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# Talking to the computer
Arduino · Writing C++
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## Today
**We are learning to** serial.begin(9600) in setup() opens a line to the computer; Serial.println(level) sends a value and starts a new line, and Serial.print stays on the same line. The serial monitor shows what the board sees, which is how you test and debug a sketch. In C++, "words" + a number does not join them.
- I can print a value to the serial monitor.
- I can use print and println to build a line.
- I can use printing to find out what my sketch sees.
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## Why it matters
> Your sketch decides from a number you can't see. When the LED does something odd, how do you find out what the board actually read?
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💬 Discussion
## Predict
**The knob is halfway. loop() runs Serial.println(level); after level = analogRead(A0);. What does the serial monitor show?**
- level
- Nothing
- 512, on a new line each time
- Level: 512
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✍️ Activity
## Try it
{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fengineering%2Farduino%2Fserial%2Fstep-2&code=widget%3A+arduino%0Aknob%3A+A0%0Alevel%3A+50%0Aedit%3A+off%0Acode%3A+int+level+%3D+0%3B%0Acode%3A%0Acode%3A+void+setup%28%29+%7B%0Acode%3A+Serial.begin%289600%29%3B%0Acode%3A+%7D%0Acode%3A%0Acode%3A+void+loop%28%29+%7B%0Acode%3A+level+%3D+analogRead%28A0%29%3B%0Acode%3A+Serial.println%28level%29%3B%0Acode%3A+delay%28250%29%3B%0Acode%3A+%7D%0A&fresh=1}}
[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/engineering/arduino/serial/?full=step-2#step-2)
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✅ Check
## What happened?
* 512 (halfway on 0 to 1023), one line every time loop() runs. println prints the variable's value, not its name.
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💬 Discussion
## Build a line
Now pick the pair that does this.
$$\text{Serial.begin(9600);}$$
1) $\text{Serial.print("Level: ");}$ — Send the words. The cursor stays on this line.
2) $\text{Serial.println(level);}$ — Send the value of level (512), then a new line.
3) $\text{monitor: Level: 512}$ — One line per loop().
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✅ Check
## Build a line
**Which pair of lines prints Level: 512 on one line, then starts a new line?**
- Serial.println("Level: "); Serial.print(level);
- Serial.println("Level: "); Serial.println(level);
- Serial.print("Level: "); Serial.print(level);
- Serial.print("Level: "); Serial.println(level);
* Answer: Serial.print("Level: "); Serial.println(level);
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✅ Check
## Find the mistake
**Sam's monitor shows bits of Level: and then nonsense instead of Level: 512. Click the line where Sam's sketch goes wrong.**
```
1 int level = 0;
2
3 void setup() {
4 Serial.begin(9600);
5 }
6
7 void loop() {
8 level = analogRead(A0);
9 Serial.println("Level: " + level);
10 delay(250);
11 }
```
* Answer: line 9
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✍️ Activity
## Vary: change one thing
- **Add a label:**
- **Nothing printed:**
[Open this step](https://studio.learningwaypoints.com/tutorials/engineering/arduino/serial/?full=step-4#step-4)
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✍️ Activity
## Build it in Tinkercad
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✅ Check
## Print and println
**loop() is Serial.print(1); Serial.print(2); Serial.println(3);. What does the monitor show after two loops?**
- 123123 on one line
- 6, then 6
- 123 on one line, then 123 on the next
- 1, 2 and 3 on separate lines, twice
* Answer: 123 on one line, then 123 on the next
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✅ Check
## Debugging
**A night light comes on in daylight. What is the most useful first step?**
- Print the reading and compare it with the threshold
- Rewrite the whole sketch
- Add a longer delay
- Swap the LED
* Answer: Print the reading and compare it with the threshold
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✍️ Activity
## Make: print a percentage
[Open this step](https://studio.learningwaypoints.com/tutorials/engineering/arduino/serial/?full=step-7#step-7)
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✅ Check
## Exit ticket
**Write the two lines in loop() that print Temp: and then the value of temp on one line.**
* Serial.print("Temp: "); Serial.println(temp);