I hope you:

1
CS110 Lecture 21 Structures With Arrays and
Pointers
Jack Tumblinjet_at_cs.northwestern.edu

• I hope you
• Are much happier with PA-3(revised), Due Today.
• Are 'almost done' with PA-4 (short due Wed.
  11/19)
• Are trying tiny practice programs using pointers
• Are reading all about structures Chap. 12 (no
  12-8)

2
Referencing and De-referencing

• Recall
• referencing get address operator
• int k2
• pInt, arr35,6,7
• pInt k / referencing / arr2 8 /
  array indexing / pInt0 1 / pointer
  indexing / pInt 1 / pointer
  de-referencing /
• Dereferencing is same as indexing at zero!
• Notation VERY confusinglooks like pointer
  declaration!
• ( pInt pInt0 ) / always true /

3
Referencing and De-referencing

• Recall
• indexing get element operator
• int k2
• pInt, arr35,6,7
• pInt k / referencing / arr2 8 /
  array indexing / pInt0 1 / pointer
  indexing / pInt 1 / pointer
  de-referencing /
• Dereferencing is same as indexing at zero!
• Notation VERY confusinglooks like pointer
  declaration!
• ( pInt pInt0 ) / always true /

4
Referencing and De-referencing

• Recall (Book does this too soon!)
• de-referencing value pointed to op
• int k2
• pInt, arr35,6,7
• pInt k / referencing / arr2 8 /
  array indexing / pInt0 1 / pointer
  indexing / pInt 1 / pointer
  de-referencing /
• Dereferencing is same as indexing at zero!
• Notation VERY confusinglooks like pointer
  declaration!
• ( pInt pInt0 ) / always true /

5
Dynamic Allocation Example
malloc() and free() are defined in stdlib.h

• include ltstdlib.hgt
• int main(void)
• int k,kmax,pKey,pDone...pKey (int
  )malloc( kmaxsizeof(int) )for(k0 kltkmax
  k) pKeyk rand()... (more work
  with pKey) ...
• pDone pKey
• free(pDone)

malloc(n) function reserve a block of n
bytes (Block has no name)
free(ptr) function release a block of bytes at
address ptr.
6
CS110 Lecture 21 Structures With Arrays and
Pointers
Jack Tumblinjet_at_cs.northwestern.edu

• I hope you
• Are much happier with PA-3(revised), Due Today.
• Are 'almost done' with PA-4 (short due Wed.
  11/19)
• Are trying tiny practice programs using pointers
• Are reading all about structures Chap. 12 (no
  12-8)

7
(Recall) Enumerated Types

• Define a new enum data type, Declare variables
  of it
• typedef enum / define a new data type /
  Sun, Mon, Tues, Wed, Thu, Fri, Sat
• weekdayT
• int main()
• weekdayT today,tomorrow / declare vars /
• today Sat
• tomorrow (today-1)7 /computed as int/...

Name of our new data type
Declare some variables of type weekdayT
8
(Recall) Enumerated Types

• Mostly a notational convenience
• Works just like define statements
• typedef enum
• Sun, Mon, Tues, Wed, Thu, Fri, Sat
• weekdayT
• enum variables given int values internally, or
  you can assign them explicitly (see book)
• No restrictions on value--today 578 allowed
  (but meaningless)

0 1 2 3 4 5 6
9
1) Define a Data Structure

• typedef struct workerT /employee record
  /
• char name / string variable /float
  salary / in dollars /char ssn11 /
  xxx-xx-xxxx /int ded / tax deductions /
• workerT
• int main()
• workerT emp1,staff30
• emp1.name Bob Cratchitemp1.salary
  10.00strncpy(emp1.ssn,022-85-7741,11)emp1.de
  d 7

define a new type for me
it is a data structure, and
The name of the new data type is
The members of the new data type are
10
2) Declare Variables of the new Type

• typedef struct workerT /employee record
  /
• char name / string variable /float
  salary / in dollars /char ssn11 /
  xxx-xx-xxxx /int ded / tax deductions /
• workerT
• int main()
• workerT emp1, pGood, staff30int i,imax
• emp1.name Bob Cratchitemp1.salary
  10.00strncpy(emp1.ssn,022-85-7741,11)emp1.de
  d 7

workerT is now a data type. Declare an
ordinary variable emp1, and a pointer-to-workerT
variable pGood, and an array of workerT elements
staff30
11
3) Initialize and Use struct Variables

