While you have one answer, I would suggest taking a hard look at the first comment posted following your question. Regardless how you slice it, you will have to parse the user input in the form of a string. While it may seem daunting to parse something like "Li3Co4CO2" to get the atomic weight for each element, multiply it by the correct multiplier and keep a running sum, it can be done with 2 pointers and a nested while loop.
Take for example your user input in argv[1] and assign it to pointer p:
char *p = argv[1];
All you need now is a while loop to check each character in the string:
while (*p) {
Within the loop, all you will need to do is to identify each element a Capital letter followed by any lower case and a terminating digit (if any) before the next Cap or the null-terminating character (end of string). So assign an end-pointer say ep and work down the string with a nested while loop. (we'll add a multiplier to hold the digit, and a symbol length for use later) We skip the first character in p, (char *ep = p + 1;), so we will initialize the symbol length to 1 as well (the multiplier always starts at 1):
char *ep = p + 1; /* end pointer */
char m = 1; /* multiplier */
size_t symlen = 1; /* symbol len */
...
/* for each char in p until next CAP */
while (*ep && (*ep < 'A' || 'Z' < *ep)) {
/* if digit */
if ('0' <= *ep && *ep <= '9')
m = *ep - '0'; /* set multiplier */
else
symlen++; /* increment len */
ep++;
}
You now have all the information you need to copy/search for the symbol and then add the weight to a running sum. To finish isolating the symbol, you can simply use your symlen along with strncpy to copy the symbol to a temporary array/string:
char srchsym[8] = {0}; /* search sym */
strncpy (srchsym, p, symlen);
srchsym[symlen] = 0; /* null-terminate */
(the manual null-termination is technically not needed since you initialized srchsym[8] = {0};, but it is a good habit)
Now all that is left is to search for srchsym within your periodic table (array of structs) and return a pointer to the element of the array that corresponds to element that matches your search symbol (the atomic symbol). Once you have a pointer to the data in your periodic table, keeping the running sum in wt is as simple as:
wt += (float)m * ptp->atwt;
(where ptp - pointer to periodic table and the element of the stucture atwt is the atomic weight.) With the calculation for the first element done, simply advance your pointer p to the next Cap in the string, and repeat:
p = ep;
}
When done, you can print your results:
printf ("\n %s (atomic weight) : %.3f\n\n", argv[1], wt);
If you like, you can also have the loop print out each element it is adding to the sum as it is returned by your search with srchsym. An example for the hypothetical "Li3Co4CO2" would be:
$ ./bin/ptable Li3Co4CO2
element : Lithium
symbol : Li
atomic num : 3
atomic wt. : 6.941
element : Cobalt
symbol : Co
atomic num : 27
atomic wt. : 58.933
element : Carbon
symbol : C
atomic num : 6
atomic wt. : 12.011
element : Oxygen
symbol : O
atomic num : 8
atomic wt. : 15.999
Li3Co4CO2 (atomic weight) : 300.565
Good luck, let me know if you need additional help fitting the pieces together.
