C++ Atoi function gives error [duplicate]

Question

I have a string which has 5 characters. I want to convert each single character to int and then multiply them with each other. This is the code :

int main()
{
    int x;
    string str = "12345";
    int a[5];
    for(int i = 0; i < 5; i++)
    {
        a[i] = atoi(str[i]);
    }
    x = a[0]*a[1]*a[2]*a[3]*a[4];
    cout<<x<<endl;
}

It gives this error for the line with atoi :

invalid conversion from 'char' to 'const char*' [-fpermissive]|

How can I fix this? Thanks.

Answer 1

You can use:

a[i] = str[i] - '0';

Does a char to digit conversion by ASCII character positions.

Answer 2

The proper way to do this is std::accumulate instead of rolling your own:

std::accumulate(std::begin(str), std::end(str), 1, [](int total, char c) {
    return total * (c - '0'); //could also decide what to do with non-digits
});

Here's a live sample for your viewing pleasure. It's worth noting that the standard guarantees that the digit characters will always be contiguous, so subtracting '0' from any of '0' to '9' will always give you the numerical value.