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CS 112

Intro to Programming

Strings

Prof. SnyderFall 2011

George Mason University

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String Data Structure

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The String data structure is:

A complex data type (i.e., it is comprised of

sub-components or elements)

A sequence (i.e., a complex data structure

that consists of sequential elements, which

implies a specific ordering.

Immutable (i.e., once created, cannot be

modified)

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Complex Data Type

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Non-complex or primitive data types consist of a

singular item of data:

integer, float, boolean, etc.

Complex data types (structures) comprise zeroor more sub elements:

Strings comprise zero (e.g., an empty string)

or more characters.

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Sequence Data Types

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Sequential data types comprise sub-elements

that possess a particular ordering:

one_string = ABC

another_string = ABC

These represent two distinct, unique string data.

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Immutable Data Types

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Immutable data types may not be modified once

they are declared:

They may, however, be re-declared:

Attempting to change an element of an immutable data type

Re-declaration of a_string

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Sequence Operations

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(all sequences, not just strings)

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Membership

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Repetition

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Indexing

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Indexing

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Accessing the 0th indexed element of

a_string

Accessing the (negative) 1st element of

a_string

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Indexing (Loops)

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The variable i takes

on the value of each

successive elementof the sequence

during each

successive iteration

of the loop

Limitation: can only iterate through

the sequence elements in order

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Length

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Number of elements in the

sequence

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Indexing (Loops)

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The variable i takes on

the value of eachsuccessive element of

the range during each

successive iteration of

the loop

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Indexing (Loops)

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The variable i takes onthe value of each

successive element of the

sequence during each

successive iteration of the

loop

More robust: can iterate through the

sequence elements in any manner

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Slicing

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Slicing

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Strange...

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Practice ProblemUsing indexes and slices, what are these expressions

values?

word = "dysfunctional" group = "team"

word[1]

word[3:6]word[3:]

word[4:11]

word[len(word)-1]

group[-1:-4:-2]

group[-1:-4:-2]+group[-2::-2]

group[:3]

group [-len(group)]

word[-6:-3:2]+group[:2]+group[3]

'y'

'fun'

'functional'

'unction'

'l'

'me'

'meat''tea'

't'

'totem'

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Min / Max

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Why?

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ord(char): converts single character to

corresponding ASCII integer value

chr(int): converts integer value tocorresponding character symbol

Based on ASCII code value

American Standard Code for Information Interchange

7 binary bits 128 unique symbols

Python also supports Unicode (16-32 bits)

chr & ord Built-In Functions

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ASCII Table

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Min / Max

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String Methods

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Method: an action or process (defined relative toa particular object) to be performed

Dot Operator: a dot (period) that is used toaccess an associated variable or call an

associated function

object_name.method_name

strVar = Test

strVar.upper() would produce TEST

Refer to Library

Reference: 5.6.1

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String Operations vs.

Methods

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sequence operations:

may be built-in functions

operate on various data types

example: len len(strVar)

methods: operate on a single data type

string object methods (e.g., capitalize)

strVar.capitalize()

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String Formatting

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def main():

x = input("Enter number: )

print "counter =", x

y,z = "counter = ", str(x); print y + z

print "counter = " + str(x)

y = "counter = " + str(x); print y

print "counter = %d" % (x)

y = "counter"; print "%s = %d" % (y,x)

z = "%s = %d" % (y,x); print z

w = " = "; z = "%s%s%d" % (y,w,x);print z

main()

Enter number: 3

counter = 3

counter = 3counter = 3

counter = 3

counter = 3

counter = 3counter = 3

counter = 3

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Tuples(a quick introduction)

A tuple is a grouping of values, with a fixedlength.

Tuples are sequences (ordered).

Tuples are immutable (can't be changed, butcan be used to construct a new value).

Syntax: (comma,separated,values)BNF: tuple ::= '(' [expr [, expr]*] ')'

Examples:(5,1,3,6) ("Mark",True,100) (x+2,z-y)

() (1,2,3,4,5,6,7,8) (range(10),"hello")

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String Formatting

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(Characters that may follow a % for special meaning)

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Practice Problem

What is the resulting string? Are there any errors?

"I have %d apples." % 15

"%d / %f = %f" % (7, 3.0, 7/3.0)x = "%d %s"; x % (3, "amigos")

"%%d prints integers." % ()

"I %s MadLibs" % (love)

"%f %d %s" % (4,4.6,"""yo""")"%s %x" % (hex(200),200)

"%d centimeters is %d inches."

'I have 15 apples.'

'7 / 3.000000 = 2.333333''3 amigos'

'%d prints integers.'

(love is not defined)

'4.000000 4 yo''0xc8 c8'

'%d centimeters is %d inches'