CS1010 AY20/21 S1 Lecture 7

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# Lecture 7

### 4 October 2021

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Admin Matters 
Unit 15: **Pointer** 
Unit 16: **Call-by-Reference** 
Unit 17: **Heap** 
Unit 18: **String** 
]
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# 💪

**121** quiz questions 
**46** programming problems 
**24** problem set questions

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### Assignment 4 and Quiz 7

- Due Wednesday night (23:59)

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### Assignment 5 onwards

- Write per-function documentation for your code

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### Tutorial 6

- Problem Sets 15-18

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### Lab 6

- Exercise on call-by-reference and dynamically allocated arrays.

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background-image: url(figures/meme/explain_pointer.jpg)

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### Variable

- a place in memory that holds one or more values
- we can access a variable through its name or its memory address

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### Address-of operator `&`

- if `x` is the name of a variable, `&x` is its memory address.
- if `x` as the type `T`, `&x` has the type `*T` (pointer to `T` / address of `T`)

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### Dereference operator `*`

- if `x` is a memory address, `*x` is the content at memory address `x`.
- if `x` as the type `*T`, `*x` has the type `T`

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### Pointers must be of the same type

```C
double p = 3.1415926;
long radius = 5;
double *addr;
addr = &p; // ok
addr = &radius; // not ok
```

(`void *` is special)

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### We cannot change the address of a variable

```C
long x = 1;
long y = 2;
&x = &y;
```

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### Pointer arithmetic

```C
long x = 1;
long *ptr = &x;
ptr += 1;
```

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`a[i]` is just another notation for `*(a + i)`

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### Pointers are variables too!

```C
long x;
long *ptr = &x;
long **ptrptr = &ptr;
```

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### The `NULL` Pointer

A special pointer value that says a pointer points to "nothing".

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## Call by Reference

(non-array variables)

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## Swap

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```C
void swap(long *a, long *b) {
  long temp = *a;
  *a = *b;
  *b = temp;
}

int main() {
  long a = 10;
  long b = -4;
  swap(&a, &b);
}
```
]
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## Heap

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- Heap: another region of memory that a program can utilize
- Program explicitly "borrows" memory space for use
- Program must "return" the memory space after use
- Lifetime of content stored on heap can exceed the lifetime of the
  function that allocates it

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| Size Known in Advanced | Size Determined During Run-time 
-------|-----------------------|-----------------------------------
Stack  | Fixed-size Array      | VLA (stay away)      
Heap   | stay away             | `malloc` / `calloc`

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## Characters and Strings

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### `char` literals

`'A'`, `'a'`, `'4'`, `'0'`, `'*'`, `'\n'`, `'\\'`

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A literal is a constant value in C program that cannot be changed (as opposed to a variable).
]

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```C
'A' + 1 == 'B'
```

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### String

- An array of `char`
- Terminated with `'\0'`

.small[
```C
char str[5] = { 'a', 'b', 'c', 'd', '\0' };
```
is the same as
```C
char str[5] = "abcd";
```
]

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### String Literals

Stored in read-only region in memory

```C
char *str1 = "Hello";
char str2[5] = "Hello";
```

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### Important Rules

- Do not access memory space you are not allowed to.
- Always return memory space that you borrow.

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### Homework

- Problem Set 15-18
- Assignment 4
- Quiz 7
- Exercise 8

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Version: 1.1

Last Updated: Mon Oct  4 17:08:27 +08 2021
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