IT 116: Introduction to Scripting

IT 116: Introduction to Scripting
Class 19

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Solution to Homework 7

I have posted a solution to homework 7 here.

Let's take a look.

Homework 9

I have posted homework 9 here.

It is due this coming Sunday at 11:59 PM.

Questions

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Tips and Examples

A Simple Function

A function is just a group of Python statements that is given a name ...
and does something useful
You should create a function whenever you do something more than once
A simple example of a function is one that converts Celsius to Fahrenheit
There is the formula for this conversion
```
fahrenheit = 9/5 * celsius + 32
```
If I only needed to calculate this value once ...
I would not write a function for it
But if I needed to do this more than once ...

I would write the following function

def celsius_to_fahrenheit(celsius):
    return round(9/5 * celsius + 32)

If I used this in more than one script I would put it in a module
It could then be imported it into any script where it was needed

Don't Use readline() in a `for` Loop

The readline() method is not often used
You could use it in a while loop to read a file
But this is usually not the best way to read a file
A for loop is the best way to work with a file ...
in most situations
But you should NEVER USE readline() in a for loop
If you do, you will not read every line
Here is an example of what happens ...

when you use a readline() in a for loop

$ cat 1-5.txt 
1
2
3
4
5

>>>  file = open('1-5.txt', 'r')
>>>  for line in file:
...     line = file.readline()
...     print(line.strip())
... 
2
4

Why did this happen?
The for loop reads the next line
But the first line in the code block calls readline
which replaces the value in the variable line ...
with that of the following line
This happens with each pass through the loop
So the code prints every other line
I have seen students make this mistake on exams more than once

Review

Getting Rid of Newline Characters

Lines in a text file have a newline, \n, at the end of each line
You must remove this newline character when printing the line ...
or there will be a blank line after each printed line
As you will see in a few weeks, strings are objects
Like most objects, they have methods
strip is a string method that removes whitespace at both ends of a string
You use it like this
```
line = line.strip()
```

Writing and Reading Numbers

In this course we will only be dealing with text files
Any numbers you read from a file will be strings
You can't perform arithmetic on strings
You need to convert them into numbers using either int or float
When writing a number to a file, you must convert it into a string
You can do this with the str conversion function
If you try to write a number to a file you have opened with "w" ...

you will get an error

>>> file = open("numbers.txt","w")
>>> file.write(5)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: write() argument must be str, not in

Appending Data to an Existing File

If you write something to a file that already exists ...
it will erase what was in the file
To avoid doing this use the append access mode, "a"
Writing to a file opened in this mode adds text to the bottom of the file

Reading Files with a `for` Loop

The easiest way to read a file in Python is with a for loop
The header of for loop has this format
```
for LOOP_VARIABLE in LIST_OF_SOME_KIND:
```
The last item is some sort of list
A text file is a list of lines
This means we can use a file object in a for loop
```
for line in file:
    print(line.strip())
```

Attendance

New Material

Running a Script Using `python3`

To write a program in the computer language C you create a text file ...
that contains statement in the C language
This file is called the source code
You cannot run this file
You have to run the source code through a program called a compiler
The compiler creates a file of binary instructions ...
that the CPU understand
These binary instructions are called machine language
Our scripts are text files containing Python statements
The computer does not understand Python text
Only the Python interpreter understands Python statements
That means to run a Python script we need two things in RAM
- The text of the script
- The binary code for the Python interpreter
The picture in memory looks like this

The Python interpreter was written in a language like C ...
and compiled into an executable file
When we type python3 at the command line ...

we are running Python in interactive mode

$ python3 
Python 3.5.2 (default, Apr 16 2020, 17:47:17) 
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>

To run a script we can type python3 followed by the name of the script
The interpreter then reads each Python statement ...
translates it into machine language ...
and runs the translated statements

If I have the Python script hello_1.py

# prints a friendly message

print("Hello world!")

I can run it on the Unix command line like this
```
$ python3 hello_1.py 
Hello world!
```

Making a Script Executable

Unix gives us another way to run a Python script

Look at the Bash script hello1.sh

# prints a friendly message

echo Hello world

I can run this script using the bash command
```
$ bash hello_1.sh
Hello world!
```
The program bash can run a Bash script
Just like the program python3 runs a Python script
But if I change the access permission on this Bash script ...
using the Unix command chmod ...
and give everyone read and execute permission
```
$ chmod 755  hello_1.sh
```
I can now run the script directly without using bash
```
$ ./hello_1.sh 
Hello world!
```
In order to run a script in my current directory ...
I have to put . / before the script name
The reason for this has to do with Unix security
If I give the same permissions to a Python script hello_1.py
```
$ chmod 755  hello_1.py
```

And try to run it, I will get an error

$ ./hello_1.py
./hello_1.py: line 3: syntax error near unexpected token `"Hello world!"
./hello_1.py: line 3: `print("Hello world!")'

