Walking Randomly

Say you have two vectors in R (These are taken from my tutorial Simple nonlinear least squares curve fitting in R)

xdata = c(-2,-1.64,-1.33,-0.7,0,0.45,1.2,1.64,2.32,2.9)
ydata = c(0.699369,0.700462,0.695354,1.03905,1.97389,2.41143,1.91091,0.919576,-0.730975,-1.42001)

We put these in a data frame with

data = data.frame(xdata=xdata,ydata=ydata)

This looks like this in R

   xdata     ydata
1  -2.00  0.699369
2  -1.64  0.700462
3  -1.33  0.695354
4  -0.70  1.039050
5   0.00  1.973890
6   0.45  2.411430
7   1.20  1.910910
8   1.64  0.919576
9   2.32 -0.730975
10  2.90 -1.420010

Exporting to a .csv file is done using the standard R function, write.csv

write.csv(data,file='example_data.csv')

The resulting .csv file looks like this:

"","xdata","ydata"
"1",-2,0.699369
"2",-1.64,0.700462
"3",-1.33,0.695354
"4",-0.7,1.03905
"5",0,1.97389
"6",0.45,2.41143
"7",1.2,1.91091
"8",1.64,0.919576
"9",2.32,-0.730975
"10",2.9,-1.42001

I don’t want to include the row numbers in my output. To achieve this, we do

write.csv(data,file='example_data.csv',row.names=FALSE)

This gets us a file that looks like this:

 "xdata","ydata"
-2,0.699369
-1.64,0.700462
-1.33,0.695354
-0.7,1.03905
0,1.97389
0.45,2.41143
1.2,1.91091
1.64,0.919576
2.32,-0.730975
2.9,-1.42001

I can also remove the quotes around xdata and ydata with quote=FALSE

write.csv(data,file='example_data.csv',row.names=FALSE,quote=FALSE)

giving the file below

xdata,ydata
-2,0.699369
-1.64,0.700462
-1.33,0.695354
-0.7,1.03905
0,1.97389
0.45,2.41143
1.2,1.91091
1.64,0.919576
2.32,-0.730975
2.9,-1.42001

Changing the separator

Despite the fact that they are asking R to write a comma separated file, some people try to change the separator. Perhaps you’d like to try changing it to a tab for example. The following looks reasonable:

write.csv(data,file='example_data.csv',row.names=FALSE,quote=FALSE,sep="\t")

Although it understands what you are trying to do, R will completely ignore your request!

Warning message:
In write.csv(data, file = "example_data.csv", row.names = FALSE,  :
  attempt to set 'sep' ignored

This is because write.csv is designed to ensure that some standard .csv conventions are followed. It’s trying to protect you against yourself!

In the UK, the convention for .csv files is to use . for a decimal point and , as a separator and that’s the convention that write.csv sticks to. Other countries have a different convention –  they use a , for the decimal point and a ; for the separator. The function write.csv2 takes care of that for you.

If you absolutely must change the separator to something else, make use of write.table instead:

write.table(data,file='example_data.csv',row.names=FALSE,quote=FALSE,sep="\t")

Now, the file will come out like this:

xdata   ydata
-2      0.699369
-1.64   0.700462
-1.33   0.695354
-0.7    1.03905
0       1.97389
0.45    2.41143
1.2     1.91091
1.64    0.919576
2.32    -0.730975
2.9     -1.42001

Further reading: Official write.table documentation in R