Repeating Patterns with Loops

A dance can have lots of steps but only one short pattern. When you spot that pattern, you can give much shorter instructions.

Look for matching groups

An instruction tells you one step to do. A sequence puts steps in order. A repeating pattern has a group that comes again in the same order. Repetition means doing something again. Another name for it is iteration. In a program, a loop repeats instructions.

Try saying this dance aloud:

"Clap, clap, stomp, clap, clap, stomp, clap, clap, stomp, clap, clap, stomp."

Start with the smallest group that fits: "clap, clap, stomp." Mark each copy with a line:

"Clap, clap, stomp | clap, clap, stomp | clap, clap, stomp | clap, clap, stomp."

There are four matching groups. Each group has three actions. Adding the group sizes gives 12 actions:

The short instruction is "repeat four times: clap, clap, stomp." The repeat number counts groups. It does not count all 12 actions.

Keep the extra steps

Now try "stand up, hop, hop, hop, hop, hop, sit down." Only "hop" repeats. Say "stand up, repeat five times: hop, then sit down." You stand up and sit down once each.

A song's chorus can repeat while its beginning and ending happen once. A robot path can repeat "forward, turn right" four times. With equal forward moves and quarter turns, that path makes a square.

Check the last group too

Look at "jump, clap, jump, clap, jump, stomp." The first two groups match: "jump, clap." The ending is different. You can repeat "jump, clap" twice, then add "jump, stomp."

To find a repeat yourself:

1. Write or say all the steps in order.
2. Mark the smallest group that repeats exactly.
3. Count the copies of that whole group.
4. Leave any extra beginning or ending outside.

A repeat block wraps around the instructions you want to do again. Its shape helps you see which blocks belong together.

Follow one round

A repeat block often looks like the letter C. Blocks fit inside its gap. Its number tells the computer how many times to run those inside blocks. A loop with a set number is a count-controlled loop.

Read this program from the top:

say "Ready!" for 1 second
repeat 3
    play drum sound
    wait 1 second
say "Done!" for 1 second

The spaces show which blocks sit inside the loop. First, the sprite says "Ready!" once. Then it plays a drum and waits. That is one whole round. It goes back to the first inside block for round two. It plays and waits again for round three. After that, it says "Done!" once.

Inside, before and after

Drag the "Done!" block inside, below the wait, and the sprite says it three times. Its words stay the same, but its place changes what happens.

Count what happens

A loop with five rounds and a 10-step move travels 50 steps along its path:

You can also think "five groups of ten." Three rounds with two drum blocks inside make six drum sounds. The number on the repeat block counts rounds, not how many blocks fit inside.

Order matters too. repeat 2 with clap then jump gives "clap, jump, clap, jump." Swapping the inside blocks gives "jump, clap, jump, clap." The totals match, but the dance changes.

Put the whole group inside

To draw a square, the sprite must move and turn each round. If only the move is inside repeat 4, it moves four times along one straight path. A turn below that loop happens just once, at the end. Put the move and the turn inside together.

Copying blocks works, but a long stack can be hard to check. A loop lets you keep one copy of the repeating group.

Count blocks you can see

Drawing a square takes four copies of move 100 steps and turn right 90 degrees. That makes eight blocks in the long stack:

The loop version has one repeat block and two inside blocks:

repeat 4
    move 100 steps
    turn right 90 degrees

Count each visible block once. There are three blocks, even though the move and turn each run four times. The saving is five blocks:

For ten copies of "move 10 steps, play pop sound," the long stack needs 20 blocks. One repeat block and those two inside blocks need three. That saves 17 blocks. More copies mean more places where a loop can save work.

Change one place

Want a bigger square? In the long stack, change all four move blocks. In the loop, change the single move from 100 to 150 steps. Every side becomes longer. Want eight hops instead of four? Change the repeat number.

A shorter, clearer program is more efficient for the person building and reading it. Fewer copied blocks make mistakes easier to spot. One copied turn might say 80 degrees when the others say 90. A loop repeats the same turn each time. If that turn is wrong, you can fix it in one place.

