### Things that can be learned from this exercise
- `serialize_word` call takes 5 steps
- `serialize_word` adds the passed felt to the output pointer
- Said command is useful when debugging

### Solution

```cairo
// Use the output builtin.
%builtins output

// Import the serialize_word() function.
from starkware.cairo.common.serialize import serialize_word

func main{output_ptr: felt*}() {
    // Output 100 by calling serialize_word.
    serialize_word(100);
    // Output 200.
    serialize_word(200);
    // Output 300.
    serialize_word(300);
    // Output 400.
    serialize_word(400);
    // Return.
    return ();
}
```

### Output
First run
```
Program output:
  100
  200
  300

Number of steps: 17
Program hash: 0x06dfe1be49c593f8b693c70a6e9ce50ea84272d76360a46dfb26d6b536d94b77
```

Second run
```
Program output:
  100
  200
  300
  400

Number of steps: 22
Program hash: 0x0542e6604d143454ef2a63b5e6762616c9999e2224bc7d6098a1b9617fa9dd72
```

Can you help how to change the code so that it outputs 400 as well?

```
// The following code outputs the numbers 100, 200, 300.
// 1. Click on "Run" to see the output.
// 2. Change it to output 400 as well.

// Use the output builtin.
%builtins output

// Import the serialize_word() function.
from starkware.cairo.common.serialize import serialize_word

func main{output_ptr: felt*}() {
// Output 100 by calling serialize_word.
serialize_word(100);
// Output 200.
serialize_word(200);
// Output 300.
serialize_word(300);
// Return.
return ();
}
```

---

> Answers to this question are a part of the ✨ **17 days of Cairo Lang with Playground & Newton.** ✨ 

*Vote for your favorite answer - the best answer will win a $10 award. A new day – a new reward! During the next 17 days, our goal is to attract more developers to the Cairo language and to systematize the knowledge of Cairo lang. [Read rules](https://www.notion.so/17-days-of-Cairo-Lang-with-Playground-Newton-550dfcda6a2d449ca468be293c776578)*

How to make output like "hello world" in Cairo Language (StarkNet)?

We can see by the name of the function (`add1`), the `assert` in the `main` function and the signature `add1(y=x)` in `add1_square` that we need to return `y + 1`.

Therefore, an implementation that would satisfy the assert in the main function would be:
```
func add1(y: felt) -> (y: felt) {
    return (y=y + 1);
}
```


Hello! This is Day 2 of the 17 days of the Cairo challenge. And I have no idea how to solve the [playground](https://www.cairo-lang.org/playground/) exercise “Functions”. Perhaps you can help me?

```python
// The following code is missing the function `add1`.
// 1. Click on "Run" to see the error.
// 2. Write the function `add1`, based on `add1_square`, so
//    that the program terminates successfully.
//
// Note that `add1` doesn't have to call other functions.
//
// You can learn more about functions [here](https://www.cairo-lang.org/docs/how_cairo_works/functions.html).

// Write your code here.

func add1_square(x: felt) -> (x: felt) {
    // Call `add1` and unpack the result into `z`.
    let (z) = add1(y=x);
    return (x=z * z);
}

func main() {
    // Call `add1_square` and save the result into `res`.
    let (res) = add1_square(x=12);

    // Verify the computation.
    assert res = (12 + 1) * (12 + 1);
    return ();
}
```

---
> Answers to this question are a part of the ✨ **17 days of Cairo Lang with Playground & Newton.** ✨ 

*Vote for your favorite answer - the best answer will win a $10 award. A new day – a new reward! During the next 17 days, our goal is to attract more developers to the Cairo language and to systematize the knowledge of Cairo lang. [Read rules](https://www.notion.so/17-days-of-Cairo-Lang-with-Newton-550dfcda6a2d449ca468be293c776578)*

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