I think you should be comparing the string WITHIN the struct.
Username[i].usernameadmin == username
This would compare string username with string usernameadmin.
The == operator can be used to compare strings in C++, thanks to overloading.
You can still use .compare as well.
Username[i].usernameadmin.compare(username)
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solved How to compare a string with a struct (which include strings too)? [closed]