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You can start working on your copy of Exercise 6 by accepting the GitHub Classroom assignment.
Exercise 6 is due by 16:00 on Wednesday 17.10.
You can also take a look at the open course copy of Exercise 6 in the course GitHub repository (does not require logging in). Note that you should not try to make changes to this copy of the exercise, but rather only to the copy available via GitHub Classroom.
Exercise 6 hints for Pandas¶
Data format for problems 1-3¶
The first 5 rows of the data file look like the following:
STATION ELEVATION LATITUDE LONGITUDE DATE PRCP TAVG TMAX TMIN ----------------- ---------- ---------- ---------- -------- -------- -------- -------- -------- GHCND:FIE00142080 51 60.3269 24.9603 19520101 0.31 37 39 34 GHCND:FIE00142080 51 60.3269 24.9603 19520102 -9999 35 37 34 GHCND:FIE00142080 51 60.3269 24.9603 19520103 0.14 33 36 -9999
As you can see, we have rainfall data (
PRCP) in inches, and temperature data (
TMIN) in degrees Fahrenheit.
Dates of the observations are given in the format YYYYMMDD.
No-data values are indicated with
Reading in fixed-width text files¶
Rather than having separation by commas, our data file this week has a variable number of spaces between values.
Previously, we read in comma-separated values using the option
sep=',' for the Pandas
For a variable number of spaces, we can simply change the
sep value to be
Skipping the second row of a file¶
skiprows=n option of the Pandas
read_csv() function is an easy way to skip the first n rows of a file when reading it.
If we wanted to skip the first two rows of our data file, we could thus use
The value for
n, however, need not be a single value, but can also be given in the form of a list.
In this way, one can skip reading the second row of a file using a list with an index value for the second row.
In other words, you can use
Joining data from one DataFrame to another¶
One quite useful functionality in Pandas is the ability to conduct a table join where data from one DataFrame is merged with another DataFrame based on a common key. Hence, making a table join requires that you have at least one common variable in both of the DataFrames that can be used to combine the data together.
Consider a following example. Let’s first create some test data to our DataFrames.
In : data1 = pd.DataFrame(data=[['20170101', 'Pluto'], ['20170102', 'Panda'], ['20170103', 'Snoopy']], columns=['Time', 'Favourite_dog']) In : data2 = pd.DataFrame(data=[['20170101', 1], ['20170101', 2], ['20170102', 3], ['20170104', 3], ['20170104', 8]], columns=['Time', 'Value']) In : data1 Out: Time Favourite_dog 0 20170101 Pluto 1 20170102 Panda 2 20170103 Snoopy In : data2 Out: Time Value 0 20170101 1 1 20170101 2 2 20170102 3 3 20170104 3 4 20170104 8
As we can see here, there different number of rows in the DataFrames. Important thing to notice is that there seems to be a common column called
Time that we can use to
join these DataFrames together. In Pandas we can conduct a table join with
merge -function. Consider following example where we join the data from
data2 DataFrame to
In : join1 = data1.merge(data2, on='Time') In : join1 Out: Time Favourite_dog Value 0 20170101 Pluto 1 1 20170101 Pluto 2 2 20170102 Panda 3
Ahaa! Now we can see that we managed to get the
Value column from
data2 in our
data1 DataFrame (here we just assigned those values to a new variable
Notice also that the
Pluto is two times in the joined DataFrame although, it was only once in the original one. Hence, Pandas automatically duplicates the values in such
columns where there are more matching values in one DataFrame compared to the other.
However, it is important to notice that there were more values in the
data2 DataFrame than in
data1. The result
join1, does not contain the values
3 and 8 that were from day
20170104 and they were omitted.
This might be okey, but in some cases it is useful to also bring all values from another DataFrame even though there would not be a matching value in the column that used for making the join (i.e. the
We can bring all the values from another DataFrame by specifyin parameter
how='outer', i.e. we will make an outer join.
Let’s consider another example with the outer join.
In : join2 = data1.merge(data2, on='Time', how='outer') In : join2 Out: Time Favourite_dog Value 0 20170101 Pluto 1.0 1 20170101 Pluto 2.0 2 20170102 Panda 3.0 3 20170103 Snoopy NaN 4 20170104 NaN 3.0 5 20170104 NaN 8.0
Cool! Nowe we have all the values included from both DataFrames and if Pandas did not find a common value in the
key column, it still kept them and inserted
NaN values into
Favourite_dog column and
Overall, knowing how to conduct a table join can be really handy in many different situations.
