In English, we often use the phrase:
Off the top of my head: from the knowledge you have in your memory.
E.g. we might say:
Off the top of my head, I don't know the answer.
I can think of one example off the top of my head.
He can recite poetry off the top of his head.
It's often used in interviews, etc., when someone asks a question and you don't immediately know the answer, but you could figure it out fairly easily. If you answer the question, you could say the wrong thing. It's better than saying a plain "I don't know", as it merely implies you haven't memorized the answer.
I'd like to express this in Chinese, but I don't know how to...
Question: How can I express "off the top of my head"?
Google Translating the above examples gives:
在我的脑海中，我不知道答案。
我能想到一个例子。
他可以在头脑中背诵诗歌。
For the second sentence, it didn't translate "off the top off my head".
Searching on YouDao and Jukuu gives:
对不起，我现在一下子不能给你精确的数字。
我不经考虑无法告诉你答案。
我没办法立刻告诉你准确消息，我得核实一下。
我随口回答了他的问题。
Maybe some of these are better than others.
