如何在Python中将字符串转换为整数？

Question

我这个小例子应用的输出结果是这样的：

Welcome to the Calculator!
Please choose what you'd like to do:
0: Addition
1: Subtraction
2: Multiplication
3: Division
4: Quit Application
0
Enter your first number: 1
Enter your second number: 1
Your result is:
11

这是因为我的 addition() 方法把输入的内容当成了字符串，而不是数字。那我该怎么把它们当作数字来用呢？

下面是我的完整代码：

def addition(a, b):
    return a + b

def subtraction(a, b):
    return a - b

def multiplication(a, b):
    return a * b

def division(a, b):
    return a / b

keepProgramRunning = True

print "Welcome to the Calculator!"

while keepProgramRunning:    
    print "Please choose what you'd like to do:"

    print "0: Addition"
    print "1: Subtraction"
    print "2: Multiplication"
    print "3: Division"
    print "4: Quit Application"



    #Capture the menu choice.
    choice = raw_input()    

    if choice == "0":
        numberA = raw_input("Enter your first number: ")
        numberB = raw_input("Enter your second number: ")
        print "Your result is:"
        print addition(numberA, numberB)
    elif choice == "1":
        numberA = raw_input("Enter your first number: ")
        numberB = raw_input("Enter your second number: ")
        print "Your result is:"
        print subtraction(numberA, numberB)
    elif choice == "2":
        numberA = raw_input("Enter your first number: ")
        numberB = raw_input("Enter your second number: ")
        print "Your result is:"
        print multiplication(numberA, numberB)
    elif choice == "3":
        numberA = raw_input("Enter your first number: ")
        numberB = raw_input("Enter your second number: ")
        print "Your result is:"
        print division(numberA, numberB)
    elif choice == "4":
        print "Bye!"
        keepProgramRunning = False
    else:
        print "Please choose a valid option."
        print "\n"

Answer 1

在你的主函数里调用子函数的时候，可以把变量转换成整数，然后再进行调用。请看下面的代码：

import sys

print("Welcome to Calculator\n")
print("Please find the options:\n" + "1. Addition\n" + "2. Subtraction\n" + 
"3. Multiplication\n" + "4. Division\n" + "5. Exponential\n" + "6. Quit\n")

def calculator():
    choice = input("Enter choice\n")

    if int(choice) == 1:
        a = input("Enter first number\n")
        b = input("Enter second number\n")
        add(int(a), int(b))

    if int(choice) == 2:
        a = input("Enter first number\n")
        b = input("Enter second number\n")
        diff(int(a), int(b))

    if int(choice) == 3:
        a = input("Enter first number\n")
        b = input("Enter second number\n")
        mult(int(a), int(b))

    if int(choice) == 4:
        a = input("Enter first number\n")
        b = input("Enter second number\n")
        div(float(a), float(b))

    if int(choice) == 5:
        a = input("Enter the base number\n")
        b = input("Enter the exponential\n")
        exp(int(a), int(b))

    if int(choice) == 6:
        print("Bye")
        sys.exit(0)



def add(a, b):
    c = a+b
    print("Sum of {} and {} is {}".format(a, b, c))

def diff(a,b):
    c = a-b
    print("Difference between {} and {} is {}".format(a, b, c))

def mult(a, b):
    c = a*b
    print("The Product of {} and {} is {}".format(a, b, c))

def div(a, b):
    c = a/b
    print("The Quotient of {} and {} is {}".format(a, b, c))

def exp(a, b):
    c = a**b
    print("The result of {} to the power of {} is {}".format(a, b, c))

calculator()

在这里，我做的就是在调用每个函数的时候，把输入的参数转换成整数。我希望这对你有帮助。

在你的情况下，可以这样修改：

 if choice == "0":
        numberA = raw_input("Enter your first number: ")
        numberB = raw_input("Enter your second number: ")
        print "Your result is:"
        print addition(int(numberA), int(numberB))

Answer 2

>>> a = "123"
>>> int(a)
123

这里有一些免费的代码：

def getTwoNumbers():
    numberA = raw_input("Enter your first number: ")
    numberB = raw_input("Enter your second number: ")
    return int(numberA), int(numberB)

Answer 3

因为你正在写一个计算器，它可能也会接受小数（比如 1.5, 0.03），所以用一个简单的辅助函数会更稳妥：

def convertStr(s):
    """Convert string to either int or float."""
    try:
        ret = int(s)
    except ValueError:
        #Try float.
        ret = float(s)
    return ret

这样如果整数转换失败，你就会得到一个小数作为返回值。

补充一下：你的 division 函数可能会让你感到困惑，如果你不完全了解 Python 2.x 是如何处理整数除法的。

简单来说，如果你想要 10/2 的结果是 2.5 而不是 2，你需要使用 from __future__ import division，或者把其中一个或两个参数转换为小数，像这样：

def division(a, b):
    return float(a) / float(b)