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# Python – Conditional String Append

Sometimes, while working with data, we have a problem in which we need to perform an append operation in a string on a particular condition. This kind of problem is common in web development and day-day programming. Let’s discuss certain ways in which this task can be performed.

Method #1: Using loop This is a brute-force way to perform this task. In this, we run a loop and check for the condition, and according to that append the string to the original string.

## Python3

 `# Python3 code to demonstrate``# Conditional String Append``# using loop` `def` `append_str(item, boy_str, girl_str):``    ``if` `len``(item) > ``4` `and` `item[``-``5``] ``=``=` `' '``:``        ``return` `item ``+` `girl_str``    ``return` `item ``+` `boy_str``    ` `# initializing list``test_list ``=` `[``'Manjeet Singh'``, ``'Harsimran Kaur'``, ``'Sarbjeet Kaur'``]` `# initializing append string``boy_str ``=` `" Boy"``girl_str ``=` `" Girl"` `# printing original list``print` `(``"The original list is : "` `+` `str``(test_list))` `# Conditional String Append``# using loop``res ``=` `[append_str(item, boy_str, girl_str) ``for` `item ``in` `test_list]` `# printing result``print` `(``"The filtered strings are : "` `+` `str``(res))`

Output

```The original list is : ['Manjeet Singh', 'Harsimran Kaur', 'Sarbjeet Kaur']
The filtered strings are : ['Manjeet Singh Boy', 'Harsimran Kaur Girl', 'Sarbjeet Kaur Girl']```

Time Complexity: O(n)

Auxiliary Space: O(n)

Method #2: Using list comprehension List comprehension is shorthand for the longer method of loops. This solved problem in similar way but in shorter constructs.

## Python3

 `# Python3 code to demonstrate``# Conditional String Append``# using list comprehension` `# initializing list``test_list ``=` `[``'Manjeet Singh'``, ``'Harsimran Kaur'``, ``'Sarbjeet Kaur'``]` `# initializing append string``boy_str ``=` `" Boy"``girl_str ``=` `" Girl"` `# printing original list``print` `(``"The original list is : "` `+` `str``(test_list))` `# Conditional String Append``# using list comprehension``res ``=` `[ele ``+` `girl_str ``if` `ele[``-``5``] ``=``=` `' '` `else` `ele ``+` `boy_str ``for` `ele ``in` `test_list]` `# printing result``print` `(``"The filtered strings are : "` `+` `str``(res))`

Output

```The original list is : ['Manjeet Singh', 'Harsimran Kaur', 'Sarbjeet Kaur']
The filtered strings are : ['Manjeet Singh Boy', 'Harsimran Kaur Girl', 'Sarbjeet Kaur Girl']```

Time Complexity: O(n)

Auxiliary Space: O(n)

Method #3: Using map function Map function is used to create a new list by updating the existing list. We solve this problem by defining the condition append function for the map function which will apply to all the elements of the list.

## Python3

 `# # Python3 code to demonstrate``# Reverse Row sort in Lists of List``# using map` `# Initializing append string``boy_str ``=` `" Boy"``girl_str ``=` `" Girl"``#  Function to append the string`  `def` `func(x):``    ``if` `x[``-``5``] ``=``=` `' '``:``        ``return` `x ``+` `girl_str``    ``else``:``        ``return` `x ``+` `boy_str`  `# initializing list``test_list ``=` `[``'Manjeet Singh'``, ``'Harsimran Kaur'``, ``'Sarbjeet Kaur'``]`  `# printing original list``print``(``"The original list is : "` `+` `str``(test_list))` `# Conditinal string append using map``res ``=` `list``(``map``(func, test_list))` `# printing result``print``(``"The filtered string are : "` `+` `str``(res))`

Output

```The original list is : ['Manjeet Singh', 'Harsimran Kaur', 'Sarbjeet Kaur']
The filtered string are : ['Manjeet Singh Boy', 'Harsimran Kaur Girl', 'Sarbjeet Kaur Girl']```

Time Complexity: O(n)

Auxiliary Space: O(n)

#### Approach#4: Using regular expression

This Approach uses regular expressions to match strings ending with “Singh” and appends “Boy” to those strings. For other strings, it appends “Girl”. The filtered strings are stored in a new list called “filtered_strings”.

#### Algorithm

1. Import the re module for regular expressions
2. Define a regular expression pattern to match strings ending with ‘Singh’
3. Create an empty list ‘filtered_strings’
4. Iterate over each string in the original list:
a. Use the re.sub() function to replace the matched pattern with ‘Boy’ and append it to ‘filtered_strings’
b. If the pattern does not match, append the string with ‘Girl’ and add it to ‘filtered_strings’
5. Return the ‘filtered_strings’ list

## Python3

 `import` `re` `original_list ``=` `[``'Manjeet Singh'``, ``'Harsimran Kaur'``, ``'Sarbjeet Kaur'``]``pattern ``=` `re.``compile``(r``'Singh\$'``)``filtered_strings ``=` `[]` `for` `string ``in` `original_list:``    ``if` `pattern.search(string):``        ``filtered_strings.append(pattern.sub(``'Boy'``, string))``    ``else``:``        ``filtered_strings.append(string ``+` `' Girl'``)` `print``(``'The filtered strings are:'``, filtered_strings)`

Output

`The filtered strings are: ['Manjeet Boy', 'Harsimran Kaur Girl', 'Sarbjeet Kaur Girl']`

Time complexity: O(n), where n is the length of the original list. We iterate over each string in the list once.

Auxiliary Space: O(n), where n is the length of the original list. We create a new list ‘filtered_strings’ which stores n strings. We also create a regular expression pattern object, which takes up some memory.

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