UVa-1586 Molar mass

#include<iostream>
#include<cstdio>
#include<cstring>
#include<ctype.h>
using namespace std;
double change(char a)
{
    if(a=='C') return 12.010;
    else if(a=='H') return 1.008;
    else if(a=='O') return 16.000;
    else if(a=='N') return 14.010;
}
bool is_digit(char a)
{
    if(a>='0'&&a<='9') return 1;
    else return 0;
}
int main()
{
    int t;
    cin>>t;
    while(t--)
    {
        char a[85]={};
        cin>>a;
        int k=strlen(a);
        double sum=0.000;
        for(int i=0;i<k;i++)
        {
            if(isalpha(a[i]))
            {
                if(is_digit(a[i+1]))
                {
                    if(is_digit(a[i+2]))
                        sum+=((a[i+1]-'0')*10+(a[i+2]-'0'))*change(a[i]);
                    else
                        sum+=(a[i+1]-'0')*change(a[i]);
                }
                else sum+=change(a[i]);
            }
        }
        printf("%.3f
",sum);
    }
}