2 7 Structured Data Types En

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Topic 2. Introduction to programming

Universidad de Oviedo

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Computer Science Fundamentals

2.7 Basic data types and structures

2.1 Problem abstraction for programming. Basic concepts. 2.2 Variables, expressions, assignment 2.3 Console input / output 2.4 Basic structures for control flow handling: sequential,

choice and repetitive.

2.5 Definition and use of subprograms and functions. Variablescope.

2.6 File input / output 2.7 Basic data types and structures 2.8 Steps in the development of a program: from high level to

CPU execution.


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Examples of data structures

From basic data types, new data structures can be defined. Asingle variable will be needed to store all the information.

Lists -> Sequences of elements of distinc type Text strings -> Sequences of characters Arrays -> Sequences of elements of the same type Dictionaries -> Values determined by keys Sets -> Sets of elements of the same type etc


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Lists

A list is an ordered sequence of data of any kind. Its length isnot fixed as they are allowed to grow. List values are

expressed between brackets, separated by commas.

>>>mylist=["speed",17,3.1416]

>>>type(mylist)

>>>len(mylist)

3


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Lists: Item access

Items in a list are ordered and can be accessed by means ofan index value which always starts in 0.

>>>mylist=["speed",17,3.1416]

>>>printmylist[0]

speed

>>>type(mylist[2])

>>>i=1

>>>printmylist[i]

17


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Lists: operators

Concatenation operator + Repetition operator * Slice operator [ : ] Relational operators == in is>>>p=[1,2,3,4]

>>>printp+p

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

>>>q=p*3

>>>printq[1,2,3,4,1,2,3,4,1,2,3,4]

>>>printq[3:7]

[4,1,2,3]

>>>print3inpandpqTrue

The result of using an operatorora function can be assigned to a

new list


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Slice operator

It returns list items from begin to end-1, step by step.

mylist[begin:end:step]

If eitherbegin orend parameters are omitted, the first and last items are used.

Negative values mean positions to be omitted from the end (-1) to the beginning.

If the step parameter is not provided, it takes the default value (1).

numbers=["one","two","three","four","five"]

numbers[1:4] ['two','three','four']

numbers[2:-1] ['three',four']numbers[:] ['one','two','three','four','five']

numbers[::2] ['one','three','five']

numbers[::-1] ['five','four','three','two','one']


7/23Topic 2. Introduction to programming


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List functions

Length len() Numeric list creation range() Numeric list functions sum()max()min() List sorting sorted()>>>p=range(1,5)>>>printp,len(p)

[1,2,3,4]4

>>>printsum(p),max(p),min(p)

1041>>>printsorted(p*3)

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

>>>names=["Paula","Luis","Juan","Ines"]

>>>printsorted(names)

['Ines','Juan','Luis','Paula']




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List modification

Lists are mutable variables. This means that the contents of a variable can

be changed without having to create a new one.

Direct item assignment p[1] = 7 Insertion methods .append() .extend() .insert() Erase methods .pop() .remove()

>>>numbers=["one","two","three","four"]

>>>numbers[2]="five"

>>>printnumbers['one','two','five','four']

>>>numbers.append("six")

>>>numbers.extend(["seven","eight","nine"])

>>>printnumbers

['one','two','five','four','six','seven','eight','nine']




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List modification

>>>numbers.pop()

'nine'

>>>printnumbers

['one','two','five','four','six','seven','eight']

>>>numbers.pop(2)

'five'

>>>printnumbers

['one','two','four','six','seven','eight']

>>>numbers.remove("seven")

>>>printnumbers

['one','two','four','six','eight']>>>numbers.insert(2,"three")

>>>printnumbers

['one','two','three','four','six','seven','eight']




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List iteration

The forloop allows us to iterate over the elements of a list. It is alsopossible to access the items using index values.

>>>p=[1,2,3,5,7,11,13,17]

>>>forninp:

...printn**2,

...

1492549121169289

>>>

>>>foriinrange(len(p)):...printp[i]**2,

...

1492549121169289




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Exercises




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Strings

A string is a special type of list, where the items are characters (digits,letters, signs and control characters, such as tabulators) and are expressed

between double quotes. Single and double quotes are equivalent.

They are inmutable, therefore their contents can not be directly modified.

