Days 41-45 lessons, for review

3 of 9 - Scenario

Nina is coding the ticket booth for the school's haunted hallway on Halloween night. She stores int tickets = 4; and int total = 20; and then writes three separate if statements in a row: if (tickets > 3) { total = total - 5; } if (total > 20) { total = total + 100; } if (tickets == 4) { total = total * 2; } Her friend Leo says total will end at 15, because once one if is true the others are skipped. Nina decides to trace each statement in order and write down what total holds after every one.

What does total hold when all three statements have been checked?

• Correct
  30

  The first condition 4 > 3 is true, so total becomes 15. The second condition 15 > 20 is false, so it is skipped. The third condition 4 == 4 is true, so total becomes 15 * 2, which is 30.

  Ties to today: Each one-way if is checked on its own, using the values at that moment. That is the heart of Topic 2.3.

• Wrong
  15

  That would be true if the other ifs were skipped after the first true one, but separate ifs are each checked on their own. The third condition is true as well.

• Wrong
  230

  The second if checks total after the discount, when total is already 15. Since 15 > 20 is false, the + 100 never happens.

• Wrong
  40

  The third if multiplies the current total, which is 15 after the discount, so it gives 30 and not 40.

3 of 8 - Scenario

Marcus is coding the costume contest for the Halloween parade. He starts with int votes = 9; and int bonus = 0; and String result = ""; and then writes: if (votes >= 10) { bonus = 5; result = "winner"; } else { bonus = 2; result = "runner-up"; } votes = votes + bonus; if (votes >= 10) { result = result + "!"; } His friend Kai says the contest will report a winner, since the final votes will be over 10. Marcus traces the code one statement at a time before the parade starts.

What do votes and result hold when the code finishes?

• Correct
  votes is 11 and result is runner-up!

  9 >= 10 is false, so the else block runs: bonus becomes 2 and result becomes runner-up. Then votes becomes 9 + 2, which is 11. The second if is a separate check made after that, and 11 >= 10 is true, so a ! is added.

  Ties to today: Each if-else makes its decision once, using the values at that moment.

• Wrong
  votes is 14 and result is winner!

  The first condition was checked while votes was still 9, and 9 >= 10 is false, so the winner block never runs.

• Wrong
  votes is 11 and result is runner-up

  Close, but the second if is its own statement. After votes becomes 11, its condition is true, so the ! gets added.

• Wrong
  votes is 9 and result is runner-up!

  votes is changed by the line votes = votes + bonus, so it does not stay at 9.

3 of 8 - Scenario

Theo is coding the candy rule for trick-or-treat night. He stores int age = 14; and boolean costume = true; and int candy = 2; and then writes: if (age < 13) { if (costume) { candy = candy + 5; } else { candy = candy + 1; } } else { if (costume) { candy = candy + 3; } } His sister Ana says anyone wearing a costume must get the big 5 candy bonus, since the costume check is right there in the code. Theo traces it from the outside in, one condition at a time.

What does candy hold when the code finishes?

• Correct
  5

  14 < 13 is false, so the outer else block runs. Inside it, costume is true, so candy becomes 2 + 3, which is 5.

  Ties to today: Working from the outside in, one condition at a time, is the key skill for nested ifs.

• Wrong
  7

  The + 5 bonus belongs to the age < 13 branch, and 14 is not less than 13.

• Wrong
  2

  The inner if inside the else block still gets checked, and costume is true there, so candy changes.

• Wrong
  3

  The 3 is added to candy, which already holds 2, so the result is 2 + 3 and not just 3.

3 of 8 - Scenario

Priya is coding the letter grades for the first quarter report cards, which close this Friday. Her first version starts with String grade = "F"; and then has three separate ifs: if (score >= 90) { grade = "A"; } if (score >= 80) { grade = "B"; } if (score >= 70) { grade = "C"; } A friend tests it with a score of 95 and the program says C. Priya changes the second and third if into else if, so it reads: if (score >= 90) { grade = "A"; } else if (score >= 80) { grade = "B"; } else if (score >= 70) { grade = "C"; } She runs both versions with score = 95.

What does grade hold for score = 95 in the first version and in the second version?

• Correct
  C in the first version and A in the second

  In the first version all three conditions are true, so the three separate ifs all run in order and the last one wins. In the second version, 95 >= 90 is true, so grade becomes A and every else-if branch is skipped.

  Ties to today: Separate ifs versus an else-if chain is the main idea of Topic 2.4.

• Wrong
  A in both versions

  The first version has three separate ifs, and nothing stops the second and third from running after the first.

