Found the info about Singapore Dos & Donts
http://www.asiarooms.com/travel-guide/singapore/useful-information/singapore-dos-&-donts.html
Here is the conclusion then:
Laws relating to jaywalking, littering and chewing gum are strictly enforced in Singapore especially in urban areas. Smoking is highly discouraged and illegal in enclosed public places (including restaurants).
If you drop a cigarette end in the street or smoking illegally, an immediate fine can up to S$500.
As you see that the laws is very strict, therefore Singapore is a neat and clean country