element symbol 
            element += char  
        elif char.isdigit():  
            number += char  
        elif char == "(":
            if element:  
                if number:  
                    value = int(number) * elements[element]  
                else:  
                    value = elements[element]  
                if stack:  
                    stack.append(value)  
                else:  
                    result += value   
                element = ""   
                number = ""   
            stack.append(char)   
        elif char == ")":
            if element:
                if number:
                    value = int(number) * elements[element]
                else:
                    value = elements[element]
                if stack:
                    stack.append(value)
                else:
                    result += value
            element = ""
            number = ""
            # Pop the values from the stack until reaching a left parenthesis
            temp_result = 0
            while stack and stack[-1] != "(":
                temp_result += stack.pop()
            # Pop the left parenthesis from the stack
            if stack and stack[-1] == "(":
                stack.pop()
            # Check if there is a number after the right parenthesis
            i = formula.index(char) + 1
            if i < len(formula) and formula[i].isdigit():
                # Multiply the temporary result by the number and append it to the stack or add it to the result
                temp_result *= int(formula[i])
                if stack:
                    stack.append(temp_result)
                else:
                    result += temp_result
                # Skip the next character as it is already processed
                continue
            else:
                # Otherwise append the temporary result to the stack or add it to the result as it is
                if stack:
                    stack.append(temp_result)
                else:
                    result += temp_result

    # Check if there is a remaining element and number at the end of the formula and add them to the result        
    if element:  
        if number:  
            value = int(number) * elements[element]  
        else:  
            value = elements[element]  
        result += value   
    return result

# Ask for user input and print out the result or error message  
def main():
    while True:
        try:  
            formula = input("Enter a chemical formula: ")
            rmm = calculate_rmm(formula)
            print("The relative molecular mass of {} is {:.3f}".format(formula,rmm))
        except KeyError as e:
            print("Unknown element: {}".format(e))
        except ValueError as e:
            print(e)

if __name__ == "__main__":
    main()
```