• typedef struct workerT /employee record
  /
• char name / string variable /float
  salary / in dollars /char ssn11 /
  xxx-xx-xxxx /int ded / tax deductions /
• workerT
• int main()
• workerT emp1,staff30int i,imax
• emp1.name Bob Cratchitemp1.salary
  10.00strncpy(emp1.ssn,022-85-7741,11)emp1.de
  d 7

Use dot operator to access the members or
fields of the data structure
12
A Beginners Mistake

• Dont confuse data type you defined with a data
  structure and the variables of that type

typedef struct workerT / employee record
/ char name / string variable
/ float salary / in dollars / char
ssn11 / xxx-xx-xxxx / int ded / tax
deductions / workerT int main() workerT
emp1,staff30int i,imax workerT.name
Bob Cratchit workerT.salary 10.00 ...
Compiler Errors!! ?What's wrong?
13
A Beginners Mistake

• Dont confuse data type you defined with a data
  structure and the variables of that type

typedef struct workerT / employee record
/ char name / string variable
/ float salary / in dollars / char
ssn11 / xxx-xx-xxxx / int ded / tax
deductions / workerT int main() workerT
emp1,staff30int i,imax workerT.name
Bob Cratchit workerT.salary 10.00 ...
Data type!
ErrorUndeclared Variable?!?
14
A Beginners Mistake

• Dont confuse data type you defined with a data
  structure and the variables of that type

typedef struct workerT / employee record
/ char name / string variable
/ float salary / in dollars / char
ssn11 / xxx-xx-xxxx / int ded / tax
deductions / workerT int main() workerT
emp1,staff30int i,imax emp1.name Bob
Cratchit emp1.salary 10.00 ...
Data type
Variable names
Correct way Variables of type 'workerT'
15
Operators Data Structures

• For the basic data types, many operators work,
  , -, , /, lt, gt, lt, , gt, !, . etc.
• Data structure types ONLY assignment () works
• () is a Member-by-Member / Field-by-field copy
• WILL copy char, int, float, double, enum values,
• WILL copy array contents (because its size is
  fixed),
• WILL copy a pointer, but
• WONT copy strings or other data located at
  pointers

16
Operators Data Structures

• typedef struct workerT / employee record
  /
• char name / string variable /float
  salary / in dollars /char ssn11 /
  xxx-xx-xxxx /int ded / tax deductions /
• workerT
• int main()workerT staff30, emp1 / copy
  Scrooges info/ staff0.name Ebenezer
  Scrooge staff0.salary 250.00
  strncpy(staff0.ssn,001-34-8902,11)
  staff0.ded 1 emp1 staff0
  printf(s, 5.2f, ssns, dedd\n,
  emp1.name,emp1.salary,emp1.ssn, emp1.ded)
• RESULT gt Ebenezer Scrooge, 250.00,
  ssn001-34-8902, ded1 gt

17
Arrays of Data Structures

• Data Structure a named group of variables
• workerT emp1... emp1.name Bob
  Cratchit emp1.salary 10.00 emp1.ssn
  022-85-774 emp1.ded 7 etc. ...

Data structure members
Array element
staff0 emp1 staff2 staff3 ...
18
Arrays of Data Structures

• Data Structure a named group of variables
• How can we describe lots of complicated
  'objects', where each 'object' has SEVERAL
  variable types? EXAMPLE employee ledger
  moving objects in a videogame...
• Answer make an array of Data Structures

staff0 staff1 staff2 staff3 ...
19
Arrays of Data Structures

• Example
• float costint k,kmaxworkerT staff30
  staff0.name Ebenezer Scrooge
  staff0.salary 250.00 strncpy(staff0.ssn
  ,001-34-8902,11) staff0.ded 1
  staff1.name Bob Cratchit
  staff1.salary 10.00 . . .

typedef struct workerT / employee record
/ char name / string variable
/ float salary / in dollars / char
ssn11 / xxx-xx-xxxx / int ded / tax
deductions / workerT
20
Pointers-to-Struct

• Pointers work on structs exactly as before...
• workerT staff30, emp1 / struct variables
  /
• workerT pGood / pointer-to-struct /
• ...
• pGood emp1 / point to 'emp1' /
• pGood staff 3 / point to staff3
  /
• Use indirection ( or 0) to get
  members pGood0.salary 1000 / Give a
  raise / / OR you can write
  / (pGood).salary 1000 / Give a raise
  /
• Operator precedence for makes a mess
  (ANOTHER reason to hate the form of
  de-referencing)
• pGood.salary 1000