Check that it still matches

Unroll a loop by writing every action in order. For repeat 3 around "forward, turn right," write:

"Forward, turn right, forward, turn right, forward, turn right."

Compare this with the long stack. Check the actions, their order and how many times they happen. A short program with only three sides still fails to draw a square.

A one-time action needs no repeat block. Wrapping it in repeat 1 adds a block without saving copies. A comment, such as "draws one square," can help a person understand a loop. The computer ignores that note.

You can make a sprite or turtle draw by giving it a pen. A repeated move and turn can draw a whole shape.

Get ready to draw

Clear old lines. Choose a start spot and a facing direction. Lower the pen with pen down. It draws as the sprite moves. Use pen up to move without leaving a line.

move 100 steps goes forward in the direction the sprite faces. turn right 90 degrees changes that direction on the spot. It draws no side by itself. A quarter turn is 90 degrees. A full turn, all the way around, is 360 degrees.

Build a square

pen down
repeat 4
    move 100 steps
    turn right 90 degrees

Follow the rounds with your finger. Round one draws a side and turns. Round two draws the next side and turns. Round three draws another side. Round four draws the last side and turns back to the starting direction. The sprite returns to its starting spot.

The four turns make one full turn:

Choose the turn for each shape

A regular shape has equal sides and equal corners. For one trip around its edge, repeat once per side. Share one full turn equally between those sides:

For a triangle, use repeat 3 with move 100 steps and turn right 120 degrees. Its three turns total 360 degrees.

The turn measures how far the sprite changes direction. The 60-degree corner inside an equal-sided triangle is different. Turning only 60 degrees three times draws half a hexagon, leaving an open path.

Change size or shape

A longer move makes each side longer. Keep the repeat number and turn the same to keep the shape. Change one number at a time and watch the result.

Changing a square to three repeats leaves one side missing. Changing it to eight repeats draws the same square twice. A tiny move repeated 36 times with a 10-degree turn makes 36 short sides. Together they look nearly round.

Left turns trace regular shapes the other way around. A five-pointed star uses five equal moves and 144-degree turns. Its turns total 720 degrees, or two full turns. Use the 360-degree rule for one trip around a regular shape's outside edge.

The core of a pattern is the part that repeats. Your loop holds that core. You can repeat lines, sounds or equal jumps on a number line.

Leave a gap after every dash

Start facing right. This loop makes a dashed line:

repeat 5
    pen down
    move 20 steps
    pen up
    move 20 steps

One round draws a 20-step dash, lifts the pen, then moves through a 20-step gap. Each round travels 40 steps. Five rounds make five dashes and travel 200 steps, including the final gap.

Put pen down inside the loop. If it sits only above the loop, the first dash appears. Then pen up leaves the pen raised for all the later rounds.

Repeat one stair

With the pen down and the sprite facing right, try:

repeat 4
    move 30 steps
    turn left 90 degrees
    move 30 steps
    turn right 90 degrees

One round goes right, turns upward, moves up, then faces right again. The opposite turns get the sprite ready for another stair. Four rounds take it 120 steps right and 120 steps up. The total path is longer because it includes both the across and upward moves.

Hear the core

A beat of "drum, drum, cymbal" has three sounds. Repeating that core four times gives eight drums and four cymbals. You hear 12 sounds altogether. Keep all three sound blocks inside and keep their order.

Jump along a number line

Start at 0. Repeat a jump of 3 five times. The landings are 3, 6, 9, 12 and 15. This is skip counting: adding the same number each time.

The jumps make five equal groups of three:

Changing the repeat number changes how many jumps you take. Changing the jump size changes the distance each time. A loop drawing two dots on each of four rounds gives four equal groups of two dots.

To build your own pattern, sketch or say it first. Circle the core and count its copies. Put the whole core inside a repeat block. Run it and compare what you see or hear with your plan.

A loop can hold another loop. The inside one finishes all its rounds every time the outside one reaches it.

Name the two loops

A nested loop is a loop inside another loop. The outside one is the outer loop. The one inside is the inner loop. Nested repetition is another name for this arrangement.

