Note_Tech

All technological notes.

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Python - Loops

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• Python - Loops
  • while Loop
  • for Loop
  • Nested Loops
  • zip: Loop multiple lists simultaneously
  • Conditional Loop: for + List Comprehension

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while Loop

• while: execute a set of statements as long as a condition is true.

  • Note: remember to increment i, or else the loop will continue forever.

• break: stop the loop.

• continue: stop the current iteration, and continue with the next iteration.

• else: run a block of code once when the condition no longer is true

print("\n--------while loop--------\n")
i = 0
while i < 6:
    i += 1
    print(i)

# 1
# 2
# 3
# 4
# 5


print("\n--------break--------\n")
i = 0
while i < 6:
    i += 1
    if i == 3:
        break
    print(i)

# 1
# 2


print("\n--------continue--------\n")
i = 0
while i < 6:
    i += 1
    if i == 3:
        continue
    print(i)

# 1
# 2
# 4
# 5
# 6


print("\n--------else--------\n")
i = 0
while i < 6:
  i += 1
  print(i)
else:
  print("i is no longer less than 6")

# 1
# 2
# 3
# 4
# 5
# 6
# i is no longer less than 6

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for Loop

• A for loop is used for iterating over a sequence (that is either a list, a tuple, a dictionary, a set, or a string).

• break: stop the loop before it has looped through all the items.

• continue: stop the current iteration of the loop, and continue with the next.

• range(): returns a sequence of numbers, starting from 0 by default, and increments by 1 (by default), and ends at a specified number.

  • To loop through a set of code a specified number of times.

• else: specifies a block of code to be executed when the loop is finished

  • The else block will NOT be executed if the loop is stopped by a break statement.

• pass: for loops cannot be empty, put in the pass statement to avoid getting an error.

print("\n--------for loop--------\n")
# loop a list
fruits = ["apple", "banana", "cherry"]
for x in fruits:
    print(x)

# apple
# banana
# cherry

# loop a string
for x in "banana":
  print(x)


# break
fruits = ["apple", "banana", "cherry"]
for x in fruits:
  if x == "banana":
    break
  print(x)
# apple


# continue
fruits = ["apple", "banana", "cherry"]
for x in fruits:
  if x == "banana":
    continue
  print(x)
# apple
# cherry


# range()
for x in range(6):
  print(x)
# 0
# 1
# 2
# 3
# 4
# 5


# start with 2, end with 5
for x in range(2, 6):
  print(x)
# 2
# 3
# 4
# 5


# start with 2, end with 29, increment the sequence with 3
for x in range(2, 30, 3):
  print(x)
# 2
# 5
# 8
# 11
# 14
# 17
# 20
# 23
# 26
# 29


# else
for x in range(6):
  print(x)
else:
  print("Finally finished!")

# 0
# 1
# 2
# 3
# 4
# 5
# Finally finished!


# break + else
for x in range(6):
  if x == 3: break
  print(x)
else:
  print("Finally finished!")
# 0
# 1
# 2


# pass
for x in [0, 1, 2]:
  pass

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Nested Loops

• A nested loop is a loop inside a loop.
  • The “inner loop” will be executed one time for each iteration of the “outer loop”:

# Nested Loops
adj = ["red", "big", "tasty"]
fruits = ["apple", "banana", "cherry"]

for x in adj:
  for y in fruits:
    print(x, y)

# red apple
# red banana
# red cherry
# big apple
# big banana
# big cherry
# tasty apple
# tasty banana
# tasty cherry

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zip: Loop multiple lists simultaneously

xList = [1, 2, 3]
yList = [1, 2, 4]

for (x, y) in zip(xList, yList):
    if x != y:
        print("difference:", x, y)      # difference: 3 4

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Conditional Loop: for + List Comprehension

xList = [1, 5, 10, 25]

target = 9

for item in [x for x in xList if x < target]:
    print(item)
# 1
# 5

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