Algorithms and Loops

01What is an algorithm?

An algorithm (say it “AL-guh-rith-um”) is a list of steps that solves a problem or gets a job done. If you follow the steps in order, you get the result, every time.

Algorithms are everywhere, not just inside computers:

• A recipe is an algorithm for pancakes.
• Directions (“turn left at the library, then go two blocks”) are an algorithm for getting somewhere.
• Long division is an algorithm for dividing big numbers. You learned it in school!
• Searching for a word in a dictionary is an algorithm, too: open near the middle, check if your word comes before or after, and keep going.

A program is an algorithm written in a language a computer can follow. The person who writes it is a programmer. That’s you, by the end of this page.

02Sequence: order matters

Sequence means the order of the steps. A computer always runs steps in the exact order you wrote them, from top to bottom. Swap two steps and you can get a very different result.

Same three steps, different order, and now your socks are on top of your shoes. When a program does something strange, one of the first things to check is the order of the steps.

03Be precise

Ask a friend to “go to the door” and they’ll figure it out. A computer can’t fill in gaps. It does exactly what you say, nothing more and nothing less. So instructions for a computer have to be precise: clear, exact, and impossible to misread.

“Turn” isn’t precise. Turn which way? How far? “Turn right 90 degrees” is. That’s why the robot in our game only knows three moves, each one exact:

forward

Move one square in the direction you’re facing. If a wall is in the way, bonk: the program stops.

turn left

Spin a quarter turn to the left, staying on the same square.

turn right

Spin a quarter turn to the right, staying on the same square.

Here’s the tricky part: “left” and “right” are from the robot’s point of view, not yours. If the robot is facing down the screen, its left is your right. Try standing up and turning with it. Programmers really do this!

04Loops: do it again

Say you want the robot to walk around a square, two steps on each side. Written out in full, that’s 12 blocks:

A loop tells the computer to run some steps more than once. The repeat 4 block means “do everything inside me 4 times.” The steps inside are called the loop body. Each time through is called an iteration.

Why loops save work

• Less to write. Walking 100 squares forward takes 100 blocks without a loop, or 2 blocks with one: repeat 100 { forward }.
• Fewer mistakes. The fewer blocks you write, the fewer places a typo can hide.
• Easy to change. Want a bigger square? Change one number instead of rewriting a dozen lines.

Loops inside loops

You can put a loop inside another loop. This is called nesting. Look at the square again: each side is “forward, forward,” which is itself a pattern. So the square can be repeat 4 { repeat 2 { forward } turn right }. The inner loop runs 2 times for every 1 time the outer loop runs, so forward happens 4 × 2 = 8 times.

05Spotting patterns

The real skill with loops is noticing when something repeats. Programmers call this pattern recognition. Here’s a way to do it:

1. Write out the steps the long way first, or say them out loud.
2. Look for a chunk that shows up again and again. That chunk is your loop body.
3. Count how many times the chunk repeats. That’s your number.
4. Check the leftovers. Steps before or after the pattern go outside the loop.

For example, climbing a staircase is “forward, turn left, forward, turn right” for every step. Four steps? repeat 4 { forward, turn left, forward, turn right }. That’s 5 blocks instead of 16.

06A first look at “if”

So far, our robot follows its program blindly. But what if it could check something first? That’s a conditional: a step that only happens if something is true.

1. if there is a wall ahead {
2. turn right
3. } otherwise {
4. forward
5. }

You use conditionals all the time: if it’s raining, take an umbrella. Put that “if” inside a loop and the robot can find its way through a maze it has never seen. Loop Bot doesn’t have “if” blocks yet. You’ll get them in the next Code Club unit, Conditionals.

07Debugging and the famous moth “bug”

A bug is a mistake in a program that makes it do the wrong thing. Debugging means finding and fixing it. Every programmer, even the best, spends a lot of time debugging. Here’s a good method:

1. Run it and watch what actually happens.
2. Compare that with what you wanted. Where did it first go wrong?
3. Step through one block at a time to find the exact block. (Loop Bot’s Step button does this.)
4. Fix one thing, then test again.

The moth in the machine

On September 9, 1947, a team at Harvard University was testing a huge computer called the Harvard Mark II. It didn’t use chips. It used relays, little electric switches that clicked open and shut.

