Colab

IMPORTANT

this is an incredibly crucial lesson. and requirs A LOT of excersizes to really get the hang of. please do all the excersizes, and if you found them challenging, keep doing more excersizes and even doing the same excersizes again and again until you can reliably solve them easily.

Where to find more excercises?

For more exercizes on loops, see this link which contains 44 exercises with solutions:

for loops

iterating over a word list with a for loop

Here’s a line from some old movie

quote = "Strange women lying in ponds, distributing swords, is no basis for a system of government!"
  1. use the .split() function to split it into a list of words.
  2. use the for loop to iterate over each word in the list
    • for each word, print the word and its len()

expected output:

Strange 7
women 5
lying 5
in 2
ponds, 6
distributing 12
swords, 7
is 2
no 2
basis 5
for 3
a 1
system 6
of 2
government! 11

comments

  • Sometimes we want to write something that will explain the code we write, for this we can use comments. 
    # this is a comment
x = 10 + 90
x = x * 2 # this is a comment at the end of a line 
print(x) # will print 200
  • comments start with the # character and continue until the end of the line
  • comments are just for reading by humans, the python interepreter completely ignores them

write helpful comments

copy this code to the answer below, and write short comments between the lines that explain the code

hamburger = 59
pizza = 39
steak = 150
soup = 42
salad = 35
chips=18

dinner = [
    hamburger,
    hamburger,
    salad,
    chips
]

cost_of_dinner = 0
for food_item_cost in dinner:
    cost_of_dinner = cost_of_dinner + food_item_cost

print(cost_of_dinner)

a nice dinner for two

  1. copy this code below 
    dinner = [
     # Sashimi plate
     60, 
     # some maki
     30, 35, 39,
     # agadashi 
     28,
     # miso
     18, 18,
     # lots of sake
     33, 39, 33,
     ]
  2. use a for loop to calculate the cost of the dinner

    this should take 3 lines

  3. this is a dinner for two. what is the average cost per person for this dinner?

    hint: divide by 2

iterating over letters in a string

  1. what does the following code do? try it 
    monty = "This is an ex parrot"
for letter in monty:
 print(letter, end=" ")
  2. what does the end=" " part do? try to chage it to different values:
    • " @ "
    • ""
    • " --- "
    • "\n"
  3. can you write a loop that prints each letter twice?

    expected output:

    TThhiiss  iiss  aann  eexx  ppaarrrroott

range() - lists of numbers

  1. run the following code 
    print(range(10))

    what does it print?

  2. the function range(10) actually returns a list-like object . iterate over all the elements of range(10) using a for loop and print each item in that list
    • repeat for range(15)
  3. what does the function range(10,20) return ? print everything in that list
    • repeat for range(1,5)
  4. what does the function range(0,10,2) return? print everything in that list
    • repeat for range(0, 100, 10)
    • repeat for range(100, 0, -10)

  5. generally speaking, can you explain what function range does?

  6. 😵 can you solve this exercise with much fewer lines of code?

    hint:

    1. put all the interesting ranges into a list of ranges
    2. iterate over the list of ranges with a for loop
    3. for each range in the list, use another nested/inner for loop to print out all the elements of the list

sum of numbers

calculate the sum of the numbers between 0 and 1000 \(0 + 1 + 2 + 3 + 4 + ... + 997 + 998 +999\)

hint:

  • you dont need math or any formula …
  • use the range() function and a for loop
  • to test your code you can see if it gets the right answer for numbers between 0 and 5 \(0+ 1+ 2+ 3+ 4 = 10\)

average grades

here are some grades for students in a class. calculate the average grade

grades = [ 60, 90, 70, 100, 65, 95, 80, 80, 80]

hints:

  1. use a for loop
  2. $average = \frac{grade_1 + grade_2 + … + grade_{n-1} + grade_n}{n}$

printing asterics

  1. what does the following code do? 
    print('*', end="")
print('*', end="")
print('*', end="")
print('*', end="")
  2. use a for loop, range() and print('*', end="") to print a line of 10 asterics

    expected output:

