Introduction to `python`¶

Basics¶

Mauricio Sevilla¶

email= j.sevillam@uniandes.edu.co

11.01.19

We have been working on the shell so far, but now, we are going to start talking about programming on python.

First of all, we are going to talk about the way python works compared with different programming languages such as C/C++

Programming languages families¶

One can divide the programming languages in two big families,

Compiled
Interpreted

and the two of them have huge differences!.

Generally compiled languages are way more dificult to write than interpreted laguages.

That is because the computer must translate the codes into binary operations so that they can be performed.

As we cannot write binary, we use a different Language!

There are two different strategies to perform this translation

Translate the complete file (code).

Translate the code line by line.

This task can is the difference between the compiled and interpreted languages.

Which is which?

Python as an interpreter¶

Python can be used as an interpreter, in the sense that makes a translation line by line of our code, so we can run one intruction at a time in the same way we did with the shell, to do so open python

Note : We are going to use python3 instead of just python Today, the latest version of python is Python 3.7.2, so we are not going to use python (which means Python 2.7).

The main reason for that, is because Python 2.7.8 was released on July 1, 2014. is too old!!, meanwhile Release Python 3.7.2 was released on Dicember 24, 2018.

Invoke python 3 by typing

python3

on the terminal

In [1]:

2+3

Out[1]:

In [2]:

2.0+3.0

Out[2]:

5.0

In [3]:

2.+3.

Out[3]:

5.0

It is also possible to save values in variables to operate them

In [4]:

a=3

In [5]:

b=2

In [6]:

print(a+b,a-b)

5 1

Operators¶

There are two different classes of operators on any programming languages,

Arithmetic

Comparison

Logical

What do you think they are?

Variables¶

To understand how the operators work, we have to study a bit deeper what a variable mean.

In [7]:

a=2
print(a)

The value 2 is saved in a.

a has a specific physical place on the memory of the computer, so that every time we type a the computer goes to that particular place and reads the value.

As the computer doesn't understand the number 2 but binary instead, there is a huge difference if we use

In [8]:

a=2

In [9]:

b=2.0

What do you think is the difference?

Arithmetic Operators¶

To test them, let us explore some operators such as +, -, *, /, **

In [10]:

a+b

Out[10]:

4.0

In [11]:

a+a

Out[11]:

NOTE: On different versions the python, you get different results.

In [12]:

1/a

Out[12]:

0.5

In [13]:

1/b

Out[13]:

0.5

Let us use the method type

In [14]:

type(a)

Out[14]:

int

In [15]:

type(b)

Out[15]:

float

which means that, a is an integer, while b can have decimals

This have a huge impact but depends on the python version we are using, on different languages such as C++, you have to say which kind of variable you want, for instance

int a=2;
float b=2.0;
double c=2.0;

where float and double are data types that allow decimal points.

But, there are some other data types, one of great interest, they are called boolean.

boolean varialbes only have two different possible values True or False.

Depending on the language, they can be written differently, for example, in languages such as c++ and julia, the possible values are written in lower case.

C++

bool test=true;

julia

test=true

While in python the first letter must be upper case,

In [16]:

test=True

Where do you think we can find bools?

Comparison Operators¶

In [17]:

a>b

Out[17]:

False

In [18]:

a<b

Out[18]:

False

Logic operators¶

Sometimes we will need to have a combination of conditions to satisfy on a particular problem,

For example, the set of people on our course is different if we ask for

Female and older than 20.

Female or older than 20.

Think about the difference.

On other languages is common to use !,||,&& for negation, or and and. python is way simpler, it uses

! for negation.
or for or.
and for and.

In [20]:

a=True
b=False

In [21]:

a and b

Out[21]:

False

In [22]:

a or b

Out[22]:

True

In [23]:

a=2
b=3

how can I ask if a and b are the same?

Comparison equality¶

In [27]:

a==b

Out[27]:

False

In [28]:

a==2.0

Out[28]:

True

Composed operators¶

In [31]:

a>=b

Out[31]:

False

You guys should take a look at Truth tables.

Now, we are ready to start working on a little bit more complex structures, such as control statements

`if` statement¶

This is used for running some part of the code, only if a condition is satisfied.

python is based on identation rather than characters to the control statements such as other languages can.

A python structure of an if structure goes as

if condition:
    inside 
outside

if the condition results to be True the inside part is executed, let see some examples.

In [37]:

if True:
    print('Inside of If')
print('Outside of If')

Inside of If
Outside of If

In [38]:

if False:
    print('Inside of If')
print('Outside of If')

Outside of If

In [43]:

a=1
b=2
if a>b:
    print('a is greater than b')
else:
    print('b is greater that a')

b is greater that a

but, what if a=b?

In [46]:

a=1
b=1
if a>b:
    print('a is greater than b')
elif a<b:
    print('b is greater that a')
else:
    print('b is equal to a')

b is equal to a

elif holds for else if

`for` statement¶

The for is one of the loop structures that can be used on python, when a procedure must be repeated

Again, we have to be careful with the indentation, so that

In [2]:

for i in range(10):
    print(i)

lists¶

The for structure, have the huge advantage compared with languages such as C++ and fortran, because of it allows to iterate on the compounds of a data type called lists.

A list is a set of things on python, the most important thing here is that one can use any kind of thing inside a list, such that

lists
strings
Numbers: int or float
objects
pointers
...

The only thing we have to consider is to make it inside of [], let see some examples