04 Control-Flow - 03 Range

The range() function generates a sequence of numbers, commonly used for looping or generating lists with specific patterns.

• range(i, j) Generates a sequence of numbers starting from i and ending before j.

range(i, j)
# Produces the sequence: i, i+1, ..., j-1

• range(j) Starts from 0 and ends before j. This is equivalent to range(0, j).

range(j)
# Like slice(:j), starts from 0, ends at j-1

Increment with range():

• range(i, j, k)
  Adds an optional third argument k for defining the step increment, useful for sequences with a specific pattern, like arithmetic progressions (AP).

range(i, j, k)
# Produces: i, i+k, i+2k, ..., i+nk

Reverse Counting:

• To count in reverse, use a negative value for k. Ensure i > j when using a negative step.

range(i, j, -1)
# i must be greater than j (i > j)

• This can be useful for iterating over a list’s valid indices:

range(0, len(l))
# Produces a valid range of indices for the list l

Converting range() to a List:

• range() creates a range object, not a list. To convert it into a list, use the list() function:

list(range(0, 5))  
# Converts range to list: [0, 1, 2, 3, 4]

Skipping Numbers in Sequences:

• Useful for generating sequences where numbers are skipped, such as even numbers or multiples:

even = list(range(2, 11, 2))
print(even)  
# Output: [2, 4, 6, 8, 10]

Using range() to Generate Sequences

range() can be particularly useful in a variety of contexts, such as repeating actions or generating specific number sequences.

Example 1: Repeating a String Multiple Times

# Repeating the string "meow" 3 times:
print("meow" * 3)  # Output: meowmeowmeow

Example 2: Using Newlines with range()

# Using newline `\n`:
print("meow\n" * 3)  # Output: meow (with new line after each "meow")

Example 3: Removing the Default Newline in print()

# Removing the default newline after the print:
print("meow\n" * 3, end="")  # Output: meowmeowmeow (without extra newline)

Example 4: Finding All Factors of a Number n

You can use range() in a for loop to find all factors of a given number n:

def factors(n):
flist = []
for i in range(1, n + 1):
	if n % i == 0:
		flist.append(i)  # Can also use flist = flist + [i]
return flist