Structure and readability of basic calculator
Minor stuff:

I'm getting several compiler warnings. Capture the returned variable
    from scanf() to check if the user actually entered a number. 

main.cpp:17: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
         scanf("%f", &amp;numtwo);
                             ^


Your if-else statement needs to be fixed.

else {
numtwo = 0;
}