day 3 – lesson 12 more about strings

Python Mini-CourseUniversity of Oklahoma

Department of Psychology

Day 3 – Lesson 12More about strings

05/02/09Python Mini-Course: Day 3 – Lesson 121

Lesson objectives

1. Use common string methods2. Use escape sequences to represent

special characters3. Use string formatting codes to

specify the format of strings for printing and file output


String methods

Like everything else in Python, strings are objectsObjects have methods that are invoked using dot notationobject.method([arguments])


Examples

word = 'banana'new_word = word.upper()print new_word

index = word.find('a')print index


String methods

For a complete listing, seehttp://docs.python.org/library/stdtypes.html#string-methods


Search methods

str.find(sub[, start[, end]]) print word.find('g',2,4)str.index(sub[, start[, end]]) print word.index('g',2,4)str.count(sub[, start[, end]]) print word.count('a')


Formatting methods(return a new string)

str.lower()str.upper()str.title()str.swapcase()str.strip([chars])str.lstrip([chars])str.rstrip([chars])


Examples: string_methods2.py

s = ' NOBODY expects the Spanish \ Inquisition!!!'

print s.lower()

print s.upper()

print s.title()

print s.swapcase()

print s.strip()

print s.lstrip()

print s.rstrip('!')


Format checking methods(return a Boolean value)

str.isalnum()str.isalpha()str.isdigit()str.islower()str.isupper()str.istitle()


Splitting and joining*

str.split([sep[, maxsplit]])str.splitlines([keepends])str.join(seq)

*We'll revisit these next week when we cover lists


Special characters

To express special characters in Python strings, use Standard C style escape sequencessee

http://docs.python.org/reference/lexical_analysis.html#string-literals



Escape Sequence

Meaning

\\ Backslash (\)

\' Single quote (')

\" Double quote (")

\b ASCII Backspace (BS)

\f ASCII Formfeed (FF)

\n ASCII Linefeed (LF)

\r ASCII Carriage Return (CR)

\t ASCII Horizontal Tab (TAB)

\uxxxx Character with 16-bit hex value xxxx (Unicode only)

\xhh Character with hex value hh (7-bit ASCII)

Example 1: squares.py

print 'x\ty'for x in range(1,10): y = x**2 print str(x) + '\t' + str(y)


Example 1: squares.py

print 'x\ty\n--\t--'for x in range(1,10): y = x**2 print str(x) + '\t' + str(y)


Example 2: square_roots.py

import mathprint 'x\ty\n--\t--'for x in range(1,10): y = math.sqrt(x) print str(x) + '\t' + str(y)


Formatting string conversions

You can use the string formatting operator % to replace values within a string

Example: s = 'The knights who say %s' % 'Ni' print s


Formatting string conversions

You can do multiple replacements using % within a single string

Example: s = 'The %s who say %s' % \ ('Knights', 'Ni') print s


Improving square_roots.py

import mathprint 'x\ty\n--\t--'for x in range(1,10): y = math.sqrt(x) print '%s\t%s' % (x, y)


String formatting codes

Syntax %[(name)][flags][width][.precision]code

Flags - left-justify

+ numeric sign (+/-)

space blank before positive numbers

0 zero fill


String formatting codes

Syntax %[(name)][flags][width][.precision]code

WidthNumber indicating the total field width

(max number of characters)Precision

Number of digits after the decimal point



Code Meaning Code

Meaning

s String e Floating-point exponent

c Character E e w/ uppercase

d Decimal (integer)

f Floating-point decimal

i Integer F f w/ uppercase

u Unsigned (integer)

g e or f

% Literal '%' G E or F

Examples: integers

x = 59print 'x = %d' % xprint 'x = %+d' % xprint 'x = %+d%%' % xprint 'x = %+6d%%' % xprint 'x = %-6d' % xprint 'x = %-6d%%' % xprint 'x = %06d' % x


Examples: floats

x = 12.3456789print 'x = %d' % xprint 'x = %f' % xprint 'x = %2.4f' % xprint 'x = %+2.4f' % xprint 'x = %06.2f' % x


Examples: exponential notation

x = 1.34e-6print 'x = %f' % xprint 'x = %e' % xprint 'x = %g' % x


Example: square_roots2.py

import mathstart, stop = 1, 9print '%s\t%s' % \ ('x'.center(3), 'y'.center(6))print '%s\t%s' % (3*'-', 6*'-')for x in range(start, stop+1): y = math.sqrt(x) print '%3d\t%2.4f' % (x, y)


day 3 – lesson 12 more about strings

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