为什么我在这里得到垃圾（不需要的）输出？ [英] Why I am getting garbage(unwanted) output here?

问题描述

每当我编写下面的代码时，都会在某些在线编译器中得到垃圾（意外）输出，但是如果我使用代码块，则会得到满意的输出。 所以我的问题是为什么我得到这种类型的输出？

Whenever I am writing this following code, I am getting garbage(unexpected) output in some online compiler, but if I use code block then getting satisfied output. So my question is why I am getting this type of output?

例如，如果我输入

5 7
+ 5
- 10
- 20
+ 40
- 20

然后我得到

22 1

在代码块中。

#include<iostream>
#include<cstdlib>
using namespace std;
int main()
{
    int have, n, i;
    int kid=0;

    cin>>n>>have;

    int line[n];

    for(i=0;i<n;i++)
    {
        cin>>line[i];

        if(line[i]>=0)
            have+=line[i];
        else
        {
            if(have>=abs(line[i]))
                have+=line[i];
            else
                kid++;
        }
    }

    cout<<have<<" "<<kid<<endl;

}

推荐答案

主要我在您的代码中看到的问题是这样的：

The main problem I can see in your code is this:

int line[n];

这被称为 VLA （可变长度数组），C ++不支持。它在C中有效。由于C ++基于C，但大多数编译器仍允许此行为，但它不是有效的C ++代码。在上一个问题中，我发现clang支持指定的初始化程序，而gcc和vc ++不支持。原因是因为某些编译器（例如clang）默认支持c99扩展名。我的观点是仅仅因为代码可以编译，并不意味着它总是正确的。

This is known as a VLA (Variable Length Array) and it is not supported in C++. It is valid in C. Most compilers still allow this behaviour due to the fact that C++ is based on C, but it is not valid C++ code. In a previous question, I found out that clang supports designated initializers, when gcc and vc++ did not. The reason is because some compilers like clang, support c99-extensions by default. My point is that just because the code compiles, it doesn't mean it's always right.

如果使用 -pedantic 参数，您将看到编译器警告您这是C99功能。在此处中查看示例。从下面的注释中，在编译器标志中使用 -pedantic-errors ，将提示错误。

If you compile with the -pedantic argument, you will see that the compiler is warning you about this being a C99 feature. Have a look at the rextester example here. From the comments below, using -pedantic-errors in the compiler flags, will prompt an error.

如果在运行时知道数组的大小，则应该使用静态数组 int line [4]; ，但如果不这样做，则需要使用动态数组。 std :: vector本质上是一个动态数组，还可以为您处理内存。易于使用且非常高效。 std :: vector< int>行；

If you know the size of the array before run-time, then you should use a static array int line[4];, but if you don't then you need to use a dynamic array. std::vector is essentially a dynamic array that also handles memory for you. It's easy to use and very efficient. std::vector<int> line;

您可以在此处详细了解矢量容器： http://www.cplusplus.com/reference/vector/vector/

You can read more about the vector container here: http://www.cplusplus.com/reference/vector/vector/

Btw，我在rextester，ideone和repl.it中尝试了您的代码，得到了相同的结果： 22 1 。我认为您正在目睹它的不确定行为。

Btw, I tried your code in rextester, ideone and repl.it and I got the same results: 22 1. I think what you are witnessing it undefined behaviour.

此外，您还可以将<$ c $ int in 限定为 constexpr 会没事的。

Also, you can qualify int n with constexpr and it'll be fine.

constexr int n = 200;
int line[n]; //now it's ok.

但这再次意味着您在编译时就知道数组的大小。

But this again means that you know the size of the array at compile time.

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