Read this from the outside inward:

repeat 3
    repeat 2
        clap
    stomp

The outer loop has three rounds. In each round, the inner loop claps twice. After that inner loop finishes, the sprite stomps once. Then the next outer round begins.

Trace one outer round per row

Count the claps as three groups of two:

That is also three times two. Adding the loop numbers would give five, but it misses a whole group. The inner loop starts again in every outer round.

The stomp runs three times. It sits inside the outer loop, after the inner loop. It does not belong to the inner loop. Six claps and three stomps make nine actions altogether.

Follow the block shapes

In a block editor, look at the two C-shaped gaps. In written code, look at the spaces before each line. The clap sits farther inward than the stomp.

Move the stomp inward beside the clap, and the inner group becomes "clap, stomp." That group runs twice per outer round. Across three outer rounds, you now get six claps and six stomps.

Decide whether nesting helps

Two outer rounds, each with three claps inside, give six claps. A single repeat 6 around clap gives the same action list. Nesting is useful when you want to show groups within groups, or do another action after each inner group. You could also write repeated actions directly inside one loop. Choose a version whose groups you can read and check.

Once you have a working shape, you can make copies with an outer loop. A move or turn between copies changes where the next shape goes.

Test one shape first

Build a square loop with four equal moves and four 90-degree turns. Lower the pen before drawing. Test that it returns to its start, facing its starting direction.

Turn squares into a flower

pen down
repeat 8
    repeat 4
        move 50 steps
        turn right 90 degrees
    turn right 45 degrees

The inner loop draws one square. After it finishes, the 45-degree turn points the sprite toward the next square. This extra turn belongs inside the outer loop, after the inner loop.

The eight copies share one starting point. Their extra turns make one full turn:

The move inside the square runs four times per square. Eight groups of four give 32 moves. The 90-degree turn also runs 32 times. The extra 45-degree turn runs only eight times.

If you remove that extra turn, every square follows the same lines. You see one square drawn over eight times. If you keep the 45-degree turn but make only six copies, the extra turns total 270 degrees. The design leaves part of the full turn unfinished.

Move copies into a row

Start facing right with the pen down:

repeat 3
    repeat 4
        move 40 steps
        turn right 90 degrees
    pen up
    move 60 steps
    pen down

Each square returns to its own start. The sprite then travels 60 steps right without drawing. Each square is 40 steps wide, so the next square starts after a 20-step gap. The sprite also makes that pen-up move after the last square.

For copies around one point, share 360 degrees between the copies. Six copies need a 60-degree turn between them. Four squares with a 90-degree turn between copies make a window-like design.

Count the saved blocks

The flower's drawing part uses five visible blocks: two repeats, one move and two turns. The long version uses eight square instructions per copy, plus an extra turn:

Including the starting pen down gives 73 blocks for the long version and six for the nested version.

Some jobs have a finish. Others keep running while your program is on. Choose a loop that matches what you want to happen.

A count or no count?

A forever loop is also called an infinite loop. It repeats the inside blocks without finishing by itself. You can stop the program with its stop button.

A windmill can keep spinning with this loop:

forever
    turn right 5 degrees

For exactly one full turn, use 72 repeats of that turn. The turns total 360 degrees. After its last turn, the count-controlled loop finishes.

Put starting steps first

A forever loop never reaches an ending. Blocks below it in the same script cannot start. Many block editors give it a flat bottom, so you cannot attach another block underneath.

If a fan should say "Starting!" before spinning, place that message above the forever loop. If it should spin a set number of times and then say "Finished!", use a count-controlled loop.

Using forever to draw a square keeps drawing over the same four sides. It never moves on to the next part of that script. Use four repeats when the drawing should end after one square.

Other scripts can still run

A script is a connected stack of instructions. A forever loop keeps its own stack busy. Another stack with its own start block can run at the same time. A game can keep music playing in one script while another script moves a character.

A planned forever loop is useful. It becomes a bug when endless repeating stops the program from doing the job you wanted.

