# Hints for Exercise 6¶

Below are some tips for working on Exercise 6.

## 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 (TAVG, TMAX, and TMIN) in degrees Fahrenheit. Dates of the observations are given in the format YYYYMMDD. No-data values are indicated with -9999.

## 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 read_csv() function. For a variable number of spaces, we can simply change the sep value to be sep='\s+'.

## Skipping the second row of a file¶

The 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 skiprows=2. 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 skiprows=[1].

## 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 [1]: data1 = pd.DataFrame(data=[['20170101', 'Pluto'], ['20170102', 'Panda'], ['20170103', 'Snoopy']], columns=['Time', 'Favourite_dog'])

In [2]: data2 = pd.DataFrame(data=[['20170101', 1], ['20170101', 2], ['20170102', 3], ['20170104', 3], ['20170104', 8]], columns=['Time', 'Value'])

In [3]: data1
Out[3]:
Time Favourite_dog
0  20170101         Pluto
1  20170102         Panda
2  20170103        Snoopy

In [4]: data2
Out[4]:
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 data1 DataFrame.

In [5]: join1 = data1.merge(data2, on='Time')

In [6]: join1
Out[6]:
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 join1). 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 key).

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 [7]: join2 = data1.merge(data2, on='Time', how='outer')

In [8]: join2
Out[8]:
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 Value column. 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.