Data Science & Statistics: Indexing an element from a matrix

Key Takeaways

all right now that we know how to create and name a matrix let's learn how to subset and select single and multiple values from it remember what we said about matrices they are natural extensions to vectors well subsetting matrices works in much the same way indexing and slicing happens with vectors we just need to take into account that matrices have two dimensions instead of one ok let's get to it I will create a matrix very very quickly you this matrix gives us the box office grosses of Oh 8 Harry Potter movies on US soil and worldwide I have intentionally left the column and row names blank we will have no difficulty adding them later okay take a look at the matrix we just made it has eight rows into columns and our index system accordingly rows are indexed with the number of the row followed by a comma in square brackets whereas columns are indexed by a comma followed by the column number again in square brackets so why is that well this is the way our notation system is all set up extracting a value happens with square brackets and the values you won't extract it in those square brackets if you're working with chloride dimensional objects think of ours extraction schema as looking like this basically the element that's always there is the comma okay let's try and call a single value from the matrix 897 it is located on the sixth row and in the second column and just like indexing vectors we can type HP mat open square bracket roll number comma column number close the square brackets and we get the value we wanted 897 great easy what if I wanted to call an entire row however with all the values on that row let's say I want to get the US and worldwide box office figures for the movie on the sixth row if I just type this our returns 290 a single value but why does that happen well here is why when we give our this specific instruction it looks for the sixth value not the row not the column so R goes through all the values in the matrix one by one column by column left to right and it counts 290 is the sixth value so this is what is returned if we really want to get the entire six row I should have typed the comma after the row number and use the blank space to indicate that I want every value on that row open bracket six comma blank space closed brackets this returns 290 and 897 all the values in the sixth row the exact same logic applies to extracting an entire column but this time the comma precedes the column index so HP mat square bracket blank space comma to close the square bracket and voila all eight values for the second column notice that our returns the result as a vector instead of a matrix this is because we're only asking for a one-dimensional object and a vector suffices to store such information which brings me to the next type of subsetting selecting multiple elements this and more will be the topic of the next lesson for more videos like this one please subscribe