Practical 1: First steps

Start up as terminal and then the python interpreter by typing ./python. You should see the version information and a command prompt:

You can run simple python code from the ">>>" command prompt.

a, b, and c are variables, 1, 2 and 2.75 are values, = is the assignment operator.

==, >, <, <= and >= are the basic logical operators used to test for equality, greater than, less than, less than or equals and greater than or equals. True and False are logical values representing truth and falsehood.

The Boolean operators and, or, and not, can be used to combine logical tests. Brackets (parentheses) can be used to group and order them.

Quit the interpretor (type quit() or Ctrl-D).

The python interpreter can also be used to execute a series of statements in a file. These statements are executed in sequence from the first line to the last. Use your favorite text editor to create a file called count.py containing the following python code:

Run this code by typing python ./count.py at the shell command prompt. What is printed to the screen? Is this what you expect?

You should see the numbers 1 to 9 and the string "The end" printed before the interpreter exits and you are returned to the command prompt. The while statement begins a block of code that is executed in a loop until some logical condition is false. What is the logical test in your count program? Is this tested before or after the block of code guarded by the while statement is run?

Modify the logic in your program to make it count to 10. Is there another way to change the logic to achieve the same thing? Now change the program to count backwards from 10 to 1. Change it again to only print the even numbers.

Often, one of the most disconcerting features of Python is that blocks of code are delimited by indentation. Most programmers will indent loops and other blocks of code they write but Python is unusual in demanding indentation and using it as part of the syntax of the language. If you’ve not already inadvertently found out, see what happens if you change your counting program so that the indentation is not consistent. For example, indent the print line more than the i = i + 1 line.

What does the following code snippet do?

Are you sure? It’s in the file count_func.py, run it to find out.

Although Python programs are executed sequentially, from the start to the end of the file, like many languages, you can specify a piece of code should form a subroutine or function that can be run later. When the interpreter reaches a block of code guarded by the def statement a function is defined. The function is not run until it is called by name. Modify count_func.py to call the count function:

What does this code do?

What happens if you try and call the function before the function has been defined? Does this work:

Functions can (optionally) have one or more arguments (in the parentheses). These allow the function to be more general. Modify your code to accept an argument, n, the upper limit of the printed numbers, and m the lower limit. Print three lists of numbers, 1 – 10, 14 – 27, and 88 – 96.

The third block statement allows pieces of code to be executed conditionally. What does the following code print?

Are you sure? It’s in the file count_if.py, run it to find out.

The block of indented code guarded by an if statement is only executed when the condition is true. Note that we can embed one indented block within another. What does the following print?

Can you condese the two nested if statments into one with more complex logic?

Now look at how python handles mathematics. Beware, there may by one or two surprises. Use the interpreter interactively to decide:

The arithmetic operators work as you may expect.

Is this what you expected?

You can also do calculations with floating point numbers. * What is 1.0 - 0.5? * What about 1.2 - 1.0? * Is that correct? what’s going on?

Complex numbers are built in. The operators work as you may expect.

Another data type in Python is the string. Literal strings are embedded in single or double quotes and strings can be assigned to variables just like the other types:

In Python strings are a sequence of characters numbered from 0. Sub-strings can be accessed using an index notation using brackets (not parentheses) containing integers separated by colons. When only one index is provided a single character is returned. When two indices are provided a series of characters are returned from the first, to one less than the second index. If left out (but with the colon in place) the indices default to the start and end of the string.

When placed between strings, + acts as a concatenation operator and * works as a repetition operator.

The third list index specifies how the sub-string is constructed by stepping through the string. Negative indices count backwards from the end, or reverse along the string. Note that strings are immutable. You cannot use the index notation to change bits of a string. We’ll cover this concept in more detail later.

Strings are the first of Python’s built in sequence types; lists are the second. To a large extent, if you know how to use one sequence type you know them all. In a list instead of each numbered "container" holding a single character, each container can hold any variable. Start up a python intepreter.

Note that lists can contain variables of different types mixed together. We can access characters within a string within a list by using multiple indices next to each other.

Python contains several other built in sequence types (there are seven in total). We’ll briefly look at them later. Some work like strings (and can only contain a particular type of data) and some work like lists (and can contain any type of data). Some sequence types are mutable and others are immutable. All sequence types support index notation with brackets and colons, along with several other operations including + and *.

We’ve seen that Python supports more than one sequence type. When writing a program, sequences (usually lists) are commonly used to store the working data. We often want to do "something" to each item of data in turn: perhaps we want to calculate the average of all the numbers in the list, maybe we want to find the maximum or add a number to each element of the list. What we need is a way to iterate through the list. A block of code using the for x in l: statement allows us to do this. What does the following program print?

The program can be found in the file for_in.py. Run it to confirm your suspicion.

Modify the program to print the numbers from 0 to 10. Use the range function to generate the list.

With one argument, n, the range function generates a list of n integers starting a zero. range() can also take two integer arguments. What is the meaning of the first and second arguments in this case? Modify the program to print the numbers from 1 to 10.

Range can also take a third argument. Modify the code to print the even numbers between 1 and 10 using the three-argument form.

Does range accept four or more arguments? Does it accept negative arguments? What do they do?

It is sometimes necessary to iterate through two sequences together in lock step. The zip() function can be used for this. What does the following program (zip.py) print? Do you see how it works?

Modify the program to calculate the average monthly rainfall in the year using two other variables and the for loop above. Print this after the monthly report. Imperial measurements are outdated. Convert the output into mm.

If rain = [10.2, 12.4, 9.4, 8.2, 8.9, 7.2, 3.9, 7.6, 8.0, 11.2, 10.5, 9.9], what is len(rain)? What is sum rain? Calculate the average monthly rainfall without using for.

What does the following program (caesar.py) do? The variable names may help.

Can you write a program to perform the inverse function?

Dictionaries are another data type found in Python. A dictionary is equivalent to a hash in Perl, std::map in C++ and "maps" in Java, all are implementations of associative arrays. Like a list a dictionary links a key to a value. A list’s key is the index and this must be an integer. This means that lists have a natural order (for the list l, l[3] comes after l[2] and before l[4]). Many data types can play the role of a key in a dictionary. This means dictionaries have no natural order and you should never assume that keys and values will be returned if you iterate through a dictionary twice. This is OK, there are many things in the world you may want to refer to in a program that don’t have a natural order (but may have an unnatural order). Words and word definitions in one of those books of the language, for example.

Dictionaries and tuples turn out to be quite important if you want to really understand Python, but you can get by without them.