如何从字符串计算浮点值

string_value = "3.14"
float_value = float(string_value)
print(float_value)  # 输出：3.14

var string_value = "3.14";
var float_value = eval(string_value);
console.log(float_value);  // 输出：3.14

#include <iostream>
#include <string>
using namespace std;

float calculateFloatValue(string str) {
    float result = 0.0;
    float decimal = 0.1;
    bool isNegative = false;

    for (char c : str) {
        if (c == '-') {
            isNegative = true;
        } else if (c == '.') {
            decimal = 0.1;
        } else {
            int digit = c - '0';
            if (digit >= 0 && digit <= 9) {
                if (decimal == 0.1) {
                    result = result * 10 + digit;
                } else {
                    result = result + digit * decimal;
                    decimal *= 0.1;
                }
            }
        }
    }

    if (isNegative) {
        result = -result;
    }

    return result;
}

int main() {
    string stringValue = "3.14";
    float floatValue = calculateFloatValue(stringValue);
    cout << floatValue << endl;  // 输出：3.14

    return 0;
}