You get this error because Unix knows the script contains text ...
not machine language instructions
Whenever you ask Unix to run a text file ...
it assumes that the file contains Unix commands
So the binary code it loads into the process RAM ...
is the binary code for Bash
But a Python script does not contain Unix commands ...
so we get an error
Unix has a special feature which we can use here
This features allows us to tell Unix to run python3 on the file ...
instead of running bash
To do this we must add a line with a special format ...
as the first line of the script
The first two characters must be
```
#!
```
Followed by the absolute pathname of the program to run the script ...
instead of bash
An absolute pathname holds the name and location of a file ...
and can be used inside any directory
On our Unix systems we use /usr/bin/python3 to run python scripts
To make Python scripts executable on Linux ...
we first give the file read and execute permission
On the Unix command line this is done using chmod
```
chmod 755 FILENAME
```
It can also be done in FileZilla
But then we must add the special line I mentioned above
Here is the line for our Unix system
```
#! /usr/bin/python3
```

The script now looks like this

#! /usr/bin/python3

# prints a friendly message

print("Hello world!")

I can now run the script without running python3
```
$ ./hello_2.py 
Hello world!
```
This special line is called the hashbang line
Since ! on Unix is often called "bang"
This is often referred to as shebang
For some reason, this word makes my skin crawl
So I use hashbang
This line must be the first line in the script
And the first two characters must be #!
If the script were written in Perl we would need a different pathname
We would have to use the absolute pathname of the Perl interpreter
We would have to write
```
#! /usr/bin/perl
```
From now on all your scripts must be executable
Or you will lose points
I have changed the Homework Script Rules to add this requirement

Why Make a Script Executable?

Why go through the trouble of making a script executable?
Typing python3 before the name of a file is no great hardship
Let me show you why making a script executable is a good idea
I have a script that sends an email to all students ...
who have not submitted a homework assignment
The script is email_missing.py
What's the big deal about typing the following at the command line?
```
python3  email_missing.py
```
The problem is this won't work
It would work if email_missing.py were in the directory ...
where I needed to run it
But the script is never in that directory
I have to run this script in a special directory ...
where I collect all your homework assignments
And I have a directory like this for each class
But email_missing.py is in the directory /home/ghoffman/bin/python

To use python3 to run the script I would have to write

python3  /home/ghoffman/bin/python/umb/email_missing.py

If I made a mistake typing the pathname ...
I would get an error
I want to be able to run this script by simply typing
```
email_missing.py
```
I can achieve this if I do three things
- Create a hashbang line
- Make the script executable
- Put the script in a directory I have added to the PATH shell variable
As you will learn in IT 244, PATH is a special Unix variable ...
that tells Unix were to look for the executable files
I have included /home/ghoffman/bin/python/umb in my PATH variable
So when I type email_missing.py on the command line ...
I can run the script from any directory

Making Scripts Executable with FileZilla

All work you create for this course must be copied to pe15.cs.umb.edu
Which you can do using FileZilla
But now you need to make your Python scripts executable
You can also do this with FileZilla
Right-click on the file and select "Permissions" from the menu
Enter 755 in the box provided
This will make the script executable

Averaging the Numbers in a File

Let's use a for loop to average the numbers in a file
Say we have a text file containing integers, one to a line
```
$ cat numbs.txt
1
2
3
4
5
6
7
8
9
10
```
We need to create a file object, so let's ask the user for the filename

We can use this to create a file object for reading

filename = input("Filename: ")
file = open(filename, "r")

We will need to total the numbers
But we also need to count them
So we must initialize two accumulators to 0
```
total = 0
count = 0
```
Now we need a for loop that will read each line
In the body of the loop we do two things
First we increment the count
```
count += 1
```
Then we convert the line to integer ...
and add it to the total
```
total += int(line)
```

Finally we calculate the average and print it

average = round(total/count, 2)
print("Average:", average)

This gives us the following script

#! /usr/bin/python3

# reads numbers from a file and averages them

filename = input("Filename: ")
file = open(filename, "r")

total = 0
count = 0
for line in file:
    count += 1
    total += int(line)

average = round(total/count, 2)
print("Average:", average)

The script has a hashbang line and I have made it executable

So I can run the script like this

$ ./for_average.py 
Filename: numbs.txt
Average: 5.5

The text in blue is user input

Records

For any sporting event involving teams we have at least 5 pieces of information
- Date
- Home team
- Opposing team
- Home team score
- Opposing team score
The collection of data about a specific event is called a record
The individual values within a record are called fields
A text file containing data usually has one record per line

For example, we might have a file of dates and the highest temperature

$ cat temps.txt 
2017-06-01 67
2017-06-02 71
2017-06-03 69
2017-06-04 88
2017-06-05 74
...

