51 lines
3.6 KiB
Markdown
51 lines
3.6 KiB
Markdown
# blocks - if statements
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An introduction to conditions for the Block Editor.
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## Introduction to conditions
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In the introduction to code, we made the BBC micro:bit automatically shows the message ‘hello, world!’:
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This statement, or code, will happen as soon as the BBC micro:bit is activated. This means it is unconditional. We can add a condition to make code function in certain ways:
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* A calculator waits for the user in input numbers and a function, before outputting a result
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* A game waits for the user to press a button at the right time before outputting their score
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* A quiz waits for the user to choose the correct option, and if they are wrong the quiz will tell the user
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In programming we use an ‘if’ statement: if this condition is met, do something. Lets add an if statement to the code we had before; the BBC Micro:bit will wait for the user to press a button before showing the image.
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### Write the code
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Click the **if** category and drag an `if/do` block. Drag the`show string` block we wrote previously into the `do` section of the block. Next click the **input** tab and drag a `button pressed` block, connect it to the open jigsaw of the `if` block. This is our criteria: `if A button is pressed`. We can change which button (button A or B) by clicking the arrow next to ‘A’ and changing the value. This means our BBC micro:bit is waiting for button A (the left button) to be pressed. Finally go to the **basic** tab and drag a `forever` block, and attach all our code inside. We add this block to ensure the BBC micro:bit is always waiting to show us this message, not just once. Your code should look like this:
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Again, test the code in the simulator. Try clicking **Button A** to display the "hello, world!" message every time the `button is pressed`.
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### More 'if' statements
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You could now add additional conditions to your 'if statement'. Here are some ideas:
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* Change the 'get button' to ask for button B
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* Add another 'if' statement within the current one, and make it so both buttons must be pressed to show the message
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* Create a mini quiz that asks for one button to be pressed which represents an answer
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## Else
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What if the user does not press a button? What if the user presses the wrong button? We call this an else, because if the criteria of the if statement are not met then something else is done.
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For example, we could make it so our BBC Micro:bit tells us to press the A button. Remove the `button pressed` and `show string` blocks from the `if` block and right click it and select **Delete**. Now click the **If** category and drag out an `else if` block. Plug the `button pressed` and `show string` blocks in the correct places.
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We want the message "Press A!" to scroll across the BBC micro:bit, so right-click the `show string` block and select **Duplicate**. Drag this new block into the `else` section and replace the “hello, world!” with "Press A!". Your code should look like this:
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So, to recap: the `forever` block makes sure our code runs forever. The BBC micro:bit checks if the user is pressing the left button, if the user is not then the “Press the button!” message will scroll across the LEDs. If the user is pressing the button then the “hello, world!” message will scroll across the screen. Check this in the simulator or attach the BBC micro:bit to the computer then click **compile** to send the code onto the BBC micro:bit.
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## What is a condition?
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A condition is criteria that the user must meet for a certain function to be carried out.
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