A sprite can change pictures while it moves. Repeating those small changes can make a bird flap or a character walk.

Pictures become movement

Animation shows pictures one after another so something seems to move. A sprite's costumes are its different pictures. Give a bird one costume with wings up and another with wings down.

forever
    next costume
    wait 0.2 seconds

next costume switches to the next picture. After the last costume, it goes back to the first. The short wait lets you see each picture. The two bird pictures keep taking turns until you stop the program.

Wait inside the loop

A computer runs blocks very quickly. Costume changes with no pause can be too fast to follow. A wait above a loop pauses only before the animation. A wait below a fixed repeat pauses only after it. Put the wait inside so it happens after every change.

A longer wait slows the changes. A shorter wait speeds them up. Try changing only that wait and watch the difference.

Move in small steps

repeat 10
    move 10 steps
    wait 0.1 seconds

Ten small moves travel 100 steps altogether. The ten waits add up to one second, so the movement takes about a second. A single move 100 steps jumps there at once.

For a walking character, put a small move, next costume and a short wait inside the loop. Bigger moves cover more ground each round; smaller moves cover less. Keep the wait the same when comparing them. A built-in edge-bounce block inside the loop can make the character turn back at the screen edge.

Plan what happens

A storyboard is a few quick drawings of your animation. Sketch the start, a repeating part and an ending. Mark which pictures keep coming back.

Choose forever for a bird that flaps throughout the game. Choose a fixed repeat for a ball that rolls through ten small moves, then stops. A storyboard helps you choose the loop before you start dragging blocks.

You can read a program like a set of directions. Predicting means saying what it will do before you press start.

Trace from the top

Read any starting blocks first. Find the loop's count, or notice that it says forever. Trace the inside blocks once in order. Then count all the rounds and include any blocks after a fixed repeat.

Try this program:

say "Listen!" for 1 second
repeat 3
    play bell sound
repeat 5
    play woodblock sound

The loops are one after the other. The sprite says "Listen!" once, plays three bells, then plays five woodblocks. The second loop starts when the first finishes. There are eight sounds altogether.

Write every action when you need to

Unrolling makes the repeats visible. For repeat 2 around move 15 steps and say "Hello!", write:

"Move 15, say Hello!, move 15, say Hello!"

Now count two moves and two messages. The total distance traveled is 30 steps. Keep an action's place in the list; all the moves do not happen before all the messages.

For nested loops, write one outer round per row. If each row has three moves and one turn, two outer rounds give six moves and two turns. Multiply the move count by the length of each move to find the distance along the whole path.

Predict a change too

Changing a square's repeat number from four to eight keeps the move and turn the same. Eight quarter turns make two full turns. The sprite draws the same square twice. The final picture can hide the extra work.

Write a prediction before running. Then watch the actions, as well as the finished picture. If they differ from your prediction, check both the code and your counting. Either one may need fixing.

A bug makes your program do something you did not want. Debugging means finding and fixing it. Watching carefully gives you clues about which block to change.

Compare the plan and the result

First say what should happen. Run the program and describe what really happens. Look for the first point where those two stories split.

A loop meant to draw a hexagon says:

repeat 5
    move 60 steps
    turn right 60 degrees

It draws five sides and leaves a gap. The turn is right for a hexagon, but there should be six sides. Change only repeat 5 to repeat 6. The six turns total 360 degrees. Run it again to check that the path closes.

Match the clue to the block

Old lines can also hide your new drawing. Clear the screen at the start, then place the sprite at its starting spot and face it the starting way.

Slow the loop down

Add wait 0.5 seconds inside a loop while testing. Count rounds aloud as you watch. You may spot a turn happening once at the end when it should happen each round. Remove the extra testing wait when you have found the bug.

Test one change at a time

Change one block or number, then run again. Changing the count, turn and move all at once makes it hard to tell which change helped.

After the repair works, try another starting spot with enough room. Try a bigger shape too. Reset the start and clear old marks for each test. Check the side lengths, turns and ending, so the fix works beyond your first run.