>>>name="Patricia"

>>>type(nombre)

>>>printnombre[2],len(nombre)

't'8

>>>nombre[2]="p"

TypeError:'str'objectdoesnotsupportitemassignment




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Special characters

Special characters are represented with a code after \

\n carriagereturn

\t tabulator

\\

backslash\\'\" Singleanddoublequotes

>>>text="firstline\nsecond\t\"line\"">>>printtext

firstlinesecond "line"

>>>text'firstline\nsecond\t"line"'




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String operators

>>>name="Patricia"

>>>print"tri"inname

True>>>years=20

>>>print"%sis%dyearsold"%(name,years)

Patriciais20yearsold

They are the same as in the case of lists +*[:]== in (it allows us to look for characters or substrings) Formatting operator % (special)




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Functions and methods

>>>printint("123")+float("12.3"),"123"+str(123)135.3123123

>>>x="Asturias"

>>>declaration="ILOVE"+x.upper()

>>>printdeclarationILOVEASTURIAS

>>>printdeclaration.replace("ILOVE","MEPRESTA")

MEPRESTAASTURIAS

>>>num=int(raw_input("Enteraninteger"))

Reading raw_input() Number / string conversion int()float()str() Special methods .lower().upper().replace()




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Arrays

An array is an ordered sequence of items of the same type. It can havemultiple variable-size dimensions.

The numpy library defines the ndarray type and several functions.

The array() function converts a sequence (e.g., a list) in an array.

>>>importnumpy

>>>v=numpy.array([1,2,3,4,5])

>>>type(v)

>>>m=numpy.array([[1,2,3],[4,5,6]],float)

>>>printm

[[1.2.3.]

[4.5.6.]]




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Arrays

An array has several attributes which define its shape and size.

o .ndim: number of dimensionso .shape: Tuple containing the length of each dimensiono .size: Total number of itemsThey also exist as functions -> ndim() shape() and size()

>>>m=numpy.array([[1,2,3],[4,5,6]],float)

>>>printm.ndim,m.shape,m.size

2(2,3)6

>>>printrows=",m.shape[0],"\ncolumns=",m.shape[1]

rows=2

columns=3




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Arrays: Accessing items

Array items are accessible using index values (initial value = 0)

A matrix can be considered as a vector when each item is a row, thereforeit is also possible to access each row.

Arrays are mutable variables, which can be directly modified.

>>>m=numpy.array([[1,2,3],[4,5,6]])

>>>printm[0,2]

3

>>>m[1,0]=9

>>>printm[1]

[9,5,6]

123

456m=




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Array creation

Several functions can be used to create arrays

o arange(): It creates a vector containing a sequence (like range())o zeros() and ones(): They create an array of zeros or ones with a

specified shape and type (float by default)

o empty(): It creates an empty array>>>printnumpy.arange(0,1,0.1)

[0.0.10.20.30.40.50.60.70.80.9]

>>>printnumpy.zeros((2,3))[[0.0.0.]

[0.0.0.]]

>>>printnumpy.ones(5,int)

[11111]




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Array operations

Arithmetic operations involving one scalar number and an array will beperformed over each element.

If two arrays are involved, the operation is performed with the correspondingitem on the second array. In this case the shape and dimensions of both arrayshave to be the same.

>>>m=numpy.array([1,2,3,4])

>>>printm+5.1,m*3,m**2

[6.17.18.19.1][36912][14916]

>>>printm>=3

[FalseFalseTrueTrue]>>>n=numpy.array([2,1,3,0])

>>>printm+n,m*n,m>n

[3364][2290][FalseTrueFalseTrue]




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Array operations

Universal functions defined in numpy, work with arrays item by item.

Ex. Evaluate the function y = 0.5+sin(2 x) , in 10 points in the range[0,1.0]

Numpy also has lots of functions from linear algebra in order to work with

vectors and matrices: product, transpose, inverse, determinant, equationsystem solving, etc.

>>>x=numpy.arange(0,1,.1)

>>>print0.5+numpy.sin(2*numpy.pi*x)

[0.51.087785251.451056521.451056521.087785250.5

-0.08778525-0.45105652-0.45105652-0.08778525]

>>>printnumpy.dot(m,n)

13


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Array iteration

The forloop, as in the case of the lists, allows us to iterate over thearray items in sequential way.

We have to take into account that each element of a matrix is a row

array.

>>>mat=numpy.array([[1,2,3],[4,5,6],[7,8,9]])

>>>forrowinmat:

...printrow,

...sum=0

...forninrow:sum=sum+n

...print"Sum=",sum

...

[123]Sum=6

[456]Sum=15

[789]Sum=24


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2 7 Structured Data Types En