See more examples and documentation from official documentation of Pandas.
Exercise 6 hints for NumPy¶
Calculating average temperatures for each month (e.g., February 1954)¶
In problem 2 you’re asked to calculate average temperatures for every month between 1952-2016. There are a number of ways you can do this, like many things in Python programming. You might be tempted to create an empty array for the temperature values for each month and year in the range of dates, but this is not an ideal solution in case there are months or years missing data (Hint: there are). Instead, I would recommend a different approach where only years with data are included in the monthly averages, and we do not create the empty array first. Below is an example of such an approach.
# Note 2016 is missing year = np.array(['2014', '2014', '2015', '2017']) month = np.array(['01', '01', '02', '03']) # Make empty lists to store temperature values and their month num_monthly =  # Loop over all unique years for year_now in np.unique(year): # Loop over all unique months for month_now in np.unique(month): # NOTE: Here you should use an array slice to get tavg values only for month_now of year_now # I am just filling in the average of 10 random values for now, since I don't have tavg defined num_m = np.random.rand(10).mean() # Add the monthly average temperature to the temp_monthly list num_monthly.append(num_m) # Finally, we can convert num_monthly to a NumPy array num_monthly = np.array(num_monthly)
This will work even if years are missing, or listed multiple times in the data you’re handling.
We could add a test to check that the array slice for a given month is not empty, to protect against the case where we were missing data for some random month during a year when we have data for other months, but don’t worry about that for now.
And in case it isn’t clear,
np.array() converts a list to a NumPy array.
There is one other thing you’ll need to do! Because we need to know which month and year the average temperatures are from, you should also make two other empty lists like you would for the monthly temperatures. In those lists you can simply store the month and year every time you store a monthly average temperature, in just the same way. You’ll also have to convert those to NumPy arrays.
Calculating average temperatures for all months (e.g., February 1952-1980)¶
In problem 3 you have to first find the average temperatures for each of the 12 months for the years 1952-1980.
For this you can simply us a
for loop to loop over each month and find the mean temperatures for that month and all years between 1952-1980.
The lesson materials should give you some idea of how to handle this, and it is less complicated than the example from problem 2 of finding monthly average temperatures for each unique month and year.
Calculating temperatures anomalies¶
To find the temperature anomalies, you will need the average temperatures for the each month in the years 1952-1980 (i.e., 12 values), and the monthly average temperatures for each year and month over the years 1952-2016 (many values). The array of anomalies itself will be the same size as the number of monthly average temperatures you found in problem 2, so you can create that in advance. Filling the array can be done several ways, but the example below is one “simple” appraach.
# Loop over all months for i in range(len(temp_monthly)): # Here we can use a cute little trick to find the current month to compare to for the anomaly calculation # month_monthly will have all of the months that correspond to the temp_monthly values. # If we convert '01' to an integer and subtract 1, that will allow us to compare to the first value in ref_temps, the one for January (i.e., index 0). ref_index = int(month_monthly[i]) - 1 ref_temp_now = ref_temps[ref_index] # Here you should calculate the temperature anomaly. I'm filling in 1.0 since I think you folks can handle this part :) anomaly[i] = 1.0
Checking your work for problem 2¶
In case you want to double check that you are getting the correct answers for problem 2, you code should produce the following when you run the commands below.
print(temp_monthly[:7]) [ 29.47826087 24.8 13.80769231 39.60714286 44.66666667 56.5 61.21428571]
print(temp_monthly_celsius[:7]) [ -1.40096618 -4. -10.10683761 4.22619048 7.03703704 13.61111111 16.23015873]
Checking your work for problem 3¶
In case you want to double check that you are getting the correct answers for problem 3, you code should produce the following when you run the commands below.
print(ref_temps[:7]) [ -5.87734242 -6.9904821 -3.84126984 2.42787524 9.52261307 14.71189774 16.49888143]
print(anomaly[:11]) [ 4.47637624 2.9904821 -6.26556777 1.79831523 -2.48557603 -1.10078663 -0.2687227 -0.86436896 -1.44938108 -2.78452381 -2.7044648 ]