• Wrong
  C in both versions

  The second version stops at the first true condition, which is score >= 90, so it never reaches the C.

• Wrong
  A in the first version and C in the second

  That is backwards. Separate ifs keep running all the way down, and the else-if chain stops early.

3 of 9 - Scenario

Omar is adding a safety check to his Mad Libs program so it rejects word lists that are empty, too short, or too long. He stores let box = "cat dog"; and let count = 2; and then writes: let tooFew = count < 3; let tooMany = count > 8; let empty = box === ""; let bad = tooFew || tooMany || empty; let good = !bad; His friend Ravi says the check should use && instead of ||, because it has to catch every problem at once. Omar traces the values one line at a time.

What do bad and good hold?

• Correct
  bad is true and good is false

  count < 3 is true because 2 is less than 3, and the other two pieces are false. true || false || false is true, and the ! flips that to false for good.

  Ties to today: || needs only one true piece, which is exactly what a safety check wants.

• Wrong
  bad is false and good is true

  That is what && would give, since every piece would have to be true. || only needs one.

• Wrong
  bad is true and good is true

  good is !bad, so the two values always disagree.

• Wrong
  bad is false and good is false

  good is !bad, so the two values can never match.

3 of 8 - Scenario

Jules is rewriting the Mad Libs fill-in code as a while loop. To understand it first, she tests a tiny version with let missing = 5; and let typed = 0; and then writes: while (missing > 2) { typed = typed + 1; missing = missing - 1; } Her friend Dev says the body will run 5 times, because missing starts at 5. Jules decides to trace each pass in a table before she runs it.

What do typed and missing hold after the loop ends?

• Correct
  typed is 3 and missing is 2

  The loop runs while missing > 2. Pass 1 makes missing 4, pass 2 makes it 3, and pass 3 makes it 2. Then 2 > 2 is false, so the loop stops after exactly 3 passes.

  Ties to today: The condition is checked before every pass, which is why the loop stops the moment it is false.

• Wrong
  typed is 5 and missing is 0

  The loop stops as soon as missing reaches 2, so it never gets that far.

• Wrong
  typed is 4 and missing is 1

  That would happen if the condition were missing >= 2. With missing > 2, the loop stops when missing is 2.

• Wrong
  typed is 3 and missing is 3

  missing goes down by 1 on every pass, so after 3 passes it is 5 - 3, which is 2 and not 3.

3 of 8 - Scenario

Ravi wants a spooky banner on his Mad Libs page that repeats the word boo three times. He writes let banner = ""; for (let i = 0; i < 3; i++) { banner = banner + "boo "; } His friend Dev bets the banner will have four boos, because i goes 0, 1, 2, and 3. Ravi counts the passes one at a time and writes down the value of i at the start of each one.

What does banner hold when the loop ends?

• Correct
  Three copies of boo, each followed by a space

  The loop runs while i < 3, so it runs for i = 0, 1, and 2. When i becomes 3, the condition 3 < 3 is false and the loop stops before a fourth pass.

  Ties to today: Counting from 0 up to, but not including, the number in the condition is how for loops work.

• Wrong
  Four copies of boo

  i does reach 3, but the condition is checked first, and 3 < 3 is false, so there is no fourth pass.

• Wrong
  One copy of boo

  The loop keeps going while i < 3, so it does not stop after the first pass.

• Wrong
  Nothing, because banner starts out empty

  The body adds to banner on every pass, so it grows each time.

3 of 8 - Scenario

Leo's Mad Libs page shows a list of spooky words. He writes let words = ["moon", "bat", "owl"]; let out = ""; for (let i = 0; i < words.length; i++) { out = out + words[i] + "!"; } His friend Mara says the loop will crash on its last pass, because words[3] does not exist and the loop goes up to 3. Leo traces the value of i and the text in out on every pass before he runs it.

What does out hold when the loop ends?

• Correct
  moon!bat!owl!

  words.length is 3, so the loop runs for i = 0, 1, and 2. When i becomes 3, 3 < 3 is false, so words[3] is never read.

  Ties to today: Pairing a loop with i < list.length visits every item and stops in the right place.

• Wrong
  moon!bat!owl!undefined!

  That would happen if the condition were i <= words.length. Here it is i < words.length, so the loop stops before index 3.

• Wrong
  bat!owl!

  The loop starts at i = 0, which is the first word, moon.

• Wrong
  moon!

  The loop keeps going until i < 3 is false, so it visits all three words.

3 of 8 - Scenario

Dana's Chromebook prints to the library printer in the same building almost instantly, but loading a website takes a little longer, because the request has to leave the school. The school's network has her Chromebook, a switch that connects the wired devices in the building, a wireless access point, and a router that connects the building to the internet service provider. Dana's teacher asks the class which device is responsible for forwarding her website request out of the school's local network and toward the internet.

