432 lines
13 KiB
Markdown
432 lines
13 KiB
Markdown
---
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title: C17
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TARGET DECK: Obsidian::STEM
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FILE TAGS: c17
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tags:
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- c17
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---
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## Overview
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> A C program can be seen as a sort of machine that manipulates values: the particular values that variables of the program have at a given time, and also intermediate values that are the result of computed expressions.
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This quote describes C's **abstract state machine**. Whatever instructions a C program compiles down to is "unimportant" provided that all **observable states** are correctly reproduced. This is the essence of optimization.
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%%ANKI
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Basic
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What feature of C's abstract state machine makes C performant?
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Back: The ability to optimize.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1723856661330-->
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END%%
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%%ANKI
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Basic
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C can compile into any sequence of instructions provided what holds?
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Back: All observable states are correctly reproduced.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1723856661337-->
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END%%
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%%ANKI
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Basic
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Why is C's abstract state machine called the way it is?
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Back: Compilers are free to realize the state machine however they see fit.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1723856661340-->
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END%%
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%%ANKI
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Basic
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What three components make up C's abstract state machine?
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Back: Values, types, and binary representations.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1723856661343-->
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END%%
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%%ANKI
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Cloze
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In C's abstract state machine, {binary representations} describe {types} which describe {values}.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1723856668033-->
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END%%
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## Values
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An **object** is a region of data storage in the execution environment, the contents of which can represent **values**. An **lvalue** is an expression (with non-`void` object type) that potentially designates an object. An **rvalue** is the "value of the expression."
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The notion of a value in C is an abstract entity. It exists beyond the program or the representation of the value in the program. For example, the value `0` (no matter how its represented) added to variable `x` should always yield result `x` regardless of platform.
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%%ANKI
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Basic
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What does an object refer to?
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Back: A region of data storage in the execution environment.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1723510994830-->
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END%%
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%%ANKI
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Basic
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What does a value refer to?
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Back: The contents of an object when interpreted as having a specific type.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1723510994835-->
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END%%
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%%ANKI
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Basic
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Why does Gustedt refer to values as abstract entities?
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Back: A value exists beyond a program or any particular representation.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1723856661349-->
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END%%
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%%ANKI
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Basic
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How does Gustedt distinguish the data of a program execution from values?
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Back: The data is the set of values of all objects at a given moment.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1723856661358-->
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END%%
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%%ANKI
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Cloze
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A {value} refers to the contents of an {object} when interpreted as having a specific type.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1723510994839-->
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END%%
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%%ANKI
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Basic
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What is an lvalue?
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Back: An expression (with non-`void` object type) that potentially designates an object.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1723510994874-->
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END%%
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%%ANKI
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Basic
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Why are lvalues named the way they are?
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Back: The name is an acronym for **l**ocator **value** (or **l**eft **value**).
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1723510994878-->
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END%%
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%%ANKI
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Basic
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What is an rvalue?
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Back: The value of an expression.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1723510994881-->
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END%%
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%%ANKI
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Basic
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Why are rvalues named the way they are?
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Back: The name is an acronym for **r**ight **value**.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1723510994886-->
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END%%
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%%ANKI
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Basic
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Can an lvalue designate an object?
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Back: Yes.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1723510994900-->
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END%%
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%%ANKI
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Basic
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Can an lvalue designate a function?
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Back: No.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1723510994907-->
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END%%
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%%ANKI
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Basic
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Is `x` an lvalue or rvalue in the following snippet?
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```c
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int x = 10;
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```
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Back: An lvalue.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1723510994914-->
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END%%
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%%ANKI
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Basic
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Is `int` an lvalue or rvalue in the following snippet?
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```c
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int x = 10;
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```
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Back: Neither.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1723510994920-->
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END%%
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%%ANKI
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Basic
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Is `10` an lvalue or rvalue in the following snippet?
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```c
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int x = 10;
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```
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Back: An rvalue.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1723510994925-->
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END%%
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%%ANKI
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Basic
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*Why* is `x` an lvalue in the following?
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```c
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int x = 10;
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```
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Back: Because `x` refers to a memory location.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1723510994932-->
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END%%
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%%ANKI
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Basic
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Is `x` an lvalue or rvalue in the following snippet?
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```c
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void x;
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```
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Back: Neither.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1723510994939-->
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END%%
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%%ANKI
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Basic
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Is `x` an lvalue or rvalue in the following snippet?
