Precedence

1
Precedence associativity

• How would you evaluate the expression 17 - 8 2
  ? Is it 17 - (8 2) or (17 - 8) 2 ?
• These two forms give different results.
• We need rules!

2
Precedence associativity

• When two operators compete for the same operand
  (e.g. in 17 - 8 2 the operators - and compete
  for 8) the rules of precedence specify which
  operator wins.
• The operator with the higher precedence wins
• If both competing operators have the same
  precedence, then the rules of associativity
  determine the winner.

3
Precedence associativity
! Unary / lt lt gt gt
!
higher precedence
Associativity execute left-to-right (except for
and unary )
lower precedence
4
Precedence associativity

• Examples X 17 - 2 8 Ans X17-(28) ,
  X1
• Y 17 - 2 - 8 Ans Y (17-2)-8, Y7
• Z 10 9 ((8 7) 6) 5 4 3 2 1 ?

Not sure? Confused? then use parentheses in your
code!
5
Statements?Functions

• Big programs get complicated very fast. Small
  programs are simpler.
• Key CS idea Decomposition
• Break big complicated sets of instructions into
  smaller, simpler modules
• In C, these modules are functions
• function a separate named block of statements
  that you call by name in order to execute

a block of statements statement1
statement2 ...
6
A function...

• ...is a "black box", a mini program with
• a simple set of inputs
• simple output
• a body of statements
• ...performs one well-defined task
• (e.g. printf only prints to the screen)

7
Function essentials

• A function has
• a unique name
• a body
• some (optional) inputs (called arguments)
• one (optional) output (called return value)
• Just like variables, before a function can be
  used it must be
• declared ( specify name, arguments, return value
  type)
• defined ( write the body)

8
Functions you know already

• main()
• the first (and last) function executed when the
  program runs
• printf(), scanf()
• You can invoke (call) other functions from within
  main()
• You can write your own functions which in turn
  may call other functions and so on.

9
How to use existing functions

• Use include directives to tell C where to find
  the declarations of these functionsinclude
  ltstdio.hgt / includeltXgt "search the usual
  places for file X"/include "coatrack.h"/
  include "X" "search current directory for
  file X"/
• Call the functions by name in your
  programs.scanf("d",numhats)isFull
  hatChek(hats)