Introduction to python

lists and strings

Mauricio Sevilla

email= j.sevillam@uniandes.edu.co

11.01.19

We have already started with some of the most basic ideas behind the lists,

Today we are going to focous on the features, advantages and disadvantages of working with lists.

lists

On python, a list is a set of things, of any kind!, and even each compound can have a different type than the others, you can have,

  • 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

In [1]:
list1=[]
print(list1)
type(list1)

[]
Out[1]:
list

An empty list, and

In [2]:
list2=[10]
print(list2,type(list2),type(list2[0]))

[10] <class 'list'> <class 'int'>

Operations such as +, * can be performed, but the result is not what one would expect,

In [3]:
list1=[1,2,3,4,5]
list2=[3,2,4,6,9]
print(list1+list2)

[1, 2, 3, 4, 5, 3, 2, 4, 6, 9]

This operation cannot be performed with an number and a list

In [4]:
print(list1+2)

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-4-f7df2789d0eb> in <module>
----> 1 print(list1+2)

TypeError: can only concatenate list (not "int") to list

We have to take into account that, not all operations are allowed between certain types

In [5]:
print(list1*list2)

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-5-2f25b862d700> in <module>
----> 1 print(list1*list2)

TypeError: can't multiply sequence by non-int of type 'list'

Sometimes they work with specific types,

In [6]:
print(list1*2)

[1, 2, 3, 4, 5, 1, 2, 3, 4, 5]

Finally

In [7]:
print(list1**2)

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-7-a2e329edf605> in <module>
----> 1 print(list1**2)

TypeError: unsupported operand type(s) for ** or pow(): 'list' and 'int'

So, we have to explore which operators can be used on which variables.

Lets create a list with different kind of data, an int, float, character, str and a list

In [8]:
list_test=[1,2.0,'c',"word",['list_a','list_b']]

Let us explore the data.

In [9]:
for i in list_test:
    print(type(i))

<class 'int'>
<class 'float'>
<class 'str'>
<class 'str'>
<class 'list'>

look that 'c' and "word" are of the same type.

In [10]:
print(list_test)

[1, 2.0, 'c', 'word', ['list_a', 'list_b']]

Let us explore some of the functions we can use on lists

  • len()
In [11]:
print(len(list_test))

5
In [12]:
print(len(list_test[0]))

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-12-cc4495ea67a3> in <module>
----> 1 print(len(list_test[0]))

TypeError: object of type 'int' has no len()

Note : Doesn't work on numbers, but on strings?

In [13]:
print(len(list_test[3]), list_test[3])

4 word

In some sense, the str and list have the same structure!

Differences on for

In [14]:
for i in list_test:
    print(i)

1
2.0
c
word
['list_a', 'list_b']
In [15]:
for i in range(len(list_test)):
    print(i) 

0
1
2
3
4

enumerate function

In [16]:
for i,j in enumerate(list_test):
    print(i,j)

0 1
1 2.0
2 c
3 word
4 ['list_a', 'list_b']

lists also can be accesed with negative values!

In [17]:
print(list_test[-1])

['list_a', 'list_b']

Sometimes, you can have more than one index

In [18]:
print(list_test[-1][1])

list_b

append method,

there are different ways to use functions on structures as lists, for example len(), or range(), but there are some such as append that are called Methods, they are the heart of python because is a language based on Object Oriented Programming

In [19]:
print(list_test)

[1, 2.0, 'c', 'word', ['list_a', 'list_b']]
In [20]:
list_test.append(1)
In [21]:
print(list_test)

[1, 2.0, 'c', 'word', ['list_a', 'list_b'], 1]
In [22]:
list_test.append([1,2,3,4,5])
In [23]:
print(list_test)

[1, 2.0, 'c', 'word', ['list_a', 'list_b'], 1, [1, 2, 3, 4, 5]]

Homework

Look for some methods to erase cells on a list

You can change the elements of a list, even if the new value have a different type.

In [24]:
list_test[6]=2
In [25]:
print(list_test)

[1, 2.0, 'c', 'word', ['list_a', 'list_b'], 1, 2]

There are other things we can do on lists, for example, how can we get more than one value of a list at a time?

In [26]:
print(list_test[1:4])

[2.0, 'c', 'word']
In [27]:
print(list_test[3][2:])

rd

When we use [2:] it means that it starts at [2] and goes until the end. we can also use [:3] and it means that goes from the begining until the 2nd compound.

Strings

A string is a set of characters,

In [28]:
test1='test'
test2="test"
In [29]:
print(test1==test2)

True

There is no difference between ', ".

And we can use the same structure than we just did with the lists!

In [30]:
len(test1)
Out[30]:
4
In [31]:
print(test1[1],test2[3])

e t
In [32]:
test1.append('a')

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-32-2fffa1be48b3> in <module>
----> 1 test1.append('a')

AttributeError: 'str' object has no attribute 'append'
In [33]:
test1=test1+'\t'+'test2'
In [34]:
print(test1)

test	test2

Strings can be multiplyed

In [35]:
print(test2*2,type(test2*2)) 

testtest <class 'str'>

How can this be useful?