Strings
Strings
• We have talked about string as sequence of
characters
• Strings are indicated by ' ' or " ".
• We have non printed characters:
● new line
'\n'
● tab
'\t'
String Representation
ord(): takes a character and returns its
Unicode integer value.
chr():takes an integer and returns the
Unicode character
String Index
• As the elements in string are stored in sequence , each element is
associated with an index (location in sequence).
• First element in sequence from the left is at index 0 and the last
element in index (size-1)
• Negative values count down from the right, starting with index -1.
my_str='Hello CS101'
print(my_str[0])
print(my_str[-1])
print(my_str[-3])
print(my_str[-12])
Output:
H
1
1
IndexError: string index out of range
String Slicing
• Strings are a sequence type, having
characters ordered by position from left to
right.
• An individual character is read using
brackets.
str='abcd'
print('str[1]=',str[1])
Output:
str[1]= b
String Slicing
• A programmer often needs to read more than
one character at a time.
• We can use slicing notation:
– str[start:end] ->which creates a new string whose
value mirrors the characters of str from positions start
to end – 1.
– my_str='abc def'
– print('my_str[1:5]=',my_str[1:5])
– Output:
– my_str[1:5]= bc d
String Slicing
my_str='abc def'
print('my_str[:-1]=',my_str[:-1])
print('my_str[3:-1]=',my_str[3:-1])
print('my_str[0:-2]=',my_str[0:-2])
Output:
my_str[:-1]= abc de
my_str[3:-1]= de
my_str[0:-2]= abc d
Stride
•
The stride determines how much to increment the index after reading
each element.
numbers = '0123456789'
print('numbers[::]: %s' % numbers[::])
print('numbers[::2]: %s' % numbers[::2])
print('numbers[::0]: %s' % numbers[::0])
print('numbers[1:9:3]: %s' % numbers[1:9:3])
Output:
numbers[::]: 0123456789
numbers[::2]: 02468
Slicing can not be 0
numbers[1:9:3]: 147
Copy Of String
• How to make a copy of a string:
str1='Hi Alex'
str2=str1[:]
print(str2)
Output:
Hi Alex
• How to reverse a string:
str1='Hi Alex'
rev=str1[::-1]
print(rev)
Output:
xelA iH
Strings
Example:
str1='Hi Alex'
rev=str1[::2]
print(rev)
Output:
H lx
rev=str1[::-2]
print(rev)
Output:
xl H
Copy and Assignment Operator
• id(): Tells us the memory address of a variable.
Memory address of S
• What will happen:
(e.g. 56547592)
s="Hello CS101"
s1=s
Hello CS101
S1
S
print("s1 is %s"%s1)
print("Location of s is",id(s),"and location of
s1 is",id(s1))
Output:
s1 is Hello CS101
Location of s is 56547592 and location of s1 is
56547592
Copy and Assignment Operator
Memory address of S
(e.g. 56547592)
s="Hello CS101"
Hello CS101
S
S2
s2=s[:]
print("s2 is %s"%s2)
print("Location of s is",id(s),"and location of s2
is",id(s2))
Output:
s2 is Hello CS101
Location of s is 56547592 and location of s2 is
56547592
Note:
Since strings are immutable, there is not a distinction between
copy and assignment.
We can't change a string, we can only assign a variable to be a new
string value.
Copy and Assignment Operator
s="Hello CS101"
s1=s
print("s1 is %s"%s1)
print("Location of s is",id(s),"and location of s1 is",id(s1))
s2=s[:]
print("s2 is %s"%s2)
print("Location of s is",id(s),"and location of s2 is",id(s2))
s="Hello again"
print(s,"and Location of s is",id(s))
print(s1,"and location of s1 is",id(s1))
print(s2,"and location of s2 is",id(s2))
Output:
s1 is Hello
Location of
s2 is Hello
Location of
Hello again
Hello CS101
Hello CS101
CS101
s is 56547592 and location of s1 is 56547592
CS101
s is 56547592 and location of s2 is 56547592
and Location of s is 59200960
and location of s1 is 56547592
and location of s2 is 56547592