!NO! -- !ERROR!
21
Pointers-to-Struct -gt

• Pointers work on structs exactly as before...
• workerT staff30, emp1 / struct variables
  /
• workerT pGood / pointer-to-struct /
• ...
• pGood emp1 / point to 'emp1' /
• pGood0.salary 1000 / Give a raise
  /
• pGood staff 3 / point to staff3 /
• (pGood).salary 1000 / Give another
  raise /
• Operator precedence for makes a mess!
  (ANOTHER reason to hate the form of
  de-referencing)
• pGood.salary 1000
• INSTEAD use this nearly-pictorial operator
  pGood-gtsalary 1000

SAME
!NO! --!ERROR!
22
Dynamic Alloc. for Structures I

• Just as we can have fixed arrays of
  structures(e.g. workerT staff30 )
• We can have dynamically allocated arrays of
  structs
• int k,kmaxworkerT pWho
• pWho (workerT )malloc(30sizeof(worker
  T)) / now pWho points to 30 workerT
  elements.../
• pWho0.name (char )malloc(31sizeof(char))
  strncpy(pWho0.name,Ebenezer Scrooge,30)
  pWho0.salary 250.00 strncpy(pWho0.ssn,00
  1-34-8902,11) pWho0.ded 1 pWho1.name
  Bob Cratchit pWho1.salary 10.00 ...

23
Dynamic Alloc. for Structures II

• Data structures can have pointer members, too...

typedef struct workerT / employee record
/ char name / string variable
/ float salary / in dollars / char
ssn11 / xxx-xx-xxxx / int ded / tax
deductions / workerT
workerT emp1 emp1.name (char
)malloc(31sizeof(char)) strncpy(emp1.name,Eb
enezer Scrooge,30) emp1.salary 250.00
strncpy(emp1.ssn,001-34-8902,11) emp1.ded
1 emp1.name Bob Cratchit emp1.salary
10.00
24
Pass by Reference

• Function arguments that are pointers
• Copy an address to the formal parameters,
• But changing formal parameter (the address) in
  the function NEVER changes the actual argument!

formal parameters
main (void) double val1.234double pd
pd val dfixit(pd) / now val42.0
/ / pd unchanged /
actual argument
void dfixit2(double x) x0 42.0
x / move!? /
NO!
25
Functions and Data Structures

• What if function arguments are 'struct'-type
  variables?
• works just fine copies struct to formal
  parameters...
• workerT give_raise(workerT b4) /costly
  prototype! /
• int main(void)
• workerT emp1
• ...
• emp1 give_raise( emp1 ) /costly fcn
  call /
• ...
• workerT give_raise(workerT b4) / costly
  fcn body /
• b4.salary 1000.00 ...
  return(b4)

COPY
26
Functions and Data Structures

• Recall that we call a function,we copy actual
  arguments to formal parameters
• workerT give_raise(workerT b4) /costly
  prototype! /
• int main(void)
• workerT emp1
• ...
• emp1 give_raise( emp1 ) /costly fcn
  call /
• ...
• workerT give_raise(workerT b4) / costly
  fcn body /
• b4.salary 1000.00 ...
  return(b4)

COMPUTE
27
Functions and Data Structures

• And when return something from a function,we
  copy it from local variables...
• workerT give_raise(workerT b4) /costly
  prototype! /
• int main(void)
• workerT emp1
• ...
• emp1 give_raise( emp1 ) /costly fcn
  call /
• ...
• workerT give_raise(workerT b4) / costly
  fcn body /
• b4.salary 1000.00 ...
  return(b4)

COPY
28
Functions and Data Structures

• Structs are just one more data type it all works
  fine.
• BUT data structures may be HUGE!
  (suppose the workerT struct includes
  a 500KB photo)do you need to copy back and forth
  to/from functions?
• workerT give_raise(workerT b4) /costly
  prototype! /
• int main(void)
• workerT emp1
• ...
• emp1 give_raise( emp1 ) /costly
  fcn call /

COPY THE BIGINPUT STRUCT to b4
COPY THE BIGRETURNED STRUCT to emp1
29
Functions and Data Structures

• Make a habit of Pass-by-Reference for structures
• Copies only the address to functionFAST!
• No return value neededEfficient!
• void give_raise(workerT pGood) / efficient
  prototype!/
• int main(void)
• workerT emp1
• ...
• give_raise(emp1) / Better! use a
  pointer / ...
• void give_raise(workerT pGood) / efficient
  function body/
• pGood-gtsalary 1000.0 / Note the
  'shorthand' / / (same as
  (pGood0).salary or (pGood).salary /