loops.md

Loops

For loops

Iterating with _

If you do not want to utilise the items in a list, it is common practice to use _ to iterate:

for _ in 'Hello world':
  print('Iterating over anonymous letter')

Tuple unpacking

Tuples inside a list can be unpacked while looping through the containing list:

e = [(1,2),(3,4),('cat','dog')]
for a,b in e:
    print(a,b)

Dictionary loop

You can access keys and values (and both) in a dictionary by using the fuctions items(), keys(), and values().

d = {'one':10,'two':'b','three':0.5}

# displays keys and values
for k,v in d.items():
    print(k,v)

# displays keys only
for k in d.keys():
    print(k)

# displays keys only
for v in d.values():
    print(v)

Interrupting loops

break: ends closest enclosing loop, ending it entirely

continue: jumps to next iteration; goes back to top of closest enclosing loop

pass: placeholder for when you want a loop but don't want it to do anything; useful to avoid syntax error in empty iteration

Useful functions

range(begin,length,step) creates a length which can be iterated over.

for num in range(0,11,2):
    print(num)
    # 0
    # 2
    # 4
    # ...etc

list() generates a list, which can then be put into a loop.

enumerate() gives every item in a list an index. Items become tuples, with index being at position 0.

for index, letter in enumerate('somestring'):
    print(index, letter)

zip() zips together two or more lists (like a zipper on a jacket).

list1 = [1,2,3]
list2 = ['a','b','c']

for item in zip(list1,list2):
    print(item)
    # (1,'a')
    # (2,'b')
    # (3,'c')

in checks if something exists in a list or dictionary. Useful to avoid errors.

Check for letter in strong:

'd' in 'dog'
# True

Check for key in dictionary:

'key' in {'key':123}
# True

Check for value in dictionary:

d = {'somekey':345}

345 in d.values()
# True

min() and max() respectively check for minimum and maximum values in a list.