No, you cannot. Functions cannot be generally declared inside another function. You can only define them. The language was designed like that.
Also see these:
I think you need a good C book or tutorial. You can choose a good book from here, or check out this tutorial. I would recommend learning from a book, but stiil if you want to learn online, the tutorial mentioned is good. I myself used to refer to it when I was learning C.
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solved Function in a switch case loop [closed]