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CS 100 Lecture 22 1

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• P5 due Thursday

• Final Exam: Tuesday August 10, 8AM - 10AM, Olin 155

• Want references? Remember to drop off a picture and note.

• Check your grades on the door of Upson 5141

CS 100 Lecture 22 2

Today’s Topics

• More C

• Data structures

• Type definitions

• Arrays

• Struct and array parameters

• Strings in C

CS 100 Lecture 22 3

Recall Pointer Parameters// Swap contents of variables pointed to by x and y

void swap (int * x, int * y){int tmp; tmp= *x; *x= *y; *y= tmp;}

Example of a call on procedure swap:

int j= 17;int k= 24;swap(&j, &k);

CS 100 Lecture 22 4

Type Definitions and structs• C provides a way to group a bunch of variables together,

in what is called a struct. • A typedef is similar to a class in Java, but the typedef

doesn’t have methods

C Corresponding Javatypedef struct { public class Pair {

int a; int a;char ch; char ch;

} Pair; }

CS 100 Lecture 22 5

Use of types/structs• Pair abc, xyz;

xyz.ch= ‘q’;abc.a = 5;

• Declaration of a struct variable in C actually creates the structure. No separate allocation is needed, as it is in Java (using new …).

• struct variables may be local to a function,just like ints, chars, and doubles.

CS 100 Lecture 22 6

Struct parametersStructs in C behave exactly like ints and chars in Java. In declaring a variable of some struct type, no reference is

created. A struct variable is passed by value.typedef struct { … } Pair; void fv (Pair p)

{p.a= 42;}

void main (void) {Pair pr;pr.a= 17;pr.ch= ‘?’;fv(pr)}

CS 100 Lecture 22 7

Struct Pointer ParametersIf you want to change a component of a struct variable,

pass a pointer to the structtypedef struct { … } Pair; // As beforevoid fr (Pair * p)

{(*p).a= 42;}void main (void) {

Pair pr;pr.a= 17;pr.ch= ‘?’;fr(&pr)}

Abbreviation. In C, instead of writing( * p) . a

one can writep ->a

CS 100 Lecture 22 8

malloc -- Dynamic Storage Allocation• The standard library function malloc is used to obtain a

block of storage. • It is an extremely low-level function ---its argument is

the number of bytes or words to be allocated (typically obtained using operator sizeof).

• It returns an untyped pointer, which must be cast to the desired type.

• Allocated storage must be explicitly freed by calling procedure free. There is no automatic garbage collection

CS 100 Lecture 22 9

Example with malloc#include <stlib.h>

Pair * p; /* p is a pointer to a Pair*/

p= (Pair *) malloc( sizeof(Pair) );p->a= 17;

…

free(p);

CS 100 Lecture 22 10

C Arrays• C arrays are syntactically similar to arrays in Java, but

the semantics (meaning) is different.• Declaring an array in C actually allocates it.• Declaring an array of structs allocates a block of storage

with space for all the structs in the array.• The array name is effectively a pointer to the first

element of the array.• Array indices start at 0.• There is no built-in function to determine the length of

an array.


Array examplesint b[10];Pair v[10];int k;for (k= 0; k < 10; k= k+1)

b[k]= 0;v[0].a= 10;v[2].ch= ‘!’;

EVIL SYNTAX THAT C ALLOWS: 1[b] = 33; x = 5[b];UGLY UGLY UGLY!


Arrays as parameters

• When an array name is used as a parameter, the “value” of the parameter is a pointer to its first element.

• Therefore, array parameters are always pointers.• The called function refers to the argument array

directly. No copy of the array is made.• An & is usually not needed or appropriate for an

array argument


Example of array parameter

/* Set s[0..n-1] to 0.0 */void zero (double a[ ] , int n) {

int k;for (k= 0; k<n; k= k+1)a[k]= 0.0;

}void main(void) {

double b[100];zero(b, 100);

}


Arrays and Pointers

• Because array names are treated as pointers, C programmers tend to use pointers and arrays somewhat interchangeably.

• Procedure zero could have been written with a pointer parameter, as shown on the next slide.

• It could still be called with an array name as argument.


Array parameter as pointer/* Set s[0..n-1] to 0.0 */

void zero (double * a , int n) {int k;for (k= 0; k<n; k= k+1)

a[k]= 0.0;}

void main(void) {double b[100];zero(b, 100);

}


Arrays of structs as parametersAn array of structs is treated like any other array. If a parameter is an

array of structs, a pointer to the begin-ning of an array (i.e.an array name) is the appropriate argument.

/* Store 0’s and blanks in pa[0..n-1] */function zap (Pair pa [ ], int n) {

int k;for (k= 0; k<n; k++)

{pa[k].a= 0; pa[k].ch= ‘ ’;}

void main( void ) {Pair ray[42];/* initialize ray[0..16] to 0’s and blanks */

zap(ray, 17);}


Structs containing arrays as parametersStructs may contain arrays --but remember that struct

arguments will be copied unless pointers are used.typedef struct {

int a[10];} Ray;

void isCopied(Ray r) {r.a[2]= -1; /* Doesn’t change the argument */ }

void notCopied(Ray * r) {r.a[2]= -1; /* Changes the argument */ }

void main (void) {Ray a;isCopied(a);notCopied(&a);

}


Strings in C

• In C, a string is an array of characters. • The array contains the characters in the string

followed by an extra char (byte) containing a binary zero.

• Example: “hello” is represented by an array:

h e l l o \0


More on Strings• Because strings are arrays, and because array names are

basically pointers, the assignment and comparison operations manipulate pointers, and not the strings themselves.

char s[ ]= “hello”;char t[ ]= s; /* s and t point to same place*/s[1]= ‘?’;/* s and t are now “h?llo” */

• Standard library strlib.h contains routines to copy string values, catenate them, compare them, etc.


END OF NEW MATERIAL• Tomorrow, Friday: review, review,

review

• Check your grades on the door of 5141 Upson this week!

• Remember: Last chance to see me about your final grade is Wednesday before 2PM

• Final is Tuesday at 8AM, Olin 155. Bright and early!

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