How to make contributed module hook execute only if conditions are met. Those functions seem to get automatically invoked. Environment is a custom module.

examples: 1.

if ($condition) {

   function mymodule_modulename_alter ($parameters) {
     //altering code here
   }

}

    if ($condition) {
           //if condition is met, i call the function included in this custom module
           mymodule_modulename_alter ($parameters)
      }

  function mymodule_modulename_alter ($parameters) {
         //but this function will get auto-invoked! I want it run only if $condition is met.
       }

Thanks