When using Copy-Paste, be especially careful with the last lines

This bug was found in Source SDK library. The error is detected by the following diagnostic: V525 The code containing the collection of similar blocks. Check items ‘SetX’, ‘SetY’, ‘SetZ’, ‘SetZ’.

inline void SetX( float val );
inline void SetY( float val );
inline void SetZ( float val );
inline void SetW( float val );

inline void Init( float ix=0, float iy=0,
                  float iz=0, float iw = 0 ) 
  SetX( ix );
  SetY( iy );
  SetZ( iz );
  SetZ( iw );


We are 100% sure this code was written with the help of Copy-Paste. One of the first lines was copied several times, with certain letters changed in its duplicates. At the very end, this technique failed the programmer: his attention weakened, and he forgot to change letter ‘Z’ to ‘W’ in the last line.

In this example, we are not concerned about the fact of a programmer making a mistake; what matters is that it was made at the end of a sequence of monotonous actions.

I do recommend reading the article “The Last Line Effect“. Due to public interest a scientific version of it also got published.

Put briefly, when copying code fragments through the Copy-Paste method, it is highly probable that you will make a mistake at the very end of the sequence of copied lines. It’s not my guess, it’s statistical data.


Correct code

  SetX( ix );
  SetY( iy );
  SetZ( iz );
  SetW( iw );


So, once again, we are dealing with the following phenomenon. When writing similarly looking code blocks, programmers copy and paste code fragments with slight changes. While doing so, they tend to forget to change certain words or characters, and it most often happens at the end of a sequence of monotonous actions because their attention weakens.

To reduce the number of such mistakes, here are a few tips for you:

  1. Arrange your similar looking code blocks in “tables”: it should make mistakes more prominent. We will discuss the “table” code layout in the next section. Perhaps in this case the table layout wasn’t of much help, but still it’s a very useful thing in programming.
  2. Be very careful and attentive when using Copy-Paste. Stay focused, and double-check the code you have written – especially the last few lines.
  3. You have now learned about the last line effect; try to keep this in mind, and tell your colleagues about it. The very fact of you knowing how such errors occur, should help you avoid them.

Written by Andrey Karpov.

This error was found with PVS-Studio static analysis tool.

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