Strings and Operations in Programming
Strings
Victor Norman
CS104
Calvin College
Reading Quiz
•
Counts toward your grade.
Collection Data Type
•
a data type that consists of multiple “smaller”
data items.
•
aka “composite data type”.
•
items may have to be one type or may be of
multiple types. Data may be ordered or not.
–
string: each item is a single character. Ordered.
–
list: items may be any type. Ordered.
–
dictionary: items may be any type. Unordered.
–
objects: any type. Unordered.
String Type
•
consists of multiple characters, in order.
•
may be empty: “”
•
is
immutable
:
–
Cannot be changed once it is created.
–
Any string methods that seem like they may be
changing the string
must
be returning a new
string.
•
newName = name.capitalize()
# returns new str, name unchanged
String Operations
•
+: concatenates two strings to make a
new
string. E.g.,
newStr = oldStr1 + oldStr2
•
[]: indexing/subscripting: stringname[index].
–
index from 0 to n – 1.
–
result is a new string (of length 1).
•
upper(), lower(), strip()
–
produce
new
strings.
Come on and
Click!
What is the value of s after the following?
s = ’abc’
s = ’d’ * 3 + s
s = s + ’’ * 3
s = s + ’q’
CQ 2
What is the value of s after this code is
executed?
s = “What is your name?”
s.upper()
Slicing Operation
•
Syntax and operation similar to indexing, but
results in shorter
new
string derived from the
original.
–
indexing: result string length is 1.
–
slicing: result string length may be > 1.
•
somestring[n:m:s]
–
start at character n in somestring.
–
go up to but not including character m.
–
by step s.
•
no n
beginning; no m
end; no s
1.
Comparing Strings
•
use == or !=.
•
can compare with <, >, etc.., but really not
used very often.
Put on your red shoes and click the
blues
CQ 1:
What is the output of this code?
sketch = “argument clinic”
print(sketch[7:-1])
CQ 2
What does t hold?
s = “Knights of Ni!”
t = s[:]
CQ 3
What does t hold?
s = “Knights of Ni!”
t = s[::-1]
Item-based vs Index-based Iteration
•
item-based:
for <item> in <sequence>:
–
<item> is each item in the sequence.
•
index-based:
for <idx> in range(len(<sequence>)):
–
code in the body has the index of what item to
deal with, as someSeq[idx]).
Examples of each
•
Item-based
for cheese in cheeses:
print(cheese)
•
Index-based
for idx in range(len(cheeses)):
print(cheeses[idx])
When to use which?
•
Item-based:
–
simpler syntax, easier to read.
–
use when code does not need to know where the item
is in the sequence.
•
Index-based:
–
harder to read.
–
accessing the item is more complicated (using
indexing operator).
–
code can know where the item is in the sequence.
–
code can access other items around the item.
Example
•
What if we want to print out the cheeses like
this:
1.
Cheddar
2.
Gouda
3.
Venezuelan Beaver Cheese
Need to use index-based:
Example continued
for idx in range(len(cheeses)):
# idx starts at 0, but we want to
# print out as if indices start at 1,
# so add 1.
print(str(idx + 1) + “.”, cheeses[idx])
Accumulator Pattern
resStr = “” # initialize var to empty or 0
for ch in someStr: # for each item
if isConsonant(ch):
resStr = resStr + ch # add to result
•
Used item-based, because didn’t care about
where we were in the string.
•
someStr
is a sequence, so syntax is legal.
•
results accumulated in
resStr
Whiteboard/IDLE Activity
Write the following function that returns a string
that is the same as
s
except that spaces are
removed.
def remove_spaces(s):
Whiteboard/IDLE Activity
Write the following function that returns a string
that is the same as
s
except that spaces are
removed.
def remove_spaces(s):
resStr = “”
for ch in s:
if ch != “ “:
resStr = resStr + ch
return resStr
while Loop vs Index-Based for Loop
for i in range(len(s)):
code here uses s[i]
i = 0
while i < len(s):
use s[i]
i = i + 1 # better: i += 1
in
and
not in
•
very useful for searching a string to see if
string is in the string or not.
•
returns Boolean: so you know if the target is in
the string, but don’t know where.
if “wherefore” in hamletText:
print(“art thou”)
Optional Parameters
•
Terminology:
–
parameters may be optional in the
call
.
–
in function definition, optional params
•
must appear on the end of the parameter list.
•
indicated by being given a
default value
.
•
Code in the function is exactly the same.
Examples
def weird(a, b, c=3):
return a + b + c
print(weird(3, 7))
print(weird(3, 7, 44))
def weirder(a=3, b=4, c=5):
return a + b + c
print(weirder())
print(weirder(7))
print(weirder(7, 8))
print(weirder(7, 8, 9))
Examples
def something(a, b, debug=False):
res = a + b
if debug:
print(“something returning “ + res)
return res
x = something(44, -10)
x = something(44, -10, True) # turn on debugging
Activity
Write a function that removes certain letters
from a given string. If no letters are given, it
removes all vowels (not including y). You can
assume everything is lowercase. The result is
returned.
def remove_chars(s, <stuff>): # remove from s
Activity
Write a function that removes certain letters from a given
string. If no letters are given, it removes all vowels (not
including y). You can assume everything is lowercase. The
result is returned.
def remove_chars(s, ch2rem=“aeiou”):
res = “”
for ch in s:
if ch not in ch2rem:
res = res + ch
return res
Activity continued
Given a string s, write code to call your function on
s to remove all vowels. Then, write a function call
to remove all letters from a to f, inclusive. Print the
results.
print(remove_chars(“Monty Python”))
# should print Mnty Pythn
print(remove_chars(“Monty Python”, “My”))
# should print ont Pthon
Assignment
•
LOTS of CodeLab questions about strings.
