Strings - Basics (in Python)

String is a data type used to store text. A String data type is built into most high-level programming languages, since storing and manipulating text is such a common need in programs.

Defining strings

Each of the following examples creates a new String with the text, bla bla bla stored in it:

Python example

Single quotes, double quotes, triple single quotes, triple double quotes... it's all good in Python.

 'bla bla bla'

 "bla bla bla"

 '''bla bla bla'''

 """bla bla bla"""

Java example

Double quotes must be used to define String literals in Java

 "bla bla bla"

Javascript example

Single quotes and double quotes are equivalent in Javascript.

"bla bla bla"

 "bla bla bla"

PHP example

Single quotes and double quotes are equivalent in PHP.

 'bla bla bla'

 "bla bla bla"

Concatenating strings

Each of the following examples creates a new String with the text, bla bla bla stored in it:

Python example

 'bla' + ' ' + 'bla' + ' ' + 'bla'

Note that Python also supports the 'multiplication' operator:

 'bla ' * 3

Java example

 "bla" + " " + "bla" + " " + "bla"

Javascript example

"bla" + " " + "bla" + " " + "bla"

PHP example

 'bla' . ' ' . 'bla' . ' ' . 'bla'

Calculating the length of a string

The following examples show how to determine how many characters are in a given string.

Python example

 len('bla bla bla')

Java example

 "bla bla bla".length()

Javascript example

"bla bla bla".length

PHP example

 strlen('bla bla bla')

String escape characters

All high-level programming languages support escape characters in Strings - these are written with more than one character, but actually represent single characters in how a string is interpreted.

Common escape characters:

\n – a newline (linefeed) character
\r – a return (carriage return) character
\t – a tab character
\' – a single quote character
\" – a double quote character
\\ – a backslash character

The following string represents the words hello and world, separated by a line break:

 "hello\nworld"

String indexing

All high-level programming languages allow individual characters within a String to be accessed. The following examples all access the first fourth character (the 'r') in the string 'fabrication'. Note that index numbers always start from zero.

Python example

 'fabrication'[3]

Python also supports negative indices

 'fabrication'[-8]

Java example

 "fabrication".charAt(3)

Javascript example

"fabrication".charAt(3)

PHP example

 'fabrication'{3}

Looping through characters in a string

All high-level languages allow easy looping through the individual characters in a string.

Python example

Forwards:

 s = 'fabrication'
 for c in s:
     print(c)

Backwards:

 s = 'fabrication'
 for c in s[::-1]:
     print(c)

Java example

Forwards:

 String s = "fabrication";
 for (int i = 0; i < s.length(); i++){
     char c = s.charAt(i);
     System.out.println(c);

Backwards:

 String s = "fabrication";
 for (int i = s.length() - 1; i >= 0; i--){
     char c = s.charAt(i);
     System.out.println(c);

Javascript example

Forwards:

 var s = "fabrication";
 for (var i = 0; i < s.length; i++){
     var c = s.charAt(i);
     console.log(c);

Backwards:

 var s = "fabrication";
 for (var i = s.length - 1; i >= 0; i--){
     var c = s.charAt(i);
     console.log(c);

PHP example

Forwards:

 $s = 'fabrication';
 for($i=0; $i < strlen($s); $i++)  {
     $c = $s[$i];
     print($c);
 }

Backwards:

 $s = 'fabrication';
 for($i = strlen($s) - 1; $i >= 0; $i--)  {
     $c = $s[$i];
     print($c);
 }

Strings are immutable

In most high-level languages, strings are immutable, meaning they cannot be modified after being created.

Python example

 s = 'fabrication'
 s[1] = 'o' #this is an error

Java example

 String s = "fabrication";
 s.charAt(1) = 'o'; //this is an error

Javascript example

var s = "fabrication"
s[1] = "o" //this is an error

PHP example

 $s = 'fabrication';
 $s{1} = 'o'; //this is not an error, but has not changed the original string, but rather the variable $s now points to a new string with an 'o' as the second character

Substrings and slicing

It is possible, in all high-level programming languages, to get a substring, or slice, of a string. All the examples below create a new string holding the text, "bric".

Python example

s1 = 'fabrication'
s2 = s1[2:6]

Python also supports negative indices:

s1 = 'fabrication'
s2 = s1[-9:-5]

Java example

String s1 = "fabrication";
String s2 = s1.substring(2, 6);

Javascript example

var s1 = "fabrication"
var s2 = s1.substring(2, 6)

PHP example

$s1 = 'fabrication';
$s2 = substr($s1, 2, 4);

PHP also supports negative indices:

$s1 = 'fabrication';
$s2 = substr($s1, -9, 4);

Comparing the values of two strings

All high-level programming languages provide a means to determine whether two strings hold the same text or not. The following expressions all evaluate to a boolean false value, since 'lipsmacking' is not the same as 'fingerlicking'.

Python example

'lipsmacking' == 'fingerlicking'

Java example

"lipsmacking".equals("fingerlicking")

Javascript example

"lipsmacking" == "fingerlicking"

PHP example

'lipsmacking' == 'fingerlicking'

Determining the position at which one string occurs within another

All of the following examples evaluate to a 3, because the string 'smack' occurs at index position 3 within the string 'lipsmackingly good'.

Python example

'lipsmackingly good'.find('smack', 0)

Java example

"lipsmackingly good".indexOf("smack")

Javascript example

"lipsmackingly good".indexOf("smack")

PHP example

strpos('lipsmackingly good', 'smack')

Determining whether one string occurs within another

All of the following examples evaluate to a boolean true value, because the text 'smack' occurs within the text 'lipsmackingly good'.

Python example

'lipsmackingly good'.find('smack') >= 0

Note Python also has an 'in' operator that can be used

'smack' in 'lipsmackingly good'

Java example

"lipsmackingly good".indexOf("smack") >= 0

Javascript example

"lipsmackingly good".indexOf("smack") >= 0

PHP example

strpos('lipsmackingly good', 'smack') != false

More string functions in Python

String functions with Boolean return values

Image you have a few strings...

x = "Геркулес"
y = "лес"
z = "Герку"

Every string has built-in methods available to it that help analyze its contents. These functions all return booleans:

x.endswith(y)
x.startswith(y)
x.islower()
x.isupper()
x.isalpha()
x.isnumeric()
x.isalnum()
x.isdigit()
x.isdecimal()

String functions that search and replace

x.find(y)
x.rfind(y)
x.replace(z, y)
x.strip()
x.count(y)

String functions that return a new copy of the string with case changes

x.capitalize()
x.lower()
x.upper()
x.swapcase()
x.title()

Formatting strings

x.format() with named keywords
x.format() with indices
see string formatting examples

String functions that split strings into lists

x.split(y)
x.rsplit(y)
see more about Lists