s [i]-'0'的使用 [英] Use of s[i] - '0'

问题描述

以下代码基本上采用字符串并将其转换为整数值。我不明白为什么需要 int digit = s [i]-‘0’; 。为什么不能使用 int digit = s [i] 来工作？ -'0'有什么用途？

The following code basically takes a string and converts it into an integer value. What I don't understand is why the int digit = s[i] - '0'; is necessary. Why can't it work by using int digit = s[i] instead? What purpose does the -'0' have?

int main(){

    string s = "123";
    int answer = 0;
    bool is_negative = false;
    if (s[0] == '-')
    {
        is_negative = true;
    }
    int result = 0;
    for (int i = s[0] == '-' ? 1 : 0; i < s.size(); ++i)
    {
        int digit = s[i] - '0';
        result = result * 10 + digit;
    }

    answer = is_negative ? -result : result;
    cout << answer << endl;
    system("pause");
}

推荐答案

首先，在您的问题标题中

Firstly, in your question title

使用'0'

the use of '0'

应写为

使用 s [i]-'0'

也就是说，通过减去 char 0的ASCII值（表示为'0'），我们得到<$以 char 格式表示的数字的c $ c> int 值（多数为ASCII值）。

That said, by subtracting the ASCII value of char 0 (represented as '0'), we get the int value of the digit represented in char format (ASCII value, mostly.)

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