[R] does there any function like sumif in excel?

[Celine]

I have a similar problem 
I have a dataframe : 

CELLCD AreaProtected 
   8928      52.39389 
   8928      41.91511 
   8929      21.21975 
   8929      63.65925 
   8930      26.08547 
   8930      14.04602 

I wouldlike to sum the AreaProtected if it is the same CELLCD in another
column : 

CELLCD AreaProtected    SumAreaProtected 
   8928      52.39389      94.309 
   8928      41.91511 
   8929      21.21975      84,879 
   8929      63.65925 
   8930      26.08547 
   8930      14.04602 

I am just started with R and I don't know how I can do that. 
Do you have any ideas ? 


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