loops in python - zamaniamin/python GitHub Wiki

1. There are two types of loops in Python: while and for:

• the while loop executes a statement or a set of statements as long as a specified boolean condition is true, e.g.:

# Example 1
while True:
    print("Stuck in an infinite loop.")

# Example 2
counter = 5
while counter > 2:
    print(counter)
    counter -= 1

• If you notice some similarities to the if instruction, that's quite all right. Indeed, the syntactic difference is only one: you use the word while instead of the word if. The semantic difference is more important: when the condition is met, if performs its statements only once; while repeats the execution as long as the condition evaluates to True.

• the for loop executes a set of statements many times; it's used to iterate over a sequence (e.g., a list, a dictionary, a tuple, or a set - you will learn about them soon) or other objects that are iterable (e.g., strings). You can use the for loop to iterate over a sequence of numbers using the built-in range function. Look at the examples below:

# Example 1
word = "Python"
for letter in word:
    print(letter, end="*")

# Example 2
for i in range(1, 10):
    if i % 2 == 0:
        print(i)

• any variable after the for keyword is the control variable of the loop; it counts the loop's turns, and does it automatically;
• the in keyword introduces a syntax element describing the range of possible values being assigned to the control variable;

2. You can use the break and continue statements to change the flow of a loop:

• You use break to exit a loop, e.g.:

text = "OpenEDG Python Institute"
for letter in text:
    if letter == "P":
        break
    print(letter, end="")

• You use continue to skip the current iteration, and continue with the next iteration, e.g.:

text = "pyxpyxpyx"
for letter in text:
    if letter == "x":
        continue
    print(letter, end="")

3. The while and for loops can also have an else clause in Python. The else clause executes after the loop finishes its execution as long as it has not been terminated by break, e.g.:

n = 0

while n != 3:
    print(n)
    n += 1
else:
    print(n, "else")

print()

for i in range(0, 3):
    print(i)
else:
    print(i, "else")

4. The range() function generates a sequence of numbers. (we can even say that it will feed the loop with) It accepts integers and returns range objects. The syntax of range() looks as follows: range(start, stop, step), where:

• start is an optional parameter specifying the starting number of the sequence (0 by default)
• stop is an optional parameter specifying the end of the sequence generated (it is not included),
• and step is an optional parameter specifying the difference between the numbers in the sequence (1 by default.)

Example code:

for i in range(3):
    print(i, end=" ")  # Outputs: 0 1 2

for i in range(6, 1, -2):
    print(i, end=" ")  # Outputs: 6, 4, 2