When i change from matrix to data frame, values are changing in R -

i have matrix read file. , sure consists of characters. matrix has 4 columns called "type","time","x value","y value". , except "type" other 3 columns values' numbers.

i want arithmetic operations subtraction on time column. gives me :

non-numeric argument binary operator error

my data is:

data <-as.matrix(read.table("data/firstdata.cal")) 

here content of data

   type     time            x           y 1   r   1383154550660   0.5638523   0.5157838 2   r   1383154550732   0.5637349   0.5152285 3   r   1383154550778   0.5634450   0.5154306 4   r   1383154550872   0.5631472   0.5145780 5   r   1383154550922   0.5633539   0.5150688 6   r   1383154550984   0.5630451   0.5148284 

i change class data frame writing:

data<-as.data.frame(data) data[, 2] <- as.numeric(as.character( data[, 2] )) 

then want subtraction between rows this:

i<-1 output<-matrix(0,nrow(data),ncol(data))  while(data[i,1]!='ss') i<-i+1 temp<-0 while(i<nrow(data)){ if(data[i,1]=='ss'){   temp<-data[i,3]   output[i+1,1]<-temp   n<-n-1 } else {   j<-i+1   while(data[j,2]-data[i,2]<=700 )   {      j<-j+1   }     k<-0   while (data[j,2]-data[i,2]<=2000){      output[k,3]<-data[i,3]     output[k,2]<-temp     output[k,1]<-data[j,2]-data[i,2]     j<-j+1     k<-k+1   }  } i<-i+1  } 

but values before changing data frame , after changing data frame different. so, should original values without getting error?