1. Initializing variables
Initialization: Giving something a value for the
first time
General rule: variables MUST be initialized
before their values are used
(Do not assume that the compiler initializes
them to 0)
Various ways to initialize
when declaring a variable
int i = 1;
assignment statement
i = 2*j*j;
scanf (or any other function call using &)
scanf("%i",&i);
as a formal parameter in a function
formal parameters are initialized with the
values of the actual parameters.
142 N -1

2. Initialization Quiz initialization Consider: of a (with actual value)
void init_example(int a) {
int b, c, d=10, e[5];
b = 5;
d = 6;
scanf("%d%d",&b,&c); }
initialization of d
initialization
of b
initialization of c
Where is each of a, b, c, d and e initialized?
Note that e is has not been initialized.
How to initialize an array?
142 N -2

3. Array Initializers When declaring: int w[4] = {1, 2, 30, -4};
w has size 4
all 4 elements are initialized
char vowel[6]={'a','e','i','o','u'};
vowel has size 6
Only 5 elements are initialized
vowel[5] is not initialized
Beware:
Can only use the above notation in a
declaration.
Illegal in an assignment statement
w = {1,2,30,-4}; /* illegal */
Can also write a declaration as:
double x[] = {1.0, 3.0, -15.0};
x has size 3
All 3 elements are initialized
However can't write (except see later...):
double x[]; /* illegal */
142 N -3

4. Array elements as parameters
When using an array element as a function
parameter, treat the array element as a
regular variable:
e.g.
printf("grades are %f and %f",
grade[0],grade[1]);
same syntax as for
a regular variable
swap(&grade[i], &grade[i+1]);
142 N -4

5. Arrays as parameters Can also pass a whole array as a parameter
#define ARRAY_SIZE 200
...
double average(int a[ARRAY_SIZE]) {
int i, total=0;
for(i=0; i<ARRAY_SIZE; i++)
total = total + a[i];
return
((double)total/(double)ARRAY_SIZE); }
/* in another function */
int x[ARRAY_SIZE];
x_avg = average(x);
optional
just write the name
of the array
142 N -5

6. Arrays as output parameters
a program to sum vectors
void vector_sum(int a[3], int b[3],
int vsum[3]) {
int i;
for(i=0; i<3; i++)
vsum[i] = a[i] + b[i]; }
int main(void)
int x[3]={1,2,3}, y[3]={4,5,6};
int z[3];
vector_sum(x,y,z);
printf("%d %d %d",z[0],z[1],z[2]);
return 0;
no &
Why?
The name of an array is a pointer to the first
element of the array.
142 N -6

7. Array parameter summary
Passing array elements:
Just like regular variables
passed by value
can pass the address if one uses &
Passing whole arrays:
Whole arrays are NOT passed by value
(not copied)
as a formal parameter
type array_name[SIZE]
or type array_name[]
as an actual parameter
array_name
no & or []
142 N -7

8. An Array as a pointer In the memory: for an array A of size n A[0]
...
pointed to by A
(the value of A is the address of the box
that holds A[0])
What is *(A+i) ?
A[i]
142 N - 8

9. Pointer Arithmetic add an integer to a pointer Can
subtract an integer from a pointer
subtract a pointer from a pointer
*((iptr1 + iptr2)/2) /* Error */
*(iptr1 + (iptr2-iptr1)/2) /* OK */
an integer
142 N -9

10. Pointer arithmetic int *iptr;
If iptr is 1031 and integers are stored on 2
bytes what is iptr + 1 ?
one byte
( = 8 bits)
1031
1032
1033
1034
1035
another
integer
iptr+1
one
integer
iptr
pointer arithmetic is done in units of the
pointer's base type.
This is why *(A+i) is the same as A[i]
142 N -10