•
Very good practice.
Whiteboard activity
Given this string, write code to print this out:
message = “greetings from the planet zorgon”
Output:
greetings
from
the
planet
zorgon
YOCO
•
“You only click once!”
This content delves into the concepts of strings, their manipulation methods, and various operations related to strings in programming. It covers collection data types, immutable nature of strings, string operations like concatenation and slicing, comparing strings, and practical examples of string manipulation through code snippets.
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Presentation Transcript
Strings Victor Norman CS104 Calvin College
Reading Quiz Counts toward your grade.
Collection Data Type a data type that consists of multiple smaller data items. aka composite data type . items may have to be one type or may be of multiple types. Data may be ordered or not. string: each item is a single character. Ordered. list: items may be any type. Ordered. dictionary: items may be any type. Unordered. objects: any type. Unordered.
String Type consists of multiple characters, in order. may be empty: is immutable: Cannot be changed once it is created. Any string methods that seem like they may be changing the string must be returning a new string. newName = name.capitalize() # returns new str, name unchanged
String Operations +: concatenates two strings to make a new string. E.g., newStr = oldStr1 + oldStr2 []: indexing/subscripting: stringname[index]. index from 0 to n 1. result is a new string (of length 1). upper(), lower(), strip() produce new strings.
Come on and Click! What is the value of s after the following? s = abc s = d * 3 + s s = s + * 3 s = s + q
CQ 2 What is the value of s after this code is executed? s = What is your name? s.upper()
Slicing Operation Syntax and operation similar to indexing, but results in shorter new string derived from the original. indexing: result string length is 1. slicing: result string length may be > 1. somestring[n:m:s] start at character n in somestring. go up to but not including character m. by step s. no n beginning; no m end; no s 1.
Comparing Strings use == or !=. can compare with <, >, etc.., but really not used very often.
Put on your red shoes and click the blues CQ 1: What is the output of this code? sketch = argument clinic print(sketch[7:-1])
CQ 2 What does t hold? s = Knights of Ni! t = s[:]
CQ 3 What does t hold? s = Knights of Ni! t = s[::-1]
Item-based vs Index-based Iteration item-based: for <item> in <sequence>: <item> is each item in the sequence. index-based: for <idx> in range(len(<sequence>)): code in the body has the index of what item to deal with, as someSeq[idx]).
Examples of each Item-based for cheese in cheeses: print(cheese) Index-based for idx in range(len(cheeses)): print(cheeses[idx])
When to use which? Item-based: simpler syntax, easier to read. use when code does not need to know where the item is in the sequence. Index-based: harder to read. accessing the item is more complicated (using indexing operator). code can know where the item is in the sequence. code can access other items around the item.
Example What if we want to print out the cheeses like this: 1. Cheddar 2. Gouda 3. Venezuelan Beaver Cheese Need to use index-based:
Example continued for idx in range(len(cheeses)): # idx starts at 0, but we want to # print out as if indices start at 1, # so add 1. print(str(idx + 1) + . , cheeses[idx])
Accumulator Pattern resStr = # initialize var to empty or 0 for ch in someStr: # for each item if isConsonant(ch): resStr = resStr + ch # add to result Used item-based, because didn t care about where we were in the string. someStr is a sequence, so syntax is legal. results accumulated in resStr
Whiteboard/IDLE Activity Write the following function that returns a string that is the same as s except that spaces are removed. def remove_spaces(s):
Whiteboard/IDLE Activity Write the following function that returns a string that is the same as s except that spaces are removed. def remove_spaces(s): resStr = for ch in s: if ch != : resStr = resStr + ch return resStr
while Loop vs Index-Based for Loop for i in range(len(s)): code here uses s[i] i = 0 while i < len(s): use s[i] i = i + 1 # better: i += 1
in and not in very useful for searching a string to see if string is in the string or not. returns Boolean: so you know if the target is in the string, but don t know where. if wherefore in hamletText: print( art thou )
Optional Parameters Terminology: parameters may be optional in the call. in function definition, optional params must appear on the end of the parameter list. indicated by being given a default value. Code in the function is exactly the same.
Examples def weird(a, b, c=3): return a + b + c print(weird(3, 7)) print(weird(3, 7, 44)) def weirder(a=3, b=4, c=5): return a + b + c print(weirder()) print(weirder(7)) print(weirder(7, 8)) print(weirder(7, 8, 9))
Examples def something(a, b, debug=False): res = a + b if debug: print( something returning + res) return res x = something(44, -10) x = something(44, -10, True) # turn on debugging
Activity Write a function that removes certain letters from a given string. If no letters are given, it removes all vowels (not including y). You can assume everything is lowercase. The result is returned. def remove_chars(s, <stuff>): # remove from s
Activity Write a function that removes certain letters from a given string. If no letters are given, it removes all vowels (not including y). You can assume everything is lowercase. The result is returned. def remove_chars(s, ch2rem= aeiou ): res = for ch in s: if ch not in ch2rem: res = res + ch return res
Activity continued Given a string s, write code to call your function on s to remove all vowels. Then, write a function call to remove all letters from a to f, inclusive. Print the results. print(remove_chars( Monty Python )) # should print Mnty Pythn print(remove_chars( Monty Python , My )) # should print ont Pthon
Assignment LOTS of CodeLab questions about strings. Very good practice.
