pxt-calliope/docs/lessons/loops.md

blocks - loops

An introduction to Loops for the Block Editor.

We may want to handle the user’s input multiple times or remain waiting for their input for a long time. We use loops to make sure that our code runs multiple times. These can be found in the Loops drawer.

Forever loops

In the Variables tutorial we utilised a forever loop to create a counter:

This allows our BBC micro:bit to wait for the user to do something forever, for example wait for the user to press the correct button as the example above shows. If you were creating a quiz, you may want to loop forever until the user presses the correct button or answers the question.

Repeat Loops

Repeat loops allow code to happen a certain amount of times. You may want to create a quiz that only gives the user a few tries to get the correct answer, for example. The number can be changed to facilitate your code.

The code above will scroll the message, “Hello world” three times.

While & Until loops

The ‘repeat while’ loop allows you to continue looping some code until a condition is met. The empty socket next to the while loop allows you to connect some Logic and construct a statement.

The code above will scroll the message, “Press it!”, while the user hasn’t pressed the button.

• Drag a set item block from the Variables drawer. Click the down arrow and click New Variable, and type "pressed". Drag a 0 block from Maths to set the variable pressed to 0.
• Drag a repeat while block from the Loops drawer and attach an = block from the Logic drawer. Drag item from the Variables drawer and click the down arrow, select ‘pressed’. Drag a 0 block from Maths and connect it to the other side of the equals. This will carry out the code until ‘pressed’ does not equal 0.
• Add a show string block from the Basic drawer and change the message to "Press it!"
• Add an if block from the Logic drawer, connect a button pressed block from the Input drawer, and add text from the Basic drawer. Change this to A to show we are waiting for button A.
• Inside the ‘do’ part of the if statement, add a set block from the Variables drawer, click the down arrow to change it to pressed and drag a 1 from the Maths drawer
• Lastly underneath the while loop, add another show string block and fill in the gaps.

Test the code above on actual hardware or on the simulator window.

We can also change the code in subtle ways to have a completely different effect:

This time we have to press the button three times to leave the while loop.

Tip

You can press the arrow next to a word in a block to change it. For example, you can change Math functions or change a Logic statement.

Count or for loops

A count loop allows you to loop a certain amount of times and to change a variable as you do so. For example, we can create a simple counting program:

The count loop will repeat a certain amount of times whilst changing a variable. You can click the arrow next to i to replace it with any of your own variables. So this program will display numbers 1 to 10.

This loop allows you to repeat code for the amount of times you want to without worrying about manually changing variables. You could use this for a counting program or a timer.