CS 280, Lesson 11 and 12: Data types

data type definition

collection of data objects and a set of predefined operations on those objects;
stored value (instructions and data) in memory are untyped, as hardware on most machines makes no attempt to keep track of which interpretations correspond to which locations in memory

type checking

activity of ensuring that the operands of an operator are of compatible types

type error

the application of an operator to an operand of an inappropriate type

strongly typed programming language

type errors are always detected

type system of a programming language

Defines how a type is associated with each expression in the language and includes its rules for type equivalence and type compatibility

descriptor, and difference between descriptor of static and dynamic variables

collection of the attributes of a variable that are used for type checking and performing allocation and deallocation operations

structured vs scalar data types

scalar data types are those that include a single value, like int, bool, double, etc.
structured data types are those which include composite values, like arrays.

primitive data types

data types that cannot be represented in terms of other data types

what does sizezof operator do in C/C++

it gives the size of the variable or data type in bytes

integer primitive type

1. what are they in non-decreasing order?

2. what is the fixed size of byte? short? int? long? in java*

3. signed vs unsigned integers, qu’est-ce que c’est?

4. can an integer be declared as unsigned

1. char, short, int, long, long long

2. 1,2,4,8

3. one of the bits in the integer is represented as a sign bit

4. yes

what is two’s complement rule?

hardware representation of signed integers in binary: take the COMPLEMENT of the binary rep of the number, and then add one.

floating point primitive type

1. what is the c++ data types of this, in non-decreasing order of size

2. defined in terms of ____ and ____.

3. what is the formula to convert a 32-bit binary string into a decimal number?

4. are they always like hardware?

1. float, double, long double

2. precision and range

3. value = sign x mantissa x 2^{exp}, where sign is a plus or minus, the 2^{exp} has exp = 8 bits after sign - 127; manitssa is the rest of the digits where you concat a “1.” at start, and then for each binary digit starting with the leftmost one (after exponent), you multiply that digit by (0.5), (0.25), (0.125), and onwards.

4. usually, but not always.

boolean primitive type

1. is it implemented as bits or bytes?

2. what are allowable ops?

1. can be implemented as bits, but usually bytes

2. the logical !, ||, &&, and relational operations

char primitive type

1. what are they stored as? (braindead) what is the most commonly used one?

2. size of char in C++ vs java

3. what is the c++ data type for unicode characters

1. numbers (numeric codings); ASCII, or Extended ASCII (128, 256 chars respectively)

2. 1 byte and 2 bytes

3. wchar_t

character string data type

1. is this primitive in C and C++?

2. is it primitive in Python?

3. how is this different than Strings in java?

4. which 3 kinds of strings are of static length?

5. which languages can implemented strings as limited dynamic and how?

6. what does the compile time descriptor look like for static strings

7. what does the run time descriptor look like for limited dynamic strings?

1. no

2. ja

3. it is not. Java string class is just character strings.

4. java’s string class, c++ standard string, and python string

5. C and C++ with the string-end being a unique character

6. static string ; length ; address

7. limited dynamic string; max length; curr length; address

enums

1. what are rules for constant appearance in declaration

2. can you set integer to enum constant? int x = Mon;

3. can you set enum constant to any datatype? how do you do it legally?

4. what is the only way to do incrementation / decrementation?

5. enums vs scoped enums in terms of what you’re allowed to do with adding consts and assigning ints

1. it can only appear once

2. ja

3. no. only way to do it legally: const = (enumName) 4; (4 is an arbitrary int)

4. const = (enumName) (const + 1);

5. enums don’t care about adding consts, they treat it as integers, but they don’t let you IMPLICITLY set consts to random nums but you can do so EXPLICITLY. scoped enums don’t let you add consts, and they work the same for setting consts to random ints.

pointers

1. what are the two possible values?

2. what is size?

3. what is indirect accessing?

4. what are dangling pointers? remember code example.

5. what is a lost heap-dynamic variable? what is another name for that variable?

6. pointer arithmetic formula?

7. what does doing ptr++ or ++ptr , ptr—, —ptr do?

8. what happens when you subtract two pointers?

9. can you add, multiply, or divide pointers?

10. can you do math on void*?

11. consider some index i inside an array arr . explain relationship with pointer.

12. what are void pointers and their rules?

13. is this legal or not, and why: int* p = malloc(sizeof(int)); ?

1. any memory address or NULL

2. 8 bytes

3. accessing a variable in two steps, first by using a pointer that gives the location of var in memory

4. pointer that points to heap dynamic variable which has been deallocated.

5. it’s a dynamic variable that is no longer being pointed to, meaning its inaccessible and GARBAGE.

6. new address = old_address + n*(sizeof(type))

7. move pointer to next element in memory or prev

8. only allowed when both point to same array, and it gives the number of elements between the two pointers

9. no. never.

10. no.

11. arr[i] = *(arr + i) = *(i + arr) + i[arr]

12. they can point to literally any type of variable. however, you cannot deference them or do ANY kind of pointer arithmetic. you must cast them into a normal pointer to deference and shit.

13. ts NOT legal in C++, but legal in C. since malloc memory is of void, you gotta cast the RHS to (int*)

can you have a reference of a reference? can you have a pointer to a reference?

no, no.

lvalue vs rvalue references

lvalue

• declared by placing a & a type

• synonym for the object it references

• cannot set equal to literal made strings “hello” or substrings

rvalue

• declared by placing && a type

• same as lvalue, but can set to literal temp strings and substrings like string && x = “hello”;

references are references to ____ instead of being ______.

objects, addresses.

pointers widen what?

range of references to memory cells that can be accessed by a variable

what data types can be used as indices for arrays

char, bool, short, int, long

what is unique about java arrays (& variables declared as arrays in Java)

they are objects. such variables are references to the array objects and have memory allocated to them. we always know how many items are in them.