Stack Block

The general shape of a Stack block.

A stack block is a rectangular block that is shaped to fit above and below other blocks. Stack blocks make up the majority of the blocks available in Scratch, being available in every non-extension category except operators.

Execution Process

When 2 stack blocks are connected to form a script their commands will execute in the order from top to bottom. The entire collective stack executes in a single frame. For example, take the following script:

reset timer
move (5) steps
move (4) steps
move (3) steps
move (2) steps
move (1) steps
set [elapsed time v] to (timer)

The entire execution process for this stack of stack blocks will take approximately 0 seconds. Opposed to visualizing the sprite move 5 steps, then 4, etc. the entire motion will be seen in one unified step; the end user will witness what appears to be the sprite move 15 steps, even though it has 5 separate blocks making up the motion. The elapsed time variable will be 0 when the script finishes executing.

Note:

Extremely long and intensive scripts could take a long time to process and may actually have a notable enough elapsed time. Do not rely on this, as it depends on the computer's speed.

Take the following code, now. While it appears to be the same, just using a loop, it actually is not all executed in a single frame this time.

reset timer
set [step move v] to (5)
repeat (5)
move (step move) steps
change [step move v] by (-1)
end
set [elapsed time v] to (timer)

This scenario should have 5 separate motions and should take up 5 frames of the 30-frame-per-second Scratch project. It should take approximately 0.166 seconds to execute. Initially, the sprite moves 5 steps. Then there is a pause because of how the repeat loop works. Then, the sprite moves 4 steps, and so forth. The only method of using a repeat loop without the delay between each cycle is to place the repeat loop inside a custom block that has the "run without screen refresh" option enabled.

Stack Blocks With Delay

Some stack blocks do execute with a delay, meaning there may be a pause between it and the next block executing. The following blocks have this behavior:

• switch backdrop to ( v) and wait
• play sound ( v) until done
• wait () seconds
• wait until <>
• ask () and wait

Blocks

Note:

Click on a block for more information.

Motion

There are 15 Motion stack blocks.

• move () steps
• turn cw () degrees
• turn ccw () degrees
• go to ( v)
• go to x: () y: ()
• glide () secs to ( v)
• glide () secs to x: () y: ()
• point in direction ()
• point towards ( v)
• change x by ()
• set x to ()
• change y by ()
• set y to ()
• if on edge, bounce
• set rotation style [ v]

Looks

There are 17 Looks stack blocks.

• say [] for () seconds
• say []
• think [] for () seconds
• think []
• switch costume to ( v)
• switch backdrop to ( v)
• switch backdrop to ( v) and wait
• next costume
• next backdrop
• change size by ()
• set size to ()%
• change [ v] effect by ()
• set [ v] effect to ()
• clear graphic effects
• show
• hide
• go to [ v] layer
• go [ v] () layers

Sound

There are eight Sound stack blocks.

• play sound ( v) until done
• start sound ( v)
• stop all sounds
• change [ v] effect by ()::sound
• set [ v] effect to ()::sound
• clear sound effects
• change volume by ()
• set volume to () %

Events

There are two Events stack blocks.

• broadcast ( v)
• broadcast ( v) and wait

Control

There are four Control stack blocks.

• wait () seconds
• wait until <>
• stop [other scripts in sprite v] // If "all" or "this script" is selected in the drop-down menu, this block becomes a Cap block.
• create clone of [ v]

Sensing

There are three Sensing stack blocks.

• ask [] and wait
• set drag mode [ v]
• reset timer

Variables

There are four Variables stack blocks.

• set [ v] to ()
• change [ v] by ()
• show variable [ v]
• hide variable [ v]

List

There are seven List stack blocks.

• add [] to [ v]
• delete () of [ v]
• delete all of [ v]
• insert [] at () of [ v]
• replace item () of [ v] with ()
• show list [ v]
• hide list [ v]

My Blocks

The My Blocks area allows the user to make their own stack blocks.

• custom block:: custom

Music Extension

There are six stack blocks in the Music Extension.

• play drum ( v) for () beats
• rest for () beats
• play note () for () beats
• set instrument to ( v)
• set tempo to ()
• change tempo by ()

Pen Extension

There are nine blocks in the Pen Extension, and all of them are stack blocks. This is the most stack blocks for any extension.

• erase all
• stamp
• pen up
• pen down
• set pen color to ()
• change pen ( v) by ()
• set pen ( v) to ()
• change pen size by ()
• set pen size to ()

Video Sensing Extension

There are two stack blocks in the Video Sensing Extension.

• turn video ( v)
• set video transparency to ()

Face Sensing Extension

There are three stack blocks in the Face Sensing Extension.

• go to [nose v]:: extension
• point in direction of face tilt:: extension
• set size to face size:: extension

Text to Speech Extension

There are three stack blocks in the Text to Speech Extension.

• speak ()
• set voice to ( v)
• set language to ( v)

Micro:bit Extension

There are three stack blocks in the micro:bit Extension.

• display ( v):: microbit
• display text ()
• clear display

LEGO MINDSTORMS EV3 Extension

There are four stack blocks in the LEGO MINDSTORMS EV3 Extension.

• motor ( v) turn this way for () seconds:: ev3
• motor ( v) turn that way for () seconds:: ev3
• motor ( v) set power ()%
• beep note () for () secs

LEGO BOOST Extension

There are seven stack blocks in the LEGO BOOST Extension.

• turn motor ( v) for () seconds
• turn motor ( v) for () rotations
• turn motor ( v) on
• turn motor ( v) off
• set motor ( v) speed to ()%
• set motor ( v) direction ( v)
• set light color to ()::boost

LEGO Education WeDo 2.0

There are six stack blocks in the LEGO Education WeDo 2.0 Extension.

• turn ( v) on for () seconds
• turn ( v) on
• turn ( v) off
• set ( v) power to ()
• set ( v) direction to ( v)
• set light color to ():: wedo

Shape

Stack blocks are fitted with a puzzle-piece like shape; the top has a notch and the bottom has a bump. Because of this shape, scripts can stretch on and on — the block tessellates.

Their shape allows them to be placed in the following areas:

• After Hat blocks

when gf clicked
say [Hi.] for (2) seconds

• Before/after other Stack blocks

go to (Sprite2 v)
play sound (meow v) until done
point towards (Sprite3 v)

• Before Cap blocks

say [Bye.] for (2) seconds
stop [this script v]

• Inside C blocks

forever
if <(loudness) > (30)> then
say [No noises.] for (2) seconds

Use

As Stack blocks are shaped to allow blocks above and below them, they are used almost everywhere in a script; most scripts have a Stack block in them. An example script:

when flag clicked
repeat until <(do_Stop) = [1]>
  move (10) steps
  change [color v] effect by (25)
  play sound (meow v) until done
  if <touching (edge v)?> then
    say [Done!] for (2) seconds
    stop [this script v]
  end
end

Note how the Stack blocks are used in the script — they make up all the commands.