Simultaneous update values in dataframe using R

a is a dataframe and I want to update 0 values in dataframe from some previous values and as soon as a 0 value is updated , I also want to update the bottom rows with zero values to that updated value. It is dependent on some relationship between rows and columns. But, I am stuck and get this error Error in x[[jj]][iseq] <- vjj : replacement has length zero. Also I am attaching a picture of the dataframe a. R programming.

a[1,2] should be replaced by a[2,1] and then if any of these are zeroes(a[4,1], a[4,2], a[8,1], a[8,2]), they take value of updated a[1,2].

Code

a=data.frame(c(1.1,0,3,0,4,0,7,8),c(9,8.4,0,9,7,0,6,0))
print(a)
for (j in 1:ncol(a))
{for( i in 1:nrow(a))
{
      if(j%%2==1)
      {if(i%%2==0)
        {if(a[i,j]==0)
        a[i,j]=a[i/2,j]
        ###
        while(i<=63)
        {
          a[i*2,j]=a[i/2,j]
          a[i*2,j+1]=a[i/2,j]
          i=i*2
        }
        ###
        }
      else if (i%%2==1)
        {if(a[i,j]==0)
        a[i,j]=a[floor(i/2),j+1]
        while(i<=63)
        {
          a[i*2,j]=a[floor(i/2),j]
          a[i*2,j+1]=a[floor(i/2),j]
          i=i*2
        }
        }
      }
  
   else if(j%%2==0)
   {if(i%%2==0)
   {if(a[i,j]==0)
    a[i,j]=a[i/2,j-1]
   while(i<=63)
   {
     a[i*2,j]=a[i/2,j-1]
     a[i*2,j+1]=a[i/2,j-1]
     i=i*2
   }
   } 
     else if (i%%2==1)
     {if(a[i,j]==0)
      a[i,j]=a[floor(i/2),j]
     while(i<=63)
     {
       a[i*2,j]=a[floor(i/2),j]
       a[i*2,j+1]=a[floor(i/2),j]
       i=i*2
     }
     }
   } 
}
}