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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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