Find the first repeated word in a string in Python using Dictionary

We are given a string that may contain repeated words and the task is to find the first word that appears more than once. For example, in the string "Learn code learn fast", the word "learn" is the first repeated word. Let's understand different approaches to solve this problem using a dictionary.

Using in keyword

The in keyword is used to check whether a specific key exists in a dictionary. It returns a boolean value True if the key is found and False otherwise. This is a safe and efficient way to prevent errors when working with dictionaries, especially before accessing or modifying a key.

import re

t = "Learn code learn fast"

# Get lowercase words
w = re.findall(r'\b\w+\b', t.lower())

d = {}
for x in w:
    # If repeated, print and stop
    if x in d:
        print(x)
        break
    else:
        # Mark as seen
        d[x] = 1

learn

Explanation: A dictionary tracks seen words. As each word is checked, if it's already in the dictionary, it's printed as the first repeat and the loop stops. Otherwise, it's added to the dictionary.

Using get() method

get() method is used to retrieve the value for a given key from a dictionary. If the key does not exist, it returns None by default or a value you specify. This method is helpful when you want to avoid exceptions and provide fallback values during lookups, such as when counting word occurrences.

import re

t = "Python is great and Python is easy"

# Get lowercase words
w = re.findall(r'\b\w+\b', t.lower())

c = {}
for x in w:
    # Count words and check for second occurrence
    c[x] = c.get(x, 0) + 1
    if c[x] == 2:
        print(x)
        break

python

Explanation: A dictionary is used to count how many times each word appears. As the loop iterates through each word, the get() method updates its count. When a word's count becomes 2, it is identified as the first word that repeats.

Using setdefault()

The setdefault() method checks if a key exists in a dictionary and if not, it inserts the key with a specified default value. It then returns the value for that key regardless of whether it was newly added or already present. This is particularly useful for initializing keys in dictionaries, especially in counting or grouping scenarios.

import re

t = "Data science is data driven"

# Get lowercase words
w = re.findall(r'\b\w+\b', t.lower())

s = {}
for x in w:
    # If seen before, print and stop
    if s.setdefault(x, 0):
        print(x)
        break
    s[x] += 1

data

Explanation: setdefault() method initializes each word with a count of 0 if it's not already in the dictionary. If the word has been seen before (i.e., its value is non-zero), it is printed as the first repeated word and the loop stops. Otherwise, its count is incremented.