Loop and perform operations on vector values

I have this vector:

a = c(6722, 487,  5919, 179, 7305, 336, 10015, 615,  8651, 253, 679, 75,  8652, 118, 11436, 229)

      

        
        
        
      

    

and I need to create a new vector with a ratio of value pairs like this:

ratio = c(a[2]/a[1], a[4]/a[3], a[6]/a[5]... etc) 

      

        
        
        
      

    

so the output should look like this:

ratio    num[1:8] 0.0724 0.03024 0.04599 etc   (edited)

      

        
        
        
      

    

I tried the following code:

ratio = c()
  for (i in length(a)){
    ratio[i] = a[2*i]/a[2*i-1]
  }

      

        
        
        
      

    

and got the following:

Error in a[0+2i] : invalid subscript type 'complex'

      

        
        
        
      

    

after i realized that R thought i meant i = sqrt (-1) i tried this:

ratio = c()
 for (i in length(a)){
    ratio[i] = a[i+i]/a[i+i-1]
  }

      

        
        
        
      

    

which produced:

> ratio
 [1] NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA

      

        
        
        
      

    

then I tried with paw:

lapply(a, function(x)
    a[x+i]/a[x+i-1])

      

        
        
        
      

    

which gave me a 16 element list full of NA.

Does anyone know how to do this?