for loops

https://docs.python.org/3/tutorial/controlflow.html

INTRODUCTION

The 'for' statement is used to iterate over the elements of an iterable object.

# Syntax.
for item in iterable:
    statements      # item is processed
    if condition1:
        break       # optional, terminate the loop without executing 'else'
    statements
    if condition2:
        continue    # optional, go to the next item from iterable
    statements
else:               # optional, terminate the loop
    statements      # executed if 'break' was not used, even for an empty iterable

# Warning: the name 'item' is created only if 'iterable' is not empty!

# Range-based for loop in C++ (since C++11)
# for (auto item : container) statement
# for (auto& item : container) statement

word = "Python"
for char in word:   # use decriptive names
    if char == "o":
        break   # characters printed: 'P', 'y', 't', 'h'
    print(char)
assert char == "o"   # 'char' is alive

fruits = ["apple", "pear", "plum", "apricot"]   # use plural when possible
for fruit in fruits:
    if fruit[0] == "a":
        continue   # skipping "apple" and "apricot"
    print(fruit)
assert fruit == "apricot"

# What if we need an index?
for i in range(len(fruits)):   # C style
    print("{} {}".format(i, fruits[i]))

# enumerate(iterable, start=0), return an enumerate object (iterator)
for (i, fruit) in enumerate(fruits):   # recommended
    print("{} {}".format(i, fruit))

[c*i for (i, c) in enumerate("abcd")]   # ["", "b", "cc", "ddd"]
[c*i for (i, c) in enumerate("abcd", start=1)]   # ["a", "bb", "ccc", "dddd"]

# loop over pairs
for t in [(1, "one"), (2, "two"), (3, "three")]:
    print("first {} second {}".format(t[0], t[1]))
    #print("first {0[0]} second {0[1]}".format(t))

# better version with unpacking
for (a, b) in [(1, "one"), (2, "two"), (3, "three")]:
    print("first {} second {}".format(a, b))

# Code that modifies a collection while iterating over that same collection
# can be tricky to get right.
# It is safe to extend the list (append at the end).
# It is not safe to add or remove an item in the middle of the list.
L = [0, 1, 2, 3, 4, 5]
for i in L:
    print(i)
    if (i % 2) == 1:      # if i is odd then extend the list
        L.append(2 * i)   # adding two even numbers
        L.append(4 * i)

L = [0, 1, 2, 3, 4, 5]
for i in L[:]:   # use copy in the loop [or list(L)]
    if (i % 2) == 1:
        L.remove(i)   # changing the original list

# Better solution (filtering).

new_list = [i for i in L if (i % 2) != 1]

# Looping over a sequence in reverse.

for char in reversed("qwerty"):   # 'reversed object'
    print(char)   # y, t, r, e, w, q

# Looping over a sequence in sorted order.

for char in sorted("qwerty"):   # ['e', 'q', 'r', 't', 'w', 'y']
    print(char)

# Looping over two or more sequences at the same time.

letters = ["a", "b", "c", "d"]
numbers = [10, 20, 30, 40]
for (letter, number) in zip(letters, numbers):
    print("{} {}".format(letter, number))

# Eliminating duplicate elements.

for char in set("abracadabra"):   # 'a', 'c', 'd', 'b', 'r'
    print(char)

# Using all() and any().

L = [1, 2, 4, 5]

if all(x % 2 == 0 for x in L):
    print("all numbers are even")

if any(x % 2 == 0 for x in L):
    print("there is an even number on the list")