C Programming Revision

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C Programming Revision

• Simple C program layout
• C data types
• printf output
• C pointers and arrays
• Stack and Heap memory
• Global vs Local variables

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C-Programming Revisions
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C-LANGUAGE STRUCTURE
/ include statements / include
ltstdio.hgt include ltstring.hgt / define
statements / define MAX 2000 / function
prototypes / void subfunc(int argument) /
global variables / int i,j char c float
x,y char s80
/ functions / void subfunc(int argument)
int i / local variables /
statements... / main program / int main()
statements...
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C-Programming Revisions
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VARIABLES
must be declared before they are used. Names
start with a letter, subsequent characters can be
letters, numbers or the underscore. Types
char (unsigned) int
(unsigned)(short, long) float (double)
pointers Assignment statements i 3 /
or i 0x03 hex / x 4.5 / x
4.5e0 scientific / c 'a' / c 0x61
c 97 /
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C-Programming Revisions
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VARIABLES
must be declared before they are used. Names
start with a letter, subsequent characters can be
letters, numbers or the underscore. Types
char (unsigned) int
(unsigned)(short, long) float (double)
pointers Assignment statements i 3 /
or i 0x03 hex / x 4.5 / x
4.5e0 scientific / c 'a' / c 0x61
c 97 /
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C-Programming Revisions
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Expressions operators
- / ltlt gtgt (left and right shift)
(one's complement) (bitwise
AND, OR and XOR) i (ij) 3 i c -
48 Shortcut operators are , - etc
i i j k
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Arrays
int m100 float f10 char s80 m4
3 fi fi1 Strings are arrays of
characters. Constants are enclosed in double
quotes. The last character in a valid string is
the NUL. Arrays may have more than one
dimension char sarray2580
sarray104 'A' strcpy(sarray3,Lucienne
") (Actions on whole arrays are via function
calls)
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Structures
struct person char surname80 char
forename80 int age struct person
x,y10 Individual elements within a structure
are called fields and are accessed using the dot
operator x.age 26 strcpy(x.surname,"Kay")

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C-Programming Revisions
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New types
New types can be defined by the
programmer typedef struct person
person_type person_type x
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INITIALIZING VARIABLES
Global variables are initialised to 0 by default.
Local variables contain random data. Variables
are initialised when they are declared char c
'A' int i 10 char s80 "hello world" int
m5 4,7,5,9,1 char sa380 "string
one", "string two", "string three"
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FUNCTIONS
Functions have arguments and can return
values int fct1(int i,int j) char fct2(char
c) void fct3(char s) Arguments which are
simple types, have their values copied into the
corresponding formal parameter. For arrays, a
pointer to the first element of the array is
copied.
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C-Programming Revisions
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STRING FUNCTIONS
strcpy(s1,s2) l strlen(s1) if
(strcmp(s1,s2) 0) strcat(s1,s2) Whe
n a return statement is executed the function
terminates. When the exit function is called the
program terminates.
FUNCTIONS FOR TERMINATING
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C-Programming Revisions
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OUTPUT
the library routine printf is used for output.
The first argument is a text string, which
contains formatting information. gets for
complete lines of text. getchar/getch for
characters. scanf for numbers and words.
INPUT
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C-Programming Revisions
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FILES
Files file I/O routines use a FILE pointer.
FILE f f fopen(filename,mode)
fprintf(f,"s",s) fgets(string,size,f)
fscanf(f,"d",i) fclose(f)
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C-Programming Revisions
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CONDITIONAL STATEMENTS
if and switch if (test)
statements... test contains logical
operators lt lt ! gt gt
! if (test) statements...
else statements...
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C-Programming Revisions
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SWITCH
switch (int or char) case number or
character statements... break
case number or character statements...
break . . default
statements...
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C-Programming Revisions
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LOOPS
while (test) statements...
for (initialisetestincrement)
statements... do statements...
while (test)
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C-Programming Revisions
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POINTERS
Pointers int p stuct person q The
operator p i q y0 /or q y
/ The operator (or -gt for structs) (p)
6 q-gtage 26
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C-Programming Revisions
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POINTERS
To alter the value of an actual parameter, a
pointer to that variable must be passed.
fct1(i) void fct1(int ip) (ip)
6 Pointers can be assigned dynamically. p
(char )malloc(128) free(p)
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RECURSION
Recursion A function may call itself (direct
recursion) or may call itself via a call to
another function (indirect recursion). A
recursive function must always have a route
through it, that does not involve any more
recursive calls.
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• Example output
• (terminated with D or Z)

