[R] working with unequal rows

[Saba Sehrish]

Hi 
I have a data frame with rows specifying companies (codes are assigned to companies) and columns specify months (monthly data). The data is based on male (M) and female (F) information for each month. Following is an example of how my data looks like: 


    01   02   03   04 
001   M   M   M   na 
001   F   M   M   M 
002   M   na   F   F 
003   F   F   F   M 
003   F   F   M   na 
003   M   M   M   M 


na= no male/female. 
Now, I want to firstly add rows with similar codes to see total number of Male and Female in each month for each company. Secondly, I need to calculate fraction of Female in each month (F/ M+F) for each one of these companies. For example, in first month of company 001, there is a male and a female working, so in this month the fraction of female is 0.5. I need to know the coding to get this fraction for my whole data.

Kindly guide me in this regard. 

Thanks 
Saba