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```c
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void *x;
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```
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Back: An lvalue.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1723510994945-->
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END%%
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%%ANKI
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Basic
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Is `y` an lvalue or rvalue in the following snippet?
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```c
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int y = x + 10;
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```
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Back: An lvalue.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1723510994952-->
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END%%
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%%ANKI
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Basic
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Is `x` an lvalue or rvalue in the following snippet?
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```c
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int y = x + 10;
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```
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Back: An rvalue.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1723510994958-->
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END%%
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%%ANKI
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Basic
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In the second line, is `ptr` an lvalue or rvalue?
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```c
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int *ptr = &x;
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*ptr = 10;
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```
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Back: An lvalue.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1723510994964-->
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END%%
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%%ANKI
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Basic
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In the second line, is `*ptr` an lvalue or rvalue?
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```c
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int *ptr = &x;
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*ptr = 10;
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```
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Back: An lvalue.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1723510994970-->
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END%%
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%%ANKI
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Basic
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Is `getValue` an lvalue or rvalue in the following snippet?
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```c
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int getValue () {
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return 42;
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}
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```
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Back: Neither.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1723510994975-->
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END%%
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%%ANKI
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Basic
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Is `getValue()` an lvalue or rvalue in the following snippet?
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```c
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int d = getValue();
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```
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Back: An rvalue.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1723510994980-->
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END%%
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%%ANKI
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Basic
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*Why* is `getValue` in the following snippet neither an lvalue nor an rvalue?
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```c
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int getValue() {
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return 42;
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}
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```
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Back: The function name is just syntax. That is, it isn't an expression.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1723510994984-->
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END%%
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%%ANKI
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Basic
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Are variables typically lvalues or rvalues?
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Back: lvalues.
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Reference: ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1723510994988-->
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END%%
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%%ANKI
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Basic
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Are constants typically lvalues or rvalues?
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Back: rvalues.
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Reference: ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1723510994992-->
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END%%
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%%ANKI
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Basic
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Are array accesses typically lvalues or rvalues?
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Back: lvalues.
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Reference: ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1723510994997-->
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END%%
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%%ANKI
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Basic
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Are dereferenced pointers typically lvalues or rvalues?
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Back: lvalues.
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Reference: ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1723510995001-->
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END%%
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%%ANKI
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Basic
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Are temporary values typically lvalues or rvalues?
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Back: rvalues.
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Reference: ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1723510995006-->
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END%%
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## Types
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Types are additional properties that C associates with values. All values have a type that is statically determined and all possible operations on a value are determined by its type.
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%%ANKI
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Cloze
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Possible operations on a {value} are determined by its {type}.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1723856661364-->
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END%%
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## Representations
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The **binary representation** of a type is the model used to represent values of said type on a given platform. The **object representation** of a type determines how values are stored in memory, disk, etc.
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%%ANKI
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Basic
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What is the binary representation of a type?
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Back: The model used to represent values of the type on a given platform.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1723856661371-->
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END%%
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%%ANKI
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Basic
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What is the object representation of a type?
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Back: How a value of a given type is actually stored in memory, disk, etc.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1723856661386-->
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END%%
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%%ANKI
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Cloze
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A {type}'s {binary representation} determines the results of all operations.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1723856661393-->
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END%%
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%%ANKI
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Basic
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Why does Gustedt refer to binary representations as abstract entities?
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Back: Binary representations don't completely determine how values are stored in memory.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1723856661399-->
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END%%
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%%ANKI
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Cloze
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A {binary} representation is abstract whereas an {object} representation is concrete.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1723856661405-->
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END%%
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%%ANKI
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Basic
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Why might the same value have different binary representations?
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Back: Because the binary representation corresponds to the type of the value.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1727432711873-->
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END%%
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%%ANKI
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Basic
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The `sizeof` operator returns values in units of what?
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Back: Bytes.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1732622657260-->
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END%%
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%%ANKI
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Basic
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Consider object `A`. What is "the type" of its object representation?
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Back: `unsigned char[sizeof A]`
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1732622657263-->
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END%%
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%%ANKI
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Basic
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Every object can be viewed as an array of what type?
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Back: `unsigned char`
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1732622657266-->
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END%%
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%%ANKI
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Basic
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Which type is considered the "atom" of all object types?
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Back: `unsigned char`
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1732622657269-->
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END%%
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## Bibliography
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* “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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* Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020). |