std :: map :: insert不起作用 [英] std::map::insert is not working

问题描述

我试图找出一个数字中每个数字的出现次数。

这个想法很简单。我保存了一个地图，其中'digit'为'key'，'出现次数'为'value'。



但是当插入该对时，它会被插入为零而不是一个。

I am trying to find out count of occurrence of each digit in a number.
The idea is simple. I kept a map with 'digit' as 'key' and 'number of occurrence' as 'value'.

But when the pair is getting inserted, it is inserted as zero instead of one.

  int x = 121123;
  map<int, int> container;
while (x > 0)
{
    int temp = x % 10;
    int b = container[temp];
    if (b > 0)
    {
        container[temp]++;
    }
    else
    {
        container.insert(pair<int, int>(temp, 1)); //value getting inserted as zero instead of one.
    }

    x = x / 10;
}

我不知道发生了什么。为什么将值插入为零。

I have no idea what is going on. Why the value is inserted as zero.

推荐答案

这是因为 map :: operator [] [ ^ ]当键到目前为止不存在时插入一个新元素并将值设置为零。因此，下面调用 map :: insert（） [ ^ ]将始终失败。



如果您想获得没有自动插入的值，请使用 map :: at（）。但请注意，如果密钥不存在，这将抛出异常。



获得您想要的设置要简单得多：

This is because the map::operator [][^] is inserting a new element when the key does not exist so far and sets the value to zero. As a result the follwing call to map::insert()[^] will always fail.

If you want to get a value without automatic insert, use map::at(). But note that this will throw en exception if the key does not exist.

Getting the settings you want is much simpler:

while (x > 0)
{
    int temp = x % 10;
    container[temp]++;
    x = x / 10;
}

如果密钥不存在，则创建值为零，然后递增。如果它已经存在，则只是递增。

If the key does not exist, it is created with value zero and then incremented. If it exists already, it is just incremented.

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