With the passage of time and with increase in the social circle, the phone book continues to become heavier to search. Especially, when number of phone book entries enter 4-digit, the search by scrolling the contacts becomes almost impossible and the built-in in Android 6.0.1 Marshmallow mechanism comes to your help.
This is easier but if you need help, just see the instructions.
Instructions
1: Tap the ‘Dialer App’ icon on the bottom left corner of your Android 6.0.1 Marshmallow smartphone or device. It will take you to the ‘Dialer App’ screen.
2: At the ‘Dialer App’ screen, tap the Search Bar, at the top of the screen, but below the status bar. It will open up keyboard at the bottom of the screen.
3: Write the name of the contact at the Search Bar with the help of the Keyboard. See; a long list of all the contacts with the same name or similarity will appear. Here, you may select any of the required contact.
4: For more refined search, tap the ‘Contacts’ and then at the Contacts screen, tap at the Search Bar. A long list of the contacts will open up.
5: Again, write the name of the searched contact with the help of the Keyboard at the bottom of the screen. 
6: See; a long list of the contacts with similar names will appear from where you may select the one you searched.
That’s all!