I have found many times nice solutions on stackoverflow. But I read somewhere you need some reputation points to add a comment and also 15 points to upvote. So until then if I want to thank the expert whose answer helped me I have to post as an answer. I did that and somebody downvoted me saying it should be a comment and not answer. Check my comment here:
So what are my options? How do I post a thank you comment? Note: I couldn't find in faqs that I cannot comment until I have some reputation.
The answer is - don't do anything.
However, if you eventually continue using the site, remember to come back and upvote the answer. You can mark the question as "favorite question" (click the star), so that it will remain in the "favorite" tab of your user profile and you could easily find it in the future.
"Thank you" comments are pointless noise. You can thank someone for answering your question by upvoting it. This is also why comments are restricted to being longer than 15 characters: precisely so that you cannot type "Thank you".
If you want to show your appreciation, that's fine. Wait until you get 15 reputation and then upvote the post in question. When you get more reputation you'll be able to comment, but be sure to contribute meaningfully with those comments. "Thank you" is not a meaningful contribution.
Later, when you've been with Stack Overflow for a long time, you'll come to see that this is a good way to handle things as when people come to visit Stack Overflow looking for answers they won't have to deal with meaningless comments inside the content they desire.
Even though you might think they can just ignore it, it does take real mental processing to determine if a comment is meaningful or not, and that slows people down which contributes to a negative mental image of Stack Overflow. We want to avoid that.
You need 15 rep to up-vote, and you need 50 rep to comment. For more information about the privileges you get on the site based on rep, see the privileges page which you can get to by clicking on your rep at the top of the page.
You can easily get this rep by posting 1 or 2 good questions and/or answers. For each up-vote on an answer, you get 10 rep. For each up-vote on a question, you'll get 5 rep. Post an answer that gets 2 up-votes, and boom! you can vote up answers that are helpful/useful.
Note that comments are not to just say thank-you. When you vote up a post, you are saying that it is helpful to you (hover over the vote up button and read the tool-tip). A comment that just says "thank you" is considered noise on these sites.
If you don't have enough rep to vote (or, if you are an anonymous user) you can click
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