• include ltstdio.hgt
• include ltmath.hgt
• int main()
• char ch / is really an
  int from -128 to 0 to 127 /
• unsigned char uch / is really an int from
  0 to 255 /
• short int si / can just say
  short si /
• int i / usually same
  as machine word ie 32 bits /
• long int li / could be 32
  but may be 64 bits /
• long long int ll / generally is 64
  bits on gcc systems /
• float f / IEEE format floating point,
  32 bits usually /
• double d / double precision - about 15
  significant figures /
• for(i0ilt5i) / loop from 0...4 ie do
  it 5 times /
• printf("d 2 d\n", i, i 2 ) /
  print a string with 2 ints /
• ch '\n' / the newline character
  /
• uch 255

0 2 0 1 2 2 2 2 4 3 2 6 4 2
8 li 2147483647 a a b b c c d d e e hello hello
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• include ltstdio.hgt
• void printArray( int iarr, int n ) /
  function to print out an int array /
• int i / print to stderr, could
  have used stdout instead /
• for(i0iltni)
• fprintf(stderr, "d\t", iarri ) /
  separate them with tabs /
• fprintf(stderr,"\n") /
  finish with a newline /
• int main()
• int i / an integer takes 32 bits
  of memory on the stack /
• int iarray4 / will take /
• int ipointer
• printf("size of int d\n", sizeof(i) )
• printf("size of iarray d\n", sizeof(iarray)
  )
• printf("size of ipointer d\n",
  sizeof(ipointer) )
• i 0
• iarrayi 42
• printArray( iarray, 4 )
• for(i0ilt4i)
• iarrayi i 10

• Example output
• size of int 4
• size of iarray 16
• size of ipointer 4
• 42 49 6 3
• 0 10 20 30
• ipointer 3221224552
• contents of ipointer 4
• 1 11 21 31

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Pointers

• Pointers in C and C are a special data type
  that is capable of pointing to the memory
  location of a normal data type.
• Pointers are typed - eg pointer to an int is
  not the same as pointer to a char
• Pointers let us do address manipulation and
  are a powerful but dangerous feature of the
  language

• In C and C arrays are implemented via pointer
  arithmetic
• int iarray10 tells the compiler you want the
  space for 10 integers set aside (not yet
  initialised)
• int iptr sets aside memory for a pointer but
  not for an actual integer!

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Stack Memory

• When you write and run a program each function
  can set up its own variable s on what is
  known as the stack - a part of the
  computers memory organised like a stack of
  cards.

• Variables set up on the stack typically last only
  for the duration of execution of the function
  that declares them.

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Stack example

• int myfunc() / prototype /
• int main()
• int i
• myfunc()
• return 0
• int myfunc()
• int j 10

• i is a stack variable set up by the function
  main()
• j is a stack variable set up by the function
  myfunc() and will last only for the duration of
  the call to myfunc()

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Global Variables - use with Caution

• int k 42
• int myfunc()
• int j 10
• k k j
• return k
• int main()
• int i 3
• int k 7
• i myfunc()
• printf(i d, k d\n, i, k )
• return 0

• k is a global variable
• It is accessible to both main() and myfunc()
• main() declares its own stack variable k which
  hides the global k
• Output would be
• 52, 7

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DO NOT USE GLOBAL VARIABLES
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Heap Memory

• The Heap is a pool of memory available to all
  functions which they must access using
  malloc() and related functions
• Mallocd memory stays allocated until you free
  it
• This must be used with great care (otherwise get
  Bus Error or Segmentation Fault)
• Remember to allocate memory before you try to
  use it
• Remember to free up memory once you are finished
  with it.
• C has neater ways of managing this

• int ia / a global pointer /
• int myfunc()
• ia (int )malloc( 10 sizeof(int) )
• ia2 7
• int main()
• myfunc()
• ia0 8
• ia9 87
• free( ia )
• return 0

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Miscellany of C Features

• if( ) else
• while( )
• do while( )
• Use of ints 0 or not zero in place of
  false/true
• Functions can call themselves (recursion)
• No string type in C so use arrays of char,
  terminated by \0 character

• Need include ltmaths.hgt
• to use the maths library functions eg
  sqrt()
• printf()/scanf() fprintf()/fscanf() I/O
  routines
• putc()/getc() for single character I/O

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Summary

• Think of pointers as memory addresses
• Memory your program uses can be stack or heap
  memory
• Stack variables last only as long as the called
  function does
• Heap variables last until you free them up

• Look at the C programming revision material on
  the web site.
• C is based around the idea of functions which
  call one another
• OO programming goes beyond this
• Next - C simple I/O