Here the fields are
- Date
- Highest temperature

Reading Records from a File

How can we capture two pieces of information from a single line?
To do this we need to know where one field ends ...
and the next begins
The characters used to separate one field value from another...
are called delimiters
In temps.txt the delimiter is a space
When you export data from Excel you can choose many formats
Very often people chose CSV
Which stands for Comma Separated Values
In these files a comma separates the field values
We know how to read in a text file line by line
But how do we get each field?
We can use a string method called split

split breaks up the line into individual fields

>>> line = '2017-06-01   67'
>>> line.split()
['2017-06-01', '67']

Now can use multiple assignment ...
to give the value of each field to a variable
```
>>> date, temp = line.split()
```

With this approach we can read each record in temps.txt

$ cat temps_print.py
#! /usr/bin/python3

# prints each field in temps.txt

file = open('temps.txt', 'r')

for line in file:
    date, temp = line.split()
    print(date, temp)
    
$ ./temps_print.py
2017-06-01 67
2017-06-02 71
2017-06-03 69
2017-06-04 88
2017-06-05 74
...

Finding the Average From a File of Records

We read a file of records to do some processing of the data
Like calculating the average
We can do this by making a copy of temps_print.py ...
and modifying the code
First we need add two accumulators
```
count = 0
total = 0
```
The for loop header stays the same
But we need to add a statement to count lines
```
count += 1
```
And we need to update total
But before we do this we must turn temp into an integer
```
temp   = int(temp)
total += temp
```
Outside the loop we need to calculate and print the average
```
average = round(total/count, 2)
print('Average:', average)
```

Putting this all together we get

$ cat temps_average.py 
#! /usr/bin/python3

# calculates the average the temperature in temps.txt

file = open('temps.txt', 'r')

count = 0
total = 0
for line in file:
    count += 1
    date, temp = line.split()
    temp   = int(temp)
    total += temp
    
average = round(total/count, 2)
print('Average:', average)

The added statements are shown in red
Now we run it
```
$ ./temps_average.py 
Average: 77.27
```

Additional Processing of Records

In the code above we only calculated the average
But we can do more than that while processing the file
Why not compute the highest temperature and the lowest?

To compute the highest temperature we use the following algorithm

set max to the lowest possible temperature
for each temperature in the file
    if the temperature is greater than max
        set the max to this temperature

Why did do we have to set the variable to a low value?
To make sure it is replaced by a real value ...
we encounter in the loop
If we set the variable to 100
It would never be replaced by any value in temps.txt
We need to set this variable before entering the loop
```
max = -100
```
Then we need to add an if statement to the loop
```
    if temp > max:
        max = temp
```

A similar algorithm will calculate the lowest temperature

set min to the highest possible temperature
for each temperature in the file
    if the temperature is less than min
        set min to this temperature

Once again we need to set the the variable before entering the loop
```
min = 200
```
And update this value inside the loop
```
    if temp < min:
        min = temp
```

Here is the script with the additions in red

$ cat temps_max_min_average.py 
#! /usr/bin/python3

# calculates the average, maximum and minimum in temps.txt

file = open('temps.txt', 'r')

count = 0
total = 0
max   = -100
min   = 200
for line in file:
    count += 1
    date, temp = line.split()
    temp  = int(temp)
    total += temp
    if temp > max:
        max = temp
    if temp < min:
        min = temp

average = round(total/count, 2)
print('Average:', average)
print('Maximum:', max)
print('Minimum:', min)

When we run this we get

$ ./temps_max_min_average.py 
Average: 77.27
Maximum: 89
Minimum: 66

Closing a File Object

All objects take up space in RAM
For small programs like ours with only 1 or 2 objects ...
this is not something we have to worry about
But in big programs with lots of objects ...
the space they take up can be significant
This can make the script run more slowly ...
of even cause the script to crash
In big programs like this you would want to get rid of an object ...
when you are done using it
To get rid of a file object use the close method
Like this
```
file.close()
```
For the small programs we write we don't have to worry about this
The interpreter will close the file object for you ...
when the script ends

Looping Through a File More Than Once

How would we calculate the number of days the temperature was above average?
We need to loop through the file more than once to do this
We loop through it once to get the average
Then we loop through it again ...
to count the days with temperatures above average
We read a text file using sequential access
That means we can only read a file in one direction
Once we reach the end of a file ...
we cannot go back to the beginning
Instead we have to create a new file object ...
for a second run through the file

The algorithm we need is similar to that for calculating a maximum

set days_above to 0
for each temperature in the file
    if the temperature is greater than the average
        add 1 to the days_above

We make a copy of temps_average.py ...

and add the following code

file = open('temps.txt', 'r')

days_above = 0
for line in file:
    date, temp = line.split()
    temp   = int(temp)
    if temp > average:
        days_above += 1

print("Days above average:", days_above)

When we run this we get

$ ./temps_days_above_average.py
Days above average: 13

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