For Loops

Part 1 Warmup (5 minutes)

1.1

count = 0
while count < 3:
    print("Hello")
    count = count + 1

Question: How many times will this code print “Hello”?

1.2

friend = "Alice"
print("Hello", friend)
friend = "Bob"
print("Hello", friend)

Question: What will this code print?

1.3

# We want to say hello to 5 friends:
# Alice, Bob, Charlie, Diana, and Eddie

count = 0
while count < 5:
    # Yikes... how do we print the right name?
    print("Hello", "???")
    count = count + 1

Question: Using only what we know about while loops and variables, how could we solve this? What makes it awkward or difficult?

Part 2. Predictions (15 minutes)

2.1

for friend in ['Joseph', 'Glenn', 'Sally']:
    print('Happy New Year:', friend)
print('Done!')

Questions:

  • a. What gets printed to the screen? Write down the exact output.
  • b. How many times does the indented line run?
  • c. What happens to the variable ‘friend’ during each run?

2.2

# Program 1
for letter in 'Hi!':
    print(letter)

# Program 2
for number in [1, 2, 3]:
    print('Count:', number)

Questions:

  • a. What’s similar about how these two programs work?
  • b. What kinds of things can we put after the word ‘in’?
  • c. In Program 1, what value does ‘letter’ have during each run?

2.3

# Program 3
for num in [1, 2]:
    print('Start')
    print(num)
    print('End')
print('All done')

# Program 4
for word in ['cat', 'dog']:
    answer = word + '!'
    print(answer)

Questions:

  • a. How many times does ‘Start’ get printed in Program 3?
  • b. What determines which lines are part of the loop?
  • c. What will Program 4 print? Be exact.

Part 3. Lecture Notes (15 minutes)

Part 4. Practice

4.1

# Program 1
for x in "wow":
    print("*")

# Program 2
for x in "wow":
    print(x)

# Program 3
for x in "wow":
    print(x + "!")

Question: What is the output of these 3 programs?

4.2

# template
for _____ in ['a', 'b', 'c']:
    ________________

Question: Adapt the template code (just above) to print this:

hello a
hello b
hello c

4.3

Question: Write a for loop that prints:

1 Mississippi
2 Mississippi
3 Mississippi
4 Mississippi
5 Mississippi

4.4

Question: Use a for loop to make python print this pattern:

(Hint: you can multiply strings! Example: "ha" * 3 gives "hahaha")

*
**
***
****

Bonus Challenge: Use a for loop to print one of these patterns:

   *
  **
 ***
****

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