Failed to use gmapping launchfile [new user]

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I have slam_gmapping package that contains these folders:

    slam_gmapping-hydro-devel

       gmapping

           launch

           src

           test

       slam_gmapping

I want to launch it with launch file with my own parameters.

the steps I have done are as follow:

• download the package

• copy the package to the catkin_ws --> src

• make a build folder in gmapping folder, so it is now look like this:

  slam_gmapping-hydro-devel
  
     gmapping
  
         launch
  
         src
  
         test
  
         build
  
     slam_gmapping
  

• build the package using make, cmake .., catkin_make (don't know exactly what they do!)

The question is after those steps, "roslaunch gmapping [Tab]" does not find any launch file But there is a launch file "slam_gmapping_pr2.launch" in launch folder I also tried to use launch file provided here . But the same problem exist. The problem is that ROS does not detect any launch file in the gmapping package! how can I solve the problem?

Thanks