For president: without 'the'

That is right: it is not necessary.

But to make mud clearer, you would need it if you were to say:

'Please recommend some students for the position of class president'

No, laogui. As you yourself have shown, 'the' is still not needed for 'president'. Your 'the' modifies 'postion'.

The original question was Is 'the not necessary in the second sentence? The question refers to the sentence not to the word president. By the way, why is president indefinite in the original sentence - is the indefinite article a understood?

The original question was Is 'the not necessary in the second sentence? The question refers to the sentence not to the word president.-- Please do not be facetious, if that is what you are doing. This shows that you are not able to grasp the poster's intent, laogui. If you wish to assist in answering questions here, then you need to be aware of what the poster is really asking, because most

I give you credit for the generosity of spirit of your Namesake so I assume your keyboard was stuck in bold, and was in no way a sign of ill temper; and perhaps you meant pedantic rather than facetious.

When anyone asks for an answer, I think they also hope for an explanation. Don't you?

The first thing I saw in the sentence was a pattern of speech that raises the question 'Why?'

I've read somewhere that this pattern can be used when talking about a position that is unique within an organization/institution/company etc.

For example

They named him head librarian/Head Librarian. (yes you can even capitalize the title)

moon I did some research and discover my error - there are many exceptions when you don't need to use articles with nouns - you do not use articles with the name of sports or school, and you usually don't need them with president or king or queen or pope for example. English articles can be as complicated as chinese measure words with nouns it seems.

Thank you so much for your in-depth research!

I often get into confusion and ask questions.

Through answers of all posters, I realized again how complicating English articles(a/an/the) are even to some native speakers.(They know but perhaps it's hard for them to put them into explanation).

I appreciate your answers and apologise for not making clearer question.