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 Is binary search always better than sequential search?
 Answer: It depends
 - How about if the sequence is not sorted?
+
+---
+
+[04/15 ->](04-15.md)
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+[\<- 04/13](04-13.md)
+
+# Abstract Data Type
+
+- An abstract data type is a data type whose **implementation has been hidden or abstracted away.**
+
+- We cannot see the implementation. Instead we have to use a set of functions called the **interface**
+
+- We say the implementation is kept private and the interface is public
+
+Encapsulating the **data** and the **operation** on the data, and then hide them from the user
+1. Declaration of data
+2. Declaration of operations
+3. Encapsulation of data and operations
+
+## Data Structure & ADT
+
+Data Structure:
+- Things to care - how to organize data in memory or disk
+- Examples: array; linked list;
+
+ADT:
+- Things to care
+	- what data
+	- what operations can be done on these data
+- Things not to care
+	- how to organize data in memory or disk (It can be done through any data structure)
+
+## Example ADT - FILE
+
+- We don't know what a FILE looks like
+- The file system are likely to change from machine to machine, e.g. MAC/Windows, 64 bit/32 bit
+- We don't want our program to have to change from machine to machine, so C hides the low level details (implementation) from us. We only use an interface as declared in `<stdio.h>`
+	- fopen, fscanf, ...
+
+## Implementing a SET Using Array
+
+Pre-knowledge
+1. Store string elements in an array
+2. Dynamically sized array
+3. Structure
+
+# Store String Elements in an Array
+
+## One String in C - Character Array
+
+Character Array
+
+|m|a|n|g|o|\0|
+|-|-|-|-|-|--|
+
+|b|a|n|a|n|a|\0|
+|-|-|-|-|-|-|--|
+
+Refer to the character array through a character pointer: `char *`
+
+## Assert
+
+- An **assertion** specifies that a program satisfies certain conditions at a particular points in its execution
+- Common uses
+	- Pointers are not `NULL`
+	- Indices and size values are non-negative and less than a known limit
+- Ex.
+	- p is a pointer: `assert(p != NULL);`
+- Make sure to test the condition **before** the usage of the variable
+
+## Strucutre in C
+
+### Concept
+- A structure is **a collection of one or more variables**, possible of different types, grouped together under a single name for convenient handling
+- e.g.
+
+```
+struct mystruct{
+	int a;
+	int b;
+	char c;
+};
+```