Singapore

Article 17, clause 2 states: 

The President shall be elected by the citizens of Singapore in accordance with any law made by the Legislature.

Excerpt from the Constitution of the Republic of Singapore (1963, last amended 2010) 

Section 5, subsection 1 states:

Any person who on the prescribed date in any year — 

• (a) is a citizen of Singapore;
• (b) is ordinarily resident in Singapore; and 
• (c) is not less than 21 years of age, shall be entitled to have his name entered or retained in a register of electors in that year…

Excerpt from the Parliamentary Elections Act (1954, last amended 2011)