When the machine had trouble, the team went looking. Stuck inside Relay #70 on Panel F, they found a real moth. They pulled it out and taped it into the machine’s logbook with a note. The entry, at 15:45 (3:45 in the afternoon), reads:

“Relay #70 Panel F (moth) in relay.
First actual case of bug being found.”

See the joke? It says “first actual case” of a bug. The team already called machine problems “bugs.” This time, for once, the bug was a real insect.

Where does Grace Hopper fit in?

Grace Hopper was a mathematician and U.S. Navy officer who worked on the Harvard Mark I and Mark II computers. She went on to become a well-known computer pioneer: she built one of the first compilers (programs that translate human-friendly code into machine code) and helped create the programming language COBOL. She retired from the Navy as a rear admiral at age 79.

The moth story is often told as “Grace Hopper found the first bug,” but that’s not quite right:

• She didn’t find the moth herself. The Smithsonian says engineers on the Mark II team found it, and that the logbook was probably not Hopper’s. She loved telling the story, though, and she helped make the words “bug” and “debug” famous.
• She didn’t invent the word “bug.” Engineers had called small flaws in machines “bugs” since the 1800s. The inventor Thomas Edison wrote about “bugs” in his inventions in letters in 1878.

09Find the bug

Each program below has exactly one buggy line. Read the goal, trace the program in your head (or with your finger), and click the line you think is wrong.

11Talk About It

There are no single right answers here. Talk with a friend, your class, or a grown-up.

• Write the algorithm for something you do every morning. Could someone else follow it exactly? Where might they get confused?
• In Loop Bot, was the shortest program always the easiest one to understand? When might a longer program be better?
• Where do you see loops in real life? Think about music, sports practice, or the seasons.
• The moth story is often told wrong: people say Grace Hopper found it, or that it’s where the word “bug” came from. Why do you think simple versions of stories spread more easily than the true ones?
• Al-Khwarizmi wrote his books about 1,200 years ago, long before computers. Why do you think his name ended up in a computer word?

12Sources and credits

1. Smithsonian, National Museum of American History, “Log Book With Computer Bug” (object nmah_334663, ID 1994.0191.01): describes the 1947 Mark II moth, the “first actual case of bug being found” label, that the log book was probably not Hopper’s, and Edison’s use of “bug” in the 1870s. americanhistory.si.edu: object nmah_334663
2. U.S. Naval History and Heritage Command, photograph NH 96566-KN, “The First ‘Computer Bug’”: moth found in Relay #70, Panel F, of the Mark II Aiken Relay Calculator during testing at Harvard, 9 September 1947. history.navy.mil: NH 96566-KN
3. Peggy Aldrich Kidwell, “Stalking the Elusive Computer Bug,” IEEE Annals of the History of Computing 20, no. 4 (1998): 5–9. Dates the logbook page to 9 September 1947 and notes the “1545” time and Hopper’s 1981 recollection of 1945. doi.org/10.1109/85.728224
4. U.S. Naval History and Heritage Command, “Rear Admiral Grace Hopper” (namesake biography): Navy service, work on the Mark I and Mark II at Harvard, compilers, COBOL, retirement in 1986 at age 79. history.navy.mil: Grace Hopper
5. Smithsonian, National Museum of American History, “Grace Hopper: The Navy and Computers.” americanhistory.si.edu: Grace Hopper
6. J. J. O’Connor and E. F. Robertson, “Al-Khwarizmi,” MacTutor History of Mathematics, University of St Andrews: the House of Wisdom, Algoritmi de numero Indorum, and the origins of “algorithm” and “algebra.” mathshistory.st-andrews.ac.uk: Al-Khwarizmi
7. Alexander B. Magoun and Paul Israel, “Did You Know? Edison Coined the Term ‘Bug’,” IEEE Spectrum, 1 August 2013: Edison’s 1878 letters about “bugs.” (Despite the headline, the word was already shop slang; see source 1.) spectrum.ieee.org: Edison and “bug”

Credits. Written for jugami.com by Grace, the Jugami Code Club teacher. The robot, game boards, logbook drawing, favicon and share images are original art made in code (hand-written SVG) for this page; the logbook drawing is our illustration, not a copy of the real page. Loop Bot runs only in your browser. It saves your best stars in this browser’s local storage and sends nothing anywhere.