Which device forwards that request out of the LAN?

• Correct
  The router

  A router connects different networks and decides where each packet goes next, so it is the one that sends the request out of the school's LAN toward the internet.

  Ties to today: Naming what each device does is the first step in understanding any network.

• Wrong
  The switch

  A switch connects devices inside the same LAN. It does not connect the LAN to the internet.

• Wrong
  The wireless access point

  An access point lets wireless devices join the LAN, but it does not route packets to other networks.

• Wrong
  The printer

  The printer is just another device on the LAN. It receives print jobs and does not forward anything.

3 of 8 - Scenario

Maya and Devon live in different houses across town, and both of their home routers hand out addresses that start with 192.168.1. Maya's laptop is 192.168.1.23, and so is Devon's. Devon worries that when both of them visit the same website at the same time, their traffic will crash into each other. Maya says the two laptops will never collide. Their teacher asks the class to explain who is right and why.

Why can two different homes use the same 192.168.1.23 address without a conflict?

• Correct
  Those are private addresses that only mean something inside each home's own network, and each router has its own public address that the internet sees

  192.168.x.x is a private range. The websites never see the laptop's private address, only the public address of each home's router.

  Ties to today: Telling private addresses from public ones is a core networking skill.

• Wrong
  Each website keeps a list that tells the two laptops apart by name

  Websites never see the private address at all. They only see the router's public address.

• Wrong
  Only one of the two laptops can be online at a time

  Both can be online at once, because private addresses are not shared between homes.

• Wrong
  192.168.1.23 is a public address that two homes are allowed to share

  192.168.x.x is a private range. Public addresses are unique across the whole internet.

3 of 9 - Scenario

Priya is reading a packet capture for the school's network team. The first 14 bytes of one frame, shown in hex, are: FF FF FF FF FF FF 3C 22 FB 9A 10 5E 08 00. She knows the first six bytes are the destination MAC address, the next six are the source MAC address, and the last two bytes say what kind of packet comes next: 08 00 means IPv4, 08 06 means ARP, and 86 DD means IPv6. She also knows that FF FF FF FF FF FF is the address that means every device on the LAN. Her teammate Leo says a frame like this must be meant for one specific laptop, because it has a MAC address in it.

What is this frame doing?

• Correct
  It is a broadcast to every device on the LAN, sent from 3C:22:FB:9A:10:5E and carrying an IPv4 packet

  The destination bytes are all FF, which means everyone. The source is the next six bytes, 3C 22 FB 9A 10 5E. The last two bytes, 08 00, mean an IPv4 packet comes next.

  Ties to today: Reading a header one byte at a time is exactly what a hex dump is for.

• Wrong
  It is sent to one specific laptop and carries an IPv4 packet

  All six destination bytes are FF, which is the broadcast address, so every device on the LAN receives it.

• Wrong
  It is a broadcast to every device and carries an ARP message

  An ARP message would end in 08 06. These bytes end in 08 00, which means IPv4.

• Wrong
  It is a broadcast to every device and carries an IPv6 packet

  An IPv6 packet would end in 86 DD. These bytes end in 08 00, which means IPv4.

3 of 8 - Scenario

Lakeside Clinic is setting up the web portal where patients read their test results. The server could be listening on four services: plain web (HTTP) on port 80, secure web (HTTPS) on port 443, file transfer (FTP) on port 21, and remote administration (SSH) on port 22. Patients need to reach the portal, staff need to manage the server remotely, and the clinic does not want passwords or medical data crossing the network in cleartext. The IT lead has to write the firewall's allow and deny rules. A new technician suggests allowing all four ports so that nothing breaks.

Which rule set best fits the clinic's goals?

• Correct
  Allow ports 443 and 22, and deny ports 80 and 21

  Port 443 serves the portal over encrypted HTTPS, and port 22 lets staff manage the server over encrypted SSH. Ports 80 and 21 send data and passwords in cleartext, so they are denied.

  Ties to today: Writing allow and deny rules by port is exactly what the firewall lessons later in Unit 3 build on.

• Wrong
  Allow ports 80 and 21, and deny ports 443 and 22

  That is backwards. It blocks the two encrypted services and allows the two cleartext ones.

• Wrong
  Allow all four ports so nothing breaks

  Allowing everything leaves the cleartext services open. A rule set should allow only what the server needs.

• Wrong
  Deny all four ports to be as safe as possible

  Denying everything would lock out both the patients and the staff, so the portal would stop working.