std::map::insert is not working

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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.

C++

  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;
}

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

Posted 14-Nov-15 1:29am

Shubha Debnath

Updated 14-Nov-15 2:29am

Richard MacCutchan

v2

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Jochen Arndt · Accepted Answer · 2015-11-14T05:59:00

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:

C++

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

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