arrays.ppt

7/18/2019 Arrays.ppt

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C arrays

(Reek, Ch. 8)

1 CS 3090: Safety Critical Programmingin C



Review of arrays

CS 3090: Safety Critical Programmingin C

2

There are no array variables in C – only arraynames Each name refers to a constant pointer

Space for array elements is allocate at eclarationtime

Can!t change "here the array name refers to# b$t yo$ can change the array elements%

via pointer arithmetic

int m[4];

(int [])

m

???(int)

???(int)

???(int)

???(int)



Subscripts and pointer arithmetic


3

array[subscript] e&$ivalent to *(array +(subscript))

Strange b$t tr$e: 'iven earlier eclaration of m%

the e(pression 2[m] is legal) *ot only that: it!s e&$ivalent to *(2+m)

*(m+2)

m[2]



Array names and pointer variables,playing together


+

int m[3];

int *mid = m + 1;

int *right = mid[1];

int *let = mid[!1];

int *bey"nd = mid[2];

(int [])

bey"nd

???(int)

???(int)

???(int)

(int [])

(int [])

(int [])

(int [])

mid

right

let

m

s$bscript ,- "ith pointer

variable

compiler may not catch this –r$ntime environment certainly

"on!t



.n C% arg$ments are passe /by val$e temporary copy of each arg$ment is create% solelyfor $se "ithin the f$nction call

#"id (int $% int *y) & '

#"id g(') & int a = 1% b = 42;

(a% b);

'

Passbyval$e is /safe in that the f$nction playsonly in its /sanbo( of temporary variables – can!t alter the val$es of variables in the callee e(cept

via the ret$rn val$e4

Array names as function arguments


5

1(int)

42(int)

g

b

1(int)

$ y

(int [])

a





6

7$t% f$nctions that ta8e arrays as arg$ments cane(hibit what looks like /passbyreferencebehavior% "here the array passe in by the callee

oes get change emember the special stat$s of arrays in C –

They are basically $st pointers;

So arrays are inee passe by val$e –

b$t only the pointer is copie% not the array elements) *ote the avantage in e<ciency avois a lot of copying4

7$t – the pointer copy points to the same elements as thecallee!s array

These elements can easily be moi=e via pointermanip$lation





>

The strcpy /string copy f$nction p$ts this/pse$o callbyreference behavior to goo$se

#"id strcpy(char *buer% char c"nst *string);

#"id (') & char "riginal[4] = d"g;

char c"py[4];

strcpy(c"py% "riginal);

(char [])

"riginal

d(char)

"(char)

g(char)

,-.(char)

(char [])

c"py

???(char)

???(char)

???(char)

???(char)

(char [])

string

(char [])

buer

strcpy

d(char)

"(char)

g(char)

,-.(char)



When can array size be omitted?


?

There are a co$ple of conte(ts in "hich anarray eclaration nee not have a si@especi=e: Parameter eclaration:

int strlen(char string[]); s "e!ve seen% the elements of the array arg$ment are

not copie% so the f$nction oesn!t nee to 8no" ho"many elements there are;

rray initiali@ation:

int #ect"r[] = &1% 2% 3% 4% /; .n this case% $st eno$gh space is allocate to =t all

=ve4 elements of the initiali@er list



Multidimensional arrays


9

Ao" to interpret a eclaration li8e:int d[2][4];

This is an array "ith t"o elements: Each element is an array of fo$r int val$es

The elements are lai o$t se&$entially inmemory% $st li8e a oneimensional array o"maor orer: the elements of the rightmost

s$bscript are store contig$o$sly

(int) (int) (int) (int) (int) (int) (int) (int)

d[0][0] d[0][1] d[0][2] d[0][3] d[1][0] d[1][1] d[1][2] d[1][3]

d[0] d[1]



int d[2][4];

d [1] [2]

Subscripting in a multidimensional array


10

(int) (int) (int) (int) (int) (int) (int) (int)

d[0][0] d[0][1] d[0][2] d[0][3] d[1][0] d[1][1] d[1][2] d[1][3]

d[0] d[1]

*(d+1)*(*(d+1)+2)

.ncrement by the si@eof

1 array of + ints

Then increment by the

si@e of 2 ints



Why do we care about storage order?


11

.f yo$ 8eep "ithin the /paraigm of them$ltiimensional array% the orer oesn!tmatter#

7$t if yo$ $se tric8s "ith pointer arithmetic%

it matters a lot .t also matters for initiali@ation To initiali@e d li8e this:

$se this:

int d[2][4] = &0% 1% 2% 3% 4% /% % ; rather than this

int d[2][4] = &0% 4% 1% /% 2% % 3% ;

0 1 2 3

+ 5 6 >



Multidimensional arrays as parameters


12

,nly the =rst s$bscript may be left $nspeci=e#"id (int matri$[][10]); * *

#"id g(int (*matri$)[10]); * *

#"id h(int matri$[][]); * n"t *

Bhy 7eca$se the other si@es are neee for scaling "hen

eval$ating s$bscript e(pressions see slie 104

This points o$t an important ra"bac8 to C:

rrays o not carry information abo$t their o"n si@es)

.f array si@e is neee% yo$ m$st s$pply it someho"e;g;% "hen passing an array arg$ment% yo$ often haveto pass an aitional /array si@e arg$ment4 –b$mmer

arrays.ppt

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