It is fairly easy but tends to involve much proofreading, editing and testing to ensure that all code is entered correctly (especially variable names). (tes.com)
The programmer can select the code from one program and an insertion point in a second program, and the system will automatically make modifications necessary — such as changing variable names — to integrate the code into its new context. (sciencedaily.com)
For example, using inconsistent formatting, lacking spacing and indentation, using unnecessarily terse variable names, or having code duplication. (inc.com)