in r programing create A function called slope that is endowed with controls to prevent the stupid things that people can do when writing a function

$($as suggested in the lecture slide$)$ and also allowing for slope to be interpreted both as a number $($like in algebra$)$ and as an angle $($such as in geometry$).$ This function should also respond to calculating the slope based on one point $($assuming the origin as the other point$)$how do I create a part of the code that accounts for two missing values for x$2$ and y$2$ and make them $0?$