Hi there,

I am working on a simple example to understand how to define a new column with mathematical operation such as sqrt, abs, and log. In particular, I found out that the log operation return nan value both in C and python.

To illuminate the issue, the C version is shown below:

```
ROOT::RDataFrame d("Events", "/media/shoh/02A1ACF427292FC0/nanov7/BDT_training_datasets/DYJetsToLL_M-50_v7_ElePromptGenMatched.root");
auto zMean = d.Define("logd" , "log(Electron_dxy)").Mean("logd");
std::cout << *zMean << std::endl;
```

zMean return nan

The goal is to evaluate log(abs(Electron_dxy)) and append as a new column in RDataframe.

Thanks and looking forward to hear from you.

Siewyan

_ROOT Version: 6.22/02

*Platform:* Not Provided

*Compiler:* Not Provided