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So this week we finally used the logical expressions that we spoke about last week.

The reason one needs logic in a program is that different things have to be

done at different times, and decisions have to be taken based on this logic.

We used the example of the if statement in this lecture.

The data that we used has things that have to be patched, but only in a few places.

So we determined if, whether or

not anything needs to be done at that particular patch of the data.

It also means that we have to to loop over all the bits of data to check

what the if statement whether in fact anything needs to be done.

So we end up embedding the if statement inside a for loop.

Multiple curves in a single diagram.

So for the last lecture in this lesson, we look at comparing the Ebola incidence,

the total number of cases for the different countries.

So now we're going to take the data that they have in Ebola and

plot many different countries.

To do this, we will need if and for.

The if statement is wonderfully important in computing.

From time to time,

when you run a computer program, there must be several different options.

Computer programs that can actually just chug through one thing at a time

are actually worse than washing machine.

So you're going to write your code so

that the computer program will choose either this way or that way.

Depending on how things are developing.

The simplest possible choice is either you do something or you don't.

And this is what we need to do because our West African data is missing data.

So let's take a look at that.

We see that here.

Some of the countries did not report any data until a bit later on.

Epidemic day zero, only one country reported.

Epidemic day nine, two countries reported.

And that was also two on epidemic day 23 up to epidemic day 51.

But then from epidemic day 66, all the countries reported.

So this bit of missing data is what we need to replace.

First let's look at how the if statement works by some examples.

So let's look at this code here,

a generate a value a using the random number generator.

And if the value that was generated is larger than .5 we print a second message.

So after a is generated we print the value of a and

then if the value is on the bigger half then we print it.

So rand is a random number generator that generates a value between zero and one.

And if I rerun this I get a different value each time.

It's quite fun to look at random number generators in action like.

If we do it in a four loop then, basically,

all that we do is to take this code, the identical code, and

we put it in a four loop the the counter for the loop is the variable K.

And this will give us eight different values.

Now, we go on and we use this to fill in the missing data.

So, we're going to loop over all the data values and we're going to use a function,

the isnumber function.

This will test whether the data value can actually be read as a number.

This is either true or false, and so it is actually perfect in an if test.

So, whenever isnumber evaluates to false,

we must replace the missing element with a number.

And I choose 0, but one can do it in many other ways.

The isnumber test works only if the argument is a string.

But any string converts to a string so we convert all of the inputs to strings.

That means that when we read isnumber, we've read the EVD data

as string using the string function to convert into string so

isnumber gets the data as if it is a streak.

And then it sees whether it can read it.

If it can't read it,

that is if it's the case that not isnumber is true then we replace it by 0.

That's what we do.

And just to confirm that it's correct

we see that the strings have now all been replaced by zeroes.

Now we've got the dot form that we need.

Now it's really easy.

We're going to use plots, so you specify that we're using plots and

we specify that the back name is pipe plot.

The x values are all the same epidays.

We've extracted it by column one in the EVDdata.

The countries are columns 4, 6, and 8.

So column 1 epidemic days.

Columns two and three are the data for all countries.

Column two the total number of cases.

Column total number of deaths.

Column four total number of cases for the Guinea.

Column six total number of cases for Liberia.

Column 8 total number of cases for Sierra Leone.

So when we do this, either of these cases by country,

then the data structure which has all of the rows and three columns.

So it's a three column array.

And very plotted, it's absolutely straight forward.

Except that we don't quite like that plot as much as the otherwise.

So let's customize it again.

But it's already pretty useful, but we do need a legend.

We do need to break up the data so that it is scattered, rather than lines.

And we do need labels, turn up the grid lines and there we go.

Now we see it's not entirely satisfactorily because of this legend.

The legend key word is the one that tells us whether there is a legend or

not and also where it is.

We've seen that if we set the legend key word equal to false,

then there is no legend.

We don't to do that, but we want to change where it goes and for

that, we use topleft.

And, of course, we might as well save the plot

because we probably will use it again.

But that's the end of the lesson for week two.

So that brings us to the end of week two.

Where you've learnt about the important data in to Julia and

perhaps a little bit about where one can find data on the web.

It's also very important that you pay attention to the particular structures in

Julia that you've started learning about.

The for loop, the if structures, the use of logical statements and perhaps

that you should go away and not only do all the quizzes but experiment a little

bit on your own with making your own programs using these elements of Julia.

And that will get you ready for week three very nicely.

Plotting Multiple Curves in a Single Diagram

Julia Scientific Programming

Meet the Instructors