Question

I am writing a functional test in R for a program in C++. The typical output file has some columns as “string” and the other columns as “double”. Then, I would like to use diff to compare the expected output file returned by R with the observed output file returned by C++.

In pseudo-C++, I simply do this:

stringstream ssTxt;
ssTxt.precision (7);
ssTxt.setf (ios::scientific);
for(i=0; i<10; ++i){
  ssTxt << names[i] << " " << values[0][i] << " " << values[1][i] << " " << values[2][i] << endl;
  // write in file and clear ssTxt
}

Here is a typical output:

item1 2.8200000e-01 500 4.1846912e-04

In R, I do that:

results <- data.frame(name="item1", val1=0.282, val2=500, val3=0.00041846912873)
write.table(x=format(results, digits=8, nsmall=7, scientific=TRUE), file=...)

Here is the output corresponding to the same data:

item1 2.82e-01 500 4.1846912e-04

As you can see, it almost works, but R doesn’t add trailing 0 after “2.82”. I prefer to change the R code rather than the C++ code. So how can I do that?

Answer 1

Have you tried
sprintf or formatC?

To be specific I think

sprintf("%e", 2.82e-01) 

should do the trick but as mentioned above have a look at help(sprintf), which describes the various formatting capabilities of this function …