为什么我无法在 while 循环内更改索引？

Question

我想在Python中实现只包含数字、“+”和“-”的基本计算器。但我发现我无法在 while 循环内更改索引 i 。

def basicCalculator(expression):
    res = 0
    sign = 1
    i = 0
    while i < len(expression): # using while loop here
        if expression[i] == '+':
            sign = 1
        elif expression[i] == '-':
            sign = -1
        elif '0' <= expression[i] <= '9':
            temp = int(expression[i])
            while i + 1 < len(expression) and '0' <= expression[i + 1] <= '9':
                i += 1             # want to increase i, but failed
                temp = temp * 10 + int(expression[i])
            res += sign * temp
    return res

s = "2+3-999"
print(basicCalculator(s))    # ouput should be -994, but infinite

然后我尝试了另一种方法：使用for循环而不是while循环，得到错误的答案。

def basicCalculator(expression):
    res = 0
    sign = 1
    for i in range(len(expression)):  # using for loop
        if expression[i] == '+':
            sign = 1
        elif expression[i] == '-':
            sign = -1
        elif '0' <= expression[i] <= '9':
            temp = int(expression[i])
            while i + 1 < len(expression) and '0' <= expression[i + 1] <= '9':
                i += 1                    # increase i
                temp = temp * 10 + int(expression[i])
            res += sign * temp
    return res

s = "2+3-999"
print(basicCalculator(s))    # ouput should be -994, but got -1102

我不知道为什么我无法更改 while 循环或 for 循环内的索引。在 for 循环中改变 i 不是一个好主意。修复该错误的最佳方法是什么？ （我使用方法 2 在 Java 中编写了相同的代码，一切都很好，有什么区别吗？）

Answer 1

我建议不要在循环内的循环中递增索引，而是在主循环中累加当前数字，这样您就无需跟踪：

def basicCalculator(expression):
    res = 0
    sign = 1
    num = ""
    for c in expression:
        if c.isdecimal():
            num += c
            continue
        res += int(num) * sign
        num = ""
        if c == "+":
            sign = 1
        elif c == "-":
            sign = -1
        else:
            raise ValueError(f"Invalid operator {c}")
    return res + int(num) * sign

print(basicCalculator("2+3-999"))  # -994