Control flow, exceptions

Control flow, exceptions

Control flow

The indentation can be either 2 or 4. In a loop, it has to be indented. The : also is necessary.

• For loops when you know the number of iterations
• While loops when there is a condition to be met

For loop

for i in range(3):
    print('hello')

hello
hello
hello

for (i in 1:3){
  print('hello')
}

# this prints 0, 1, 2, 3, 4
for i in range(5):
    print(i)

0
1
2
3
4

Combined with lists

for i in range(1,3):
  for j in range(2,4):
    print(i, j)

1 2
1 3
2 2
2 3

scores = [45, 67, 89, 34, 56, 77, 49, 91, 52]

for score in scores:
  if score >= 70:
    print(score)

89
77
91

Combined with counter

results = ['Hit', 'Miss', 'Miss', 'Hit', 'Miss']
count = 0
for i in results:
  if i == 'Hit':
    count +=1
print(count)

2

While loop

n = 4
while n >0:
  print('continue')
  n = n-1

continue
continue
continue
continue

In R

n <- 4
while(n>0){
  print('continue')
  n <- n-1
}

If else

Conditions

rating = 74
views = 5400
print(rating>70 and views > 5000)
print(rating>70 or views > 6000)

True
True

age = 21
if age <= 18:
  print('discount')
#elif age > 18 & age <= 21
#  print('semi-discount')
else:
  print('original price')

original price

Break, continue

break needs to be inside the loop. Typically used with while

songs = ["Hello", "Yesterday", "Happy", "Hallelujah"]

for song in songs:
  print("Searching")
  if song == "Happy":
    print("Playing " + song)
    break

Searching
Searching
Searching
Playing Happy

continue skips when the condition is not met

ages = [13,19, 22, 8, 75, 34, 26, 41]
for age in ages:
  if age < 18:
    continue
  print(age)

19
22
75
34
26
41

Exception handling

Types of exceptions:

• NameError
• SyntaxError
• ValueError
• IndexError
• TypeErorr

try block: test a block of code for errors

except block: handle the error

try:
  print(k) # k is not defined
except:
  print('an exception')

an exception

If we do not know what kind of exception we are expecting,

try:
  print(k) # k is not defined, hence NameError
except:
  print('Something is wrong')

Something is wrong

try:
  print(k)
except NameError:
  print("Variable x is not defined")
except:
  print("Something else went wrong")

Variable x is not defined

try:
  print(3 + "3")
except ValueError:
  print("Cannot add different types")
except TypeError:
  print("Type mismatch error")

Type mismatch error

else block: execute code when there is no error

finally block: execute code regardless of result of the try/except blocks

books = ['HP', 'dune']
try:
  print(books[5])
except IndexError:
  print('out of range')
finally:
  print('happy reading')

out of range
happy reading

Raise an exception

x = -1
if x<0:
  raise Exception('need x to be above 0') # can also be ValueError

Raise a type error

x = 'hello'
if not type(x) is int:
  raise TypeError('needs to be an integer')