I have a UN-Adjusted Population Count dataset, for the year 2015, stored in a TIFF file format (~450MB) downloaded from http://sedac.ciesin.columbia.edu/data/collection/gpw-v4/sets/browse

In R I am able to read the data using raster package. I am also able to plot it as follows,

    #read your TIFF file
    
    worldpopcount <- raster("gpw-v4-population-count-adjusted-to-2015-unwpp-country-totals_2015.tif")
    
    worldpopcount
    
    plot(worldpopcount)

Now I want to crop the West African region (covering the three countries Guinea, Liberia and Sierra Leone). My coordinates for West Africa are,

    # extent format (xmin,xmax,ymin,ymax)
    
    # WestBound 	-16
    # EastBound 	 -7.25
    # SouthBound 	  4
    # NorthBound 	 12.75

HOW DO I CROP THE DATA USING EXTENT/CROP? Once the region of interest has been cropped I would like to export it to an asc (ESRI ASCII) using the code

    #writeRaster(r, filename="westAfrica.asc", format = "ascii", datatype='INT4S', overwrite=TRUE)