Python – Expand Character Frequency String

Given a string, which characters followed by its frequency, create the appropriate string.

Examples:

Input : test_str = ‘g7f2g3i2s2b3e4’ 
Output : gggggggffgggiissbbbeeee 
Explanation : g is succeeded by 7 and repeated 7 times.

Input : test_str = ‘g1f1g1’ 
Output : gfg 
Explanation : f is succeeded by 1 and repeated 1 time. 

Method #1: Using zip() + join()

This is one of the ways in which this task can be performed. In this, the task of joining appropriate characters is done using join() and zip() is used to convert different frequency and character strings. The drawback is that frequency of character is restricted to a 1-digit number in this.

Python3

# Python3 code to demonstrate working of
# Expand Character Frequency String
# Using join() + zip()
import re

# initializing string
test_str = 'g7f2g3i2s2b3e4s5t6'

# printing original string
print("The original string is : " + str(test_str))

# using zip() to pair up numbers and characters
# separately
res = "".join(a *int(b) for a, b in zip(test_str[0::2], test_str[1::2]))

# printing result
print("The expanded string : " + str(res))

Output

The original string is : g7f2g3i2s2b3e4s5t6
The expanded string : gggggggffgggiissbbbeeeessssstttttt

Time Complexity: O(n)

Auxiliary Space: O(n)

Method #2: Using regex() + join()

This is yet another way in which this task can be performed. In this task of pairing numbers and characters to different strings is performed using regex() and the advantage is that it can take numbers with digits more than 2.

Python3

# Python3 code to demonstrate working of
# Expand Character Frequency String
# Using regex() + join()
import re

# initializing string
test_str = 'g7f2g3i2s2b3e4s5t10'

# printing original string
print("The original string is : " + str(test_str))

# using findall to pair up numbers and characters
# separately, can include longer digit strings
res = ''.join(chr * int(num or 1)
              for chr, num in re.findall(r'(\w)(\d+)?', test_str))

# printing result
print("The expanded string : " + str(res))

Output

The original string is : g7f2g3i2s2b3e4s5t10
The expanded string : gggggggffgggiissbbbeeeessssstttttttttt

Time Complexity: O(n)

Auxiliary Space: O(n)

Method #3: Without using any built-in methods

Python3

# Python3 code to demonstrate working of
# Expand Character Frequency String

# initializing string
test_str = 'g7f2g3i2s2b3e4s5t6'

# printing original string
print("The original string is : " + str(test_str))
character=[]
frequency=[]
for i in range(0,len(test_str)):
    if(i%2==0):
        character.append(test_str[i])
    else:
        frequency.append(int(test_str[i]))
res=""
for i in range(0,len(character)):
    res+=character[i]*frequency[i]
# printing result
print("The expanded string : " + str(res))

Output

The original string is : g7f2g3i2s2b3e4s5t6
The expanded string : gggggggffgggiissbbbeeeessssstttttt

Time Complexity: O(n)

Auxiliary Space: O(n)

Method #4: Using itertools.repeat()

In this method, we use the itertools.repeat() method to repeat the characters according to their frequency. The itertools.repeat() method takes two arguments, the first argument is the element to repeat and the second argument is the number of times to repeat the element.

Python3

#Python3 code to demonstrate working of
#Expand Character Frequency String
#Using itertools.repeat()
import re
import itertools

#initializing string
test_str = 'g7f2g3i2s2b3e4s5t6'
#
#printing original string
print("The original string is : " + str(test_str))

#using itertools.repeat to repeat characters
#according to their frequency
res = ''.join("".join(itertools.repeat(c, int(num))) for c, num in zip(test_str[::2], test_str[1::2]))



#printing result
print("The expanded string : " + str(res))

Output

The original string is : g7f2g3i2s2b3e4s5t6
The expanded string : gggggggffgggiissbbbeeeessssstttttt

Time Complexity: O(n)
Auxiliary Space: O(n)

Method 5 :  without using itertools.repeat() 

Create an empty string to store the expanded string.
Loop through the string test_str with a step of 2 to iterate over the characters.
For each character, get the corresponding frequency from the next character in the string (test_str[i+1]).
Loop i times and append the character to the expanded string.
Return the expanded string.

Python3

#Python3 code to demonstrate working of
#Expand Character Frequency String
#Without using itertools.repeat()

#initializing string
test_str = 'g7f2g3i2s2b3e4s5t6'

#printing original string
print("The original string is : " + str(test_str))

#expanding the string
expanded_str = ""
for i in range(0, len(test_str), 2):
    char = test_str[i]
    freq = int(test_str[i+1])
    for j in range(freq):
        expanded_str += char

#printing result
print("The expanded string : " + str(expanded_str))

Output

The original string is : g7f2g3i2s2b3e4s5t6
The expanded string : gggggggffgggiissbbbeeeessssstttttt

The time complexity of this approach is O(n^2), where n is the length of the input string. 

The auxiliary space complexity is O(n), where n is the length of the input string.