Molar mass UVa1586

code:

//The second one
#include<stdio.h>
#include<string.h>
#include<ctype.h>
#include<math.h>
#define maxn 110

char molar[5] = "CHON";
double weight_n[4]={ 12.01, 1.008, 16.00, 14.01};

//return the weight of the molecule(such as "CHON")
double weight(char c)
{
    int num = strchr(molar, c)-molar;
    return weight_n[num];
}

//return the digit of the number after the molecule.
int sum_n(int d[], int n)
{
    int num = n-1;
    int sum = 0;
    for(int j =0 ; j < n; j++)
    {
        sum += d[j] * (int)pow(10,num--);
    }
    return sum;
}


int main()
{
    int T(0);
    scanf("%d", &T);
    while(T--)
    {
        double sum(0);
        char a[maxn];
        scanf("%s",a);
        int n = strlen(a);
        int flag = 1;          //default: everytime puch back 1 element.
        int i = 0;
        for(; i < n; i += flag)
        {
            int num[maxn];
            int index = 1;
            int number_n(1);
            int count = 0;
            if(isdigit(a[i+1]))           //if there is number after the molecule, then count the number 
            {
                while(isdigit(a[i+index]))
                {                    
                    num[count] = (a[i+index]-48);
                    index++;
                    count++;
                }
                number_n = sum_n(num,index-1);
                flag = index;
            }
            else flag = 1;
            sum += weight(a[i])*number_n;      //add the weight of molecules to the value sum;
        }
        printf("%.3f\n",sum);
    }
    
    return 0;

}

 

1.  第一部分是两个函数比较重要，一个是用来计算某个元素的摩尔质量；另一个是用来返回分子数目。
2. 时刻要保持头脑清醒。