你可以解释如何布尔可以控制回路？ [英] Can you explain HOW bool can control loops?

问题描述

我在C ++（目前）初学者程序员，我有一个概念上的问题。

I'm a beginner programmer in C++ (currently), and I've got a conceptual question.

我试图过滤一霉素输入，以确保它是01-04之间的一或二位的整数，并且如果它不是，以产生一个错误，并要求一个新的输入。

I'm trying to filter a cin input to ensure that it is a one-or-two-digit integer between 01-04, and if it isn't, to produce an error and ask for a new input.

我还使用地图来给用户一个选项列表，在有效的选择，线路输入（整数），通过任何几种方法制作一个相关的结果，但我会问其他地方这个问题的一个更具体的版本。

I'm also using map to give the user a list of options that, upon valid selection, routes inputs (integers) through any of several methods to produce a relevant result, but I'll ask a more specific version of this question elsewhere.

我发现code的片段在 http://www.cplusplus.com /论坛/新手/ 26821 / ，是为了验证输入。我有点明白了，除非布尔条件设置while循环中。因为我不明白，这使得它很难编辑或确保我操纵它的权利。

I found a snippet of code at http://www.cplusplus.com/forum/beginner/26821/ that is meant to validate an input. I sort of get it, except where the boolean condition is set inside the while loop. Because I don't understand it, it makes it very difficult to edit or make sure that I'm manipulating it right.

下面是例子code：

int main()
{
    int num;
    bool valid = false;

    while (!valid)
    {
        valid = true; //Assume the cin will be an integer.

        cout << "Enter an integer value: " << endl;
        cin >> num;

        if(cin.fail()) //cin.fail() checks to see if the value in the cin
                    //stream is the correct type, if not it returns true,
                    //false otherwise.
        {
            cin.clear(); //This corrects the stream.
            cin.ignore(); //This skips the left over stream data.
            cout << "Please enter an Integer only." << endl;
            valid = false; //The cin was not an integer so try again.
        }
    }

    cout << "You entered: " << num << endl;

    system("PAUSE");
    return 0;

这是我的code（整个事情，给上下文）。我不认为这是完整的，我只是想确保我使用布尔权利。

And here is my code (the entire thing, to give context). I don't think it's complete, I just want to make sure I'm using the boolean right.

float _tmain(float argc, _TCHAR* argv[])
{
    bool validInput = !true;

    map<string,int> Operations;
    Operations.insert(pair<string, int>("Addition", 01));
    Operations.insert(pair<string, int>("Subtraction", 02));
    Operations.insert(pair<string, int>("Multiplication", 03));
    Operations.insert(pair<string, int>("Division", 04));

    cout << "Welcome to OneOpCalc, what operation would you like to perform?" << endl;

    for(map<string, int>::iterator ii=Operations.begin(); ii!=Operations.end(); ++ii)
    {
        cout << (*ii).second << ": " << (*ii).first << endl;
    }

    while (!validInput)
    {
        cin >> operatorSelection;

        if (cin.fail() || operatorSelection < 4 || operatorSelection > 1)
        {
            cout << "Error: Invalid selection. Please choose a valid number." << endl << endl;
            cin.clear();
            cin.ignore();
        }
    }
}

确实而（！有效）的意思是当有效是假的？在我的头，它说虽然有效是！有效，这很明显，将永远是假的。

Does while (!valid) mean "While valid is false"? In my head, it's saying "While valid is !valid", which obviously, would always be false.

编辑：感谢球员，我期待通过他们所有的答案。一个答案我一直得到那张过于笼统;我明白那个 ！是不是，我理解翻转使用它的布尔的概念。然而，隐含的逻辑含义是什么让我困惑。在任何给定的声明，我习惯了的思维有效作为翻转有效价值的一种方式！;未测试的条件。它是用它来测试一个条件，即招数我的语法。换句话说，写而（！有效）字面上读我为而（NOTvalid），而不是而（有效== FALSE）。我不能让自己明白，为什么在这种情况下，！有效读为条件，而不是只是有点翻转。

Thanks for the answers guys, I'm looking through them all. One answer I keep getting goes too general; I understand that ! is NOT, and I understand the concept of flipping the bool using it. However the implicit logical implications are what confuse me. In any given statement, I am used to thinking of !valid as a way of flipping the valid value; Not testing a condition. It's the syntax of using it to test a condition that tricks me. In other words, writing while(!valid) reads literally to me as while(NOTvalid), not while(valid==false). I can't get myself to understand why in this case, !valid reads as a condition and not just a bit-flip.

推荐答案

循环（和IFS）按类型的前pression controled 布尔。
在而（！有效），前pression是！有效，操作员
不会应用到变量有效的价值。
而（！有效）办法（字面），而前pression
！有效（意思是无效）是真实的。

Loops (and ifs) are controled by an expression of type bool. In while ( !valid ), the expression is !valid, the operator not applied to the value of the variable valid. while ( !valid ) means (literally) while the expression !valid (which means "not valid") is true.

对于剩下的，你复制code是pretty坏。我不会
使用它作为一个例子，如果我是你。

For the rest, the code you're copying is pretty bad. I wouldn't use it as an example if I were you.

对于自己的code：

• _tmain 非常特别的微软。你不想来
  用它。如果你写一个控制台应用程序，只需使用主。
  （同样的事情适用于 _TCHAR ，而不是字符）

• _tmain is very particular to Microsoft. You don't want to use it. If your writing a console application, just use main. (Same thing holds for _TCHAR, rather than char.)

无论是 _tmain 也不主可以返回浮动。回报
主要类型应始终 INT 。我不熟悉
_tmain ，但它要么 INT 或无效。 （大概 INT ，如果
你在一个控制台程序。）

Neither _tmain nor main can return a float. The return type of main should always be int. I'm less familiar with _tmain, but it's either int or void. (Probably int, if you're in a console mode program.)

！真是假。总是。 （编程比不同
真实的世界。有没有maybes。）为什么会更
比需要复杂？

!true is false. Always. (Programming is different than the real world. There are no maybes.) Why be more complicated than necessary?

有没有必要为标志变量都没有。您
可以这样写：

There's no need for the flag variable at all. You can just write:

CIN >> operatorSelection;
而（CIN || operatorSelection> 4 || operatorSelection＆LT;！1）{
    // ...
}

cin >> operatorSelection; while ( !cin || operatorSelection > 4 || operatorSelection < 1 ) { // ... }

在错误的情况下，你目前只忽略一个
字符。你可能想忽略直到并包括
行尾。
（的std :: cin.ignore（的std :: numeric_limits＆LT;的std :: streamsize可＆GT; :: MAX（））;

In case of error, you currently only ignore a single character. You probably want to ignore up to and including the end of line. (std::cin.ignore( std::numeric_limits<std::streamsize>::max() );.

而在你的，如果将​​永远是真实的情况。 （见我
以上版本。）如果你希望找到一个号码是
既不小于也不大于一？

And the condition in your if will always be true. (See my version above.) Where do you expect to find a number which is neither less than for nor greater than one?

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