diff --git a/pipreqs/pipreqs.py b/pipreqs/pipreqs.py
index 0186832..d60ac4a 100755
--- a/pipreqs/pipreqs.py
+++ b/pipreqs/pipreqs.py
@@ -349,7 +349,10 @@ def compare_modules(file_, imports):
 
     imports = [imports[i]["name"] for i in range(len(imports))]
     modules = [modules[i]["name"] for i in range(len(modules))]
-    modules_not_imported = set(modules) - set(imports)
+    # modules => declared in requirements
+    # imports => all modules needed
+    # not imported => all modules - declared
+    modules_not_imported = set(imports) - set(modules)
 
     return modules_not_imported
 
@@ -359,7 +362,7 @@ def diff(file_, imports):
     modules_not_imported = compare_modules(file_, imports)
 
     logging.info(
-        "The following modules are in {} but do not seem to be imported: "
+        "The following modules are in {} but do not seem to be imported: \n"
         "{}".format(file_, ", ".join(x for x in modules_not_imported)))
 
 
@@ -432,8 +435,12 @@ def init(args):
             os.path.join(input_path, "requirements.txt"))
 
     if args["--diff"]:
-        diff(args["--diff"], imports)
-        return
+        if os.path.exists(args["--diff"]):
+            diff(args["--diff"], imports)
+            return
+        else:
+            logging.info("The file [ %s ] not exist" % args["--diff"])
+            return
 
     if args["--clean"]:
         clean(args["--clean"], imports)