std :: cin用于双精度和字符串 [英] std::cin for double and string

问题描述

我正在进行货币兑换练习。程序应读取输入形式的货币金额和名称，并以本国货币返回其价值。

I am doing an exercise on currency exchange. Program should read amount and name of currency form input stream and return its value in national currency.

double amount = 0.0;
std::string currency = " ";

std::cout << "Please enter amount and currency ('usd','eur' or 'rub'):" << std::endl;
std::cin >> amount >> currency;
std::cout << amount << currency << std::endl;

if ( currency == "usd") {
    ...;
} else if ( currency == "eur" ) {
    ...;
} else if ( currency == "rub" ) {
    ...;
} else {
    std::cout << "Input error: unknown currency..." << std::endl;
}

我在此程序中遇到了std :: cin怪异的问题。当键入 100usd或 100rub时，程序分别回显 100usd或 100rub，并继续正常工作。但是，当我键入 100eur时，它回显 0并给出 Input error ...行。同时，如果我键入 100 eur，程序将回显 100eur并正常工作。在前两种情况下，是否放置空格都没有区别。

I've encountered a weird problem with std::cin in this program. When typing in "100usd" or "100rub" the program echoes "100usd" or "100rub" respectively and continues to work normally. But when I type "100eur" it echoes "0" and gives out the "Input error..." line. At the same time, should I type "100 eur", the program echoes "100eur" and works fine. In first two cases it makes no difference if I put whitespace or not.

我在做什么错了？

推荐答案

在100eur的情况下，它认为您正在尝试用科学表示法写一个双精度符号： 1.0e-10 ，但无法解析其余的（因为ur对于指数 section无效）。

In the 100eur case it thinks you're trying to write a double in scientific notation: 1.0e-10 but fails to parse the rest of it (because ur is not valid for the exponent "section").

在其他情况下，它停止在100（到达u / r时），对double有效，因此解析成功。

In the other cases it stops at 100 (when it reaches u/r) which is valid for a double so the parse succeeds.

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