Write a function called lucky_sevens that takes in one #parameter, a string variable named a_string. Your function #should return True if there are exactly three '7's in #a_string. If there are less than three or more than three #'7's, the function should return False.
For example:
# - lucky_sevens("happy777bday") should return True.
# - lucky_sevens("h7app7ybd7ay") should also return True.
# - lucky_sevens("happy77bday") should return False.
# - lucky_sevens("h777appy777bday") should also return False.
Hint: Remember in Chapter 3.3, we covered how to use a loop to look at each character in a string. #Write your function here!
Below are some lines of code that will test your function.
You can change the value of the variable(s) to test your function with different inputs.
If your function works correctly, this will originally #print: True, True, False, False, each on their own line.

Here is the Python program:

def lucky_sevens(a_string):  # function to return true if exactly 3 sevens

   counter = 0  # counts the number of times 7 occurs in the a_string

   for i in a_string:  # loops through the a_string

       if i == "7":  # if 7 occurs in the a_string

           counter = counter + 1  #counter increases by 1 every time 7 occurs

   if counter == 3:  #if 7 occurs exactly 3 times in a_string

       return True  #returns true

   else:  # 7 occurs less or more than 3 times in a_string

       return False  # returns false

#tests the function lucky_sevens with different inputs      

print(lucky_sevens("happy777bday"))

print(lucky_sevens("h7app7ybd7ay"))

print(lucky_sevens("happy77bday"))

print(lucky_sevens("h777appy777bday"))

Explanation:

This program has a method lucky_sevens that has a parameter a_string which contains a string of characters. The counter variable stores the number of times 7 occurs in the string. The loop has variable i that works like an index and moves through the entire string looking for 7 in the a_string. If element at i-th index is 7 then the counter variable increments by 1. It increments by 1 at every occurrence of 7. Then if condition checks if the value of count is equal to 3. Which means that 7 occurred 3 times in the a_string. If the condition is true then True is displayed otherwise False is displayed on output screen.

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