      **********

    hint: this takes just two lines

use a for loop

squares

here’s a square made of the '*' character with length 5

*****
*****
*****
*****
*****
  1. use the input() function to take a number from the user 
    num = int(input('how big? '))
  2. can you use for loop(s) to print such a square of length num?

hints:

  • use print('*', end="") to print a single '*' character
  • you need more than one for loop

triangles

here’s a triangle made of the '*' character with length 5

 *
 **
 ***
 ****
 *****

can you use for loop(s) to print such a triangle of length 10?

for loop - multiplication matrix

  • this question is slightly harder 🤯
  • multiplication matrix = לוח הכפל

this time, use for loop(s) and range to print the multiplication matrix in the following way (total of 100 lines)

  1 x 1 = 1
  1 x 2 = 2
  ...
  1 x 10 = 10
  2 x 1 = 1
  2 x 2 = 2
  ...
  2 x 10 = 20
  ...
  ...
  10 x 1 = 10
  10 x 2 = 20
  ...
  10 x 10 = 100

for loop - multiplication matrix with nice formatting

  • this question is harder 🤯🤯

  1. copy your for loop multiplication matrix solution above

  2. modify the solution so that it actually prints the multiplication table in table form
    hint:  » print(1,2,3, end=”\t”)  » print(4,5,6) 1 2 3 4 5 6

expected output:

 1    2     3     4     5     6     7     8     9     10    
 2    4     6     8     10    12    14    16    18    20    
 3    6     9     12    15    18    21    24    27    30    
 4    8     12    16    20    24    28    32    36    40    
 5    10    15    20    25    30    35    40    45    50    
 6    12    18    24    30    36    42    48    54    60    
 7    14    21    28    35    42    49    56    63    70    
 8    16    24    32    40    48    56    64    72    80    
 9    18    27    36    45    54    63    72    81    90    
 10   20    30    40    50    60    70    80    90    100

conditionals and while loop

if/elif/else - checking a password

  1. use the input() function to take a password from the user

  2. check that the password is “good” according to the following criterions:
    1. the password length must be at least 4
    2. the password length must be no more than 12
    3. the password must not be just numbers
    4. the password must have both lowercase and uppercase letters

  3. if the password is good, print “excellent password”. otherwise, print the reason why the password was rejected

HINTS:

  • use the len(), .isdigit(), .isupper(), .islower() functions

While loop - until we find a good password

  1. copy the password testing code you wrote above

  2. now use a while loop to continue asking for a new password until a password that passes all the tests is given

While loop - password with maximum 3 attemps

  1. copy your answer for the while loop above

  2. now modify the code such that no more than 3 attemps are possible after 3 failed attemps, the program should say “too many attemps, access denied”

while loop - numbers 0-100 divisible by 7

using a while loop, print all the numbers between 0 and 100 that are divisible by 7

we say that x is divisible by 7 when x%7 == 0

for loop - numbers 0-100 divisible by 7

Just like the question above, this time with a for loop

print all the numbers between 0 and 100 that are divisible by 7 using a for loop and range(100)

While loop - multiplication matrix

  • this question is harder 🤯🤯🤯

  • multiplication matrix = לוח הכפל

using while loop(s), print the multiplication matrix in the following way (total of 100 lines)

  1 x 1 = 1
  1 x 2 = 2
  ...
  1 x 10 = 10
  2 x 1 = 1
  2 x 2 = 2
  ...
  2 x 10 = 20
  ...
  ...
  10 x 1 = 10
  10 x 2 = 20
  ...
  10 x 10 = 100

loop to find long words

print all the words in the following sentence that are longer than 4 characters:

"The quick brown fox jumped over the lazy dog"

expected output:

quick
brown
jumped

enumerate

  1. split the string “the quick brown fox jumped over the lazy dog” into words using the .split() function, and put the list in the variable ‘words’

  2. use the enumerate() function to print each word on its own line, along with the index of the word in the ‘words’ variable 

    0 the
1 quick
2 brown
3 fox
4 jumped
5 over
6 the
7 lazy
8 dog

  3. (harder) replace words that are longer than 4 characters to the string “too long” use enumerate() to do this with minimum number of code lines

    ['the', 'too long', 'too long', 'fox', 'too long', 'over', 'the', 'lazy', 'dog']