UVA - 1586 Molar mass

An organic compound is any member of a large class of chemical compounds whose molecules contain carbon. The molar mass of an organic compound is the mass of one mole of the organic compound. The molar
mass of an organic compound can be computed from the standard atomic weights of the elements.



When an organic compound is given as a molecular formula, Dr. CHON wants to find its molar mass. A molecular formula, such as C3H4O3 ,
identifies each constituent element by its chemical symbol and indicates the number of atoms of each element found in each discrete molecule of that compound. If a molecule contains more than one atom of a particular element, this quantity is indicated using
a subscript after the chemical symbol.

In this problem, we assume that the molecular formula is represented by only four elements, `C' (Carbon), `H' (Hydrogen), `O' (Oxygen), and `N' (Nitrogen) without parentheses.

The following table shows that the standard atomic weights for `C', `H', `O', and `N'.

Atomic Name	Carbon	Hydrogen	Oxygen	Nitrogen
Standard Atomic Weight	12.01 g/mol	1.008 g/mol	16.00 g/mol	14.01 g/mol

For example, the molar mass of a molecular formula C6H5OH is 94.108 g/mol which is computed by 6 × (12.01 g/mol) + 6 × (1.008
g/mol) + 1 × (16.00 g/mol).

Given a molecular formula, write a program to compute the molar mass of the formula.

Input

Your program is to read from standard input. The input consists of T test cases. The number of test cases T is given in the first line of the input. Each test case is given in a single line,
which contains a molecular formula as a string. The chemical symbol is given by a capital letter and the length of the string is greater than 0 and less than 80. The quantity number n which is represented after the chemical
symbol would be omitted when the number is 1 (2

n

99) .

Output

Your program is to write to standard output. Print exactly one line for each test case. The line should contain the molar mass of the given molecular formula.

Sample
Input

4
C
C6H5OH
NH2CH2COOH
C12H22O11

Sample
Output

12.010
94.108
75.070
342.296

首先呢，要知道求什么元素，
然后要找出次元素后面的个数，个数处理就利用一直向前找是否是数字
是数字就前，不是就跳出，然后进行计算个数，算出综合

<pre name="code" class="cpp">#include <iostream>
#include <string.h>
#include <stdio.h>
#include <iomanip>
#define maxn 85

using namespace std;

int main(){
int T;
while(cin>>T){
while(T--){
char s[85];
scanf("%s",s);
int len = strlen(s);
double sum = 0.0;
double w;
for(int i = 0; i < len ;){
switch(s[i]){
case 'O': w=16.00;break;
case 'C': w=12.01;break;
case 'H': w=1.008;break;
case 'N': w=14.01;break;
}
if( i+1<len && s[i+1] <= '9'){   //如果是数字就向后移位
int tail = i+1;
while(tail<len && (s[tail] <= '9'))   //如是后面也是数字就向后
tail++;
int ans = tail-1;
int n = 0;
int y = 1;
while(ans != i){    //求个数
n += ((s[ans]-'0')*y);
y*=10;
ans--;
}
sum+=(w*n);
i = tail;
}else{
sum+=w;
i++;
}
}
printf("%.3f\n", sum);
}
}
return 0;
}