Variable Names

1
CS 110 Lecture 03Literals Variables
Jack Tumblinjet_at_cs.northwestern.edu

• I hope you have already
• Read through Chapter 2.4,
• Skimmed the rest of Chapter 2, and
• Visited a computer lab to try PA-0.

2
Recall That

• C ignores / comments / and blank space //
  (C comments also OK, but NOT part of C!)
• A statement is one complete rule, andC
  statements always end with semicolon
• A block of statements is always enclosed within
  braces (curly brackets)
• A function is a named block of statements
• (more about functions later)
• All C programs have a main( ) function
• computing always begins and ends in main( )
• Bits, Bytes Memory is a numbered list of bytes

3
How C is Organized Nesting

• Literals and Variables (with their data
  types)
• Operators
• Expressions
• Statements
• Functions
• Libraries
• Programs
• Systems

4
How C is Organized Nesting

• Literals and Variables (with their data
  types)
• Operators
• Expressions
• Statements
• Functions
• Libraries
• Programs
• Systems

5
Know these Definitions

• LiteralA fixed number(data) written in a
  progm.
• Variable A named, reserved piece of memory
• Operator A non-numeric calculator button
• Expression The smallest grain of computing
• Statement Zero or more expressions ended by
• Function A named block of statements
• Library A collection of re-usable functions in a
  file
• Program a main() function any functions it
  calls.
• SystemA collection of program(s) that work
  together

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Variables and Literals

• Same idea used in algebra y 3x 5 x and
  y are the variables 3,5 are the constants
  oops! No! the literals (CS jargon
  disastermore about this on Friday)
• But Generalized a variable is a placeholder
  for one piece of data numbers, letters, sets,
  arrays, etc. Its value may be unknown until the
  program is actually running (CS jargon at
  run-time).
• Inside the computer a variable is a reserved
  location in memory, and holds data that are
  allowed to change.

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Variables

• ALWAYS declare a variable before use!
• forgot? error message, or ugly weirdness
• ALWAYS set a variables value before use!
• forgot? starting value will be unpredictable
• What does declaring a variable do?
• reserve a memory location that will store the
  value of the variable (jargon assign an
  address)
• assign the variable name to that location.

8
/ Hydr
a.c -- a program with variables.
/include lt stdio.h
gtmain()int heads int eyes heads
3 / assign a value / eyes heads
2 / compute a value / printf(It has d
heads and d eyes! \n", heads, eyes)
declare each variable write a statement that
to set its type and name
Output gt It has 3 heads have 6 eyes!
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/ Hydr
a.c -- a program with variables.
/include lt stdio.h
gtmain()int heads int eyes heads
3 / assign a value / eyes heads
2 / compute a value / printf(It has d
heads and d eyes! \n", heads, eyes)
After that, set values of variables
Output gt It has 3 heads have 6 eyes!
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How to declare Variables

• Make declaration statement (always ends with
  )
• The format data_type var, var,
• Example int counter1, counter2
• Where? Declare variables at the start of a
  function, right after the opening brace

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Variable Names

• In C, variable names are built from
• the letters of the alphabet
• the digits 0 through 9
• the underscore. (no other special characters!)
• A variable name must start with a letter or an
  underscore.
• Only the first 31 characters of a variable name
  are significant. The rest are ignored.
• A variable name must not be the same as a
  reserved word used by the C language.

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Variable Names

• Good, Valid variable names
• totalArea _main
• counter1 isEmpty
• Count_trees pNuts
• temp_in_F m_size
• Invalid variable names
• product main not-this
• total 3rd
• Legal, badly-chosen variable names
• l11 (is it L11, L1L, LL1, or LLL?)
• x (what does it mean?)
• maximum_number_of_students_in_my_class
• a23456789_123456789_123456789_12345678

13
Basic Types of Variables

• There are 4 basic built-in data types in C

Type Integer Floating point Double Character
C keyword to use int float double char
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Basic Types of Variables

• int
• Integer variables hold signed whole numbers (i.e.
  with no fractional parts). E.g. 10, -8439
• In environments such as WinNT, integers typically
  store values in the range
• From -2,147,483,648, or or about
• to 2,147,483,648 or or about -

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Basic Types of Variables

• float and double
• Used to store floating-point numbers, i.e.
  numbers with a fractional part. E.g. 12.475
• double provides twice the precision of float
• e.g. float can represent values up to about 2127
  whereas double can represent values up to about
  21023 .

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Basic Types of Variables

• float
• size 4 bytes
• approximate range () 10-44.85 to 1038.53
• double
• size 8 bytes
• approximate range () 10-323.3 to 10 308.3

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Basic Types of Variables

• char
• used to store single characters. E.g. a , R.
• (Note that the value of the character is enclosed
  in single quotes).
• Remember that the string in our earlier example
  was enclosed in double quotes. Therefore
• characters single quotes
• strings double quotes

18
More Types of Variables

• How many bytes for each type?
• A MESS! No standards in the language.
• But nowadays its almost always
• char 1 or 2 bytes (ASCII or Unicode)
• int 4 bytes32 bits, signed.
• float 4 bytes32 bits, ? (gtPentium? use double)
• double 8 bytes64 bits.
• Leftover keywords from the Dark Ages
• short int, long int, unsigned int, signed int,
  unsigned char, signed char, double double.
• Dont use in new programs (except by gunpoint)

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Literals

• Fixed values written into your program.
• Same basic data types
• int cnt ... for(cnt0 cntltmax cnt)
• char lastKey ... lastKey y
• float velocity ... velocity 0.0f
• double atoms ... atoms 6.23E23
• Can be Dangerous! Use Sparingly or not at
  all!(use define instead, coming next class)

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Literals

• Fixed values written into your program.
• Details
• 1-Character constants written with single
  quote keypressd y
• Constant phrases, or strings (more about this
  later) written with double quotes printf(Huh.
  Dude typed a c!\n, keypressd)
• Default decimal points make a double
  value.You can FORCE float data type like
  thisvelocity 3.141f

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For Next Time

• Be sure you know the definitions of the parts of
  C. Everybody uses these words. Dont ask me
  about the constant variable type thingie.
• Read ahead. Skim chapter 3, so that my lectures
  will be boring.
• Try things. Can you write a program that prints
  a 2D multiplication table?