2.1 - Key Words Types

Keywords – Qualifiers

• const 

It specifies the value of a field or a local variable that cannot be modified.

• volatile 

- Instructs the compiler not to optimise the variable qualified with it

- Indicates that the variable is asynchronous, and system or external sources may change its value

Keywords – Loops

• for

- Could be used when the number of passes is known in advance (but not necessarily)

• while

- Could be used when the number of passes is not known in advance (but not necessarily)

- Entry controlled looping mechanism

• do

- Could be used when the number of passes is not known in advance (but not necessarily)

- Exit controlled looping mechanism

Keywords – Storage Class

auto -                                Storage:                           Memory

- Default Initial Value:                     Unpredictable

- Scope:                                      Local

- Life:                                                 Within the Block

register - Storage:                                       CPU Register's

- Default Initial Value:                    Garbage

- Scope:                                           Local

- Life:                                              Within the Block

static - Storage:                                       Memory

- Default Initial Value:                   Zero

- Scope:                                          Local

- Life:                                             Across Function Calls

- Other's:                                    Limits the Function scope to the current file.

extern - Storage:                                      Memory

- Default Initial Value:                 Zero

- scope:                                         Global

- Life:                                            Program Life

Keywords – Decision

• if 

- Simple conditional branching statement

- Any non-zero value is treated as a true value and will lead to execution of this statement

• else

- Used with if statements

- Else part is executed when the condition in if becomes false

• switch

- A specialised version of an if-else cascade

- Equality checks only with integral type constants

- Simplified control-flow by generating much faster code

- Break keyword is must to end the current case

• case

- Used in switch statements for selecting a particular case

- The case should be followed by a constant integral expression.

• default

- This label is used in switch statements

- The statements after this label will be executed only when there is no match found in the case labels.

Keyword - Jump

• goto

- Take the control to required place in the program

• break

- Force immediate termination of a loop, bypassing the conditional expression and any remaining code in the body of the loop

• continue

- Take the control to the beginning of the loop bypassing the statements inside the loop

Keywords – Derived types

• struct

- struct keyword provides support for aggregate types

• unions

- Can be considered a special case of structures 

- The syntax for both is mostly the same and only the semantics differ

- Memory is allocated such that it can accommodate the biggest member

- There is no in-built mechanism to know which union member is currently used to store the value

Keywords – User Defined

• typedef

- Do not create new types - they just add new type names

- Helpful in managing complex declarations

- Increase portability of the code

- Obey scoping rules and are not textual replacements (as opposed to #defines)

• enum

- A set of named constants that are internally represented as integers

- Makes the code more readable and self-documenting

Keywords – Others

• void

- Non-existent or empty set of values

- It is used in the case of void pointers as a generic pointer type in C

- Return type of a function to specify that the function returns nothing

- You cannot have objects of type void, and hence this type is sometimes called a pseudo-type.

• return

- To return control back from the called method

• sizeof

- Used to obtain the size of a type or an object

- Can be used for portable code, since the size of a data type may differ depending on the implementation.

 

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