Notes 01/13/2020

> Review from COEN10
# Pointers

	int x;
	x = 6;
	int *px;
	px = &x;

- The pointer `*px` is "pointing" to the memory address where x is located (`&x`)
- The pointer must be assigned a data type (in this case `int`) that matches the memory type that it is pointing to

```
	x = 5;
	*px = 1
	printf("%d", x);
```

- The code above will print `1` because `*px` (the value in the memory address that px is pointing to) was changed to 1, so x was changed to 1
- You can change where the pointer is pointing to: `px++` changes the pointer to point to the next memory address
	- This is useful when dealing with arrays:

```
	int x[5] = {1, 2, 3, 4, 5}
	int *px;
	px = &x[0]; //can also be written px = x
	printf("%d", *px);
	px++;
	printf("%d", *px);
```

The code above would print `1` and then `px++` would move the pointer to point to `x[1]` and the second number printed would be `2`


	int i;
	px = &x[0];
	for(i = 0; i < 5; i++){
		printf("%d\n", *px);
		px++;
	}


The code above would result in:


	1
	2
	3
	4
	5


Another Example:


	int x = 2, y = 2;
	int *p, *q;
	p = &y;
	q = &y;
	//(p == q) is false (comparison of addresses)
	//(*p == *q) is true (comparison of values)


Visual Representation of Memory:

	int x[7] = {1, 2, 3, 4, 5, 6, 7};
	int *px;
	px = &x[0];

`x[]` in memory

| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 
| - | - | - | - | - | - | - | 
| px |  |  |  |  |  |  | 

	px+=3;

| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 
| - | - | - | - | - | - | - | 
|  |  |  | px |  |  |  | 

---

# Using Pointers with Strings

Example:

	char my_string[] = "String!"
	char *p = my_string //Single line pointer definition. can also be written *p = &my_string[0]

Visual Representation of `my_string[]` in memory:

| S | T | R | I | N | G | ! | \0 |
| - | - | - | - | - | - | - | -  |
| p |   |   |   |   |   |   |    |

We can print the string like this:

	//don't need i
	while(*p != '\0'){
		printf("%c", *p);
		p++;
	}

The above code would print `STRING!`

---

# Pointers and Functions

- A function can also receive an array as an argument in the form of a pointer
	- For Example:

```
	void my_function(char *c){
		...
	}
```

- The function would receive a memory address pointing to a char. It would be beneficial to pass a memory address pointing to a string. 
	- Let's flesh-out the example:

```
	void my_function(char *c){
		printf("%s\n", *c);
		c++;
		printf("%s\n", *c);
	}
```

- Let's say we pass a memory address `&my_string[0]` and my_string is defined as `char my_string[] = "Hello World"`
	- The code would output:

```
	Hello World
	ello World
```

---

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