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C Programming Language
OUTLINE
Repetition and Loop Statements
Arrays
Multi-Dimensional Arrays
Repetition and Loop
Statements
Categories of Flow Controls
•Relational, Equality, Logical Operators are provided
to facilitate flow controls.
•Sequential - Statements in a program are executed
one after another.
•Selection - A choice of alternative actions (if, if-else,
and switch).
•Repetition - Repeat certain actions (while, for, and
do).
3
Loops
So far our programs are very limited - they can only run
“straight through” the code.
But many situations require programs to repeat actions.
Repeating actions is called iteration
The control structures that we use to perform iteration are
called loops - we speak of programs “looping”.
4
A “loop” is a repeated (“iterated”) sequence of
statements.
Like conditionals, loops let us control the flow of our
program in powerful ways.
Like functions, loops take code that has been generalized
and execute it many times.
Block of code
within loop is
known as the
loop body
5
We will study three types of loops:
- while loops
- for loops
- do…while loops
We will also look at nested loops - what happens when
you put one loop inside another
6
• Counter-controlled repetition
– Definite repetition: know how many times
loop will execute
– Control variable used to count repetitions
• Sentinel-controlled repetition
– Indefinite repetition
– Used when number of repetitions not
known
– Sentinel value indicates "end of data"
7
• Counter-controlled repetition requires
– The name of a control variable (or loop counter).
– The initial value of the control variable.
– A condition that tests for the final value of the
control variable (i.e. whether looping should
continue).
– An increment (or decrement) by which the
control variable is modified each time through the
loop.
8
The while loop
loop repetition condition
The syntax is
while (condition) {
statements;
}
loop body
Note indentation!
Its meaning is
(a) evaluate the condition
(b) if its value is true (i.e., not zero) do the statements, and go
back to (a)
The while loop will repeatedly execute the statements in
the loop body until the condition is false.
Something in the body of the while loop must change
the condition, or the loop will never end!
9
Steps in using while loop
1. Initialize loop count (loop control variable)
2. Test loop repetition condition for termination
3. Update loop count
10
Example - Add five integers:
int i = 0;
// initialize i
int temp, sum = 0;
while (i < 5) {
// initialize sum
// test if i < 5
scanf(“%d”, &temp);
sum = sum + temp;
i++;
// read an integer
// add to sum
// increment i
}
Important to initialize the loop count before starting the
loop, and to increment the count within the loop.
11
Example - Accumulating a sum
int sum, x, count;
int number_inputs; /* Number of inputs */
sum = 0;
printf("How many numbers? ");
scanf("%d", &number_inputs);
printf("Enter %d numbers: ", number_inputs);
count = 1;
Initialization
while ( count <= number_inputs ) {
scanf("%d", &x);
Condition
sum = sum + x;
count = count + 1;
Update
}
12
Example - Calculate pay
count_emp = 0;
/* no employees processed yet */
while (count_emp < num_emp) {
/* test value of count_emp
printf("Hours> ");
scanf("%d", &hours);
printf("Rate> ");
scanf("%lf", &rate);
pay = hours * rate;
printf("Pay is $%6.2f\n", pay);
count_emp = count_emp + 1; /* increment count_emp */
}
printf("\nAll employees processed\n");
13
*/
Example - Computing a Factorial
7! = 1 * 2 * 3 * 4 * 5 * 6 * 7
We could write this without a loop (Yuck!). But easier to
write it with a loop.
x = 1 * 2 * 3 * 4 * 5 * 6 * 7;
printf ( "%d", x ) ;
int i = 1;
int product = 1;
while(i <= 7){
product = product * i;
i++;
}
14
Example - Factorial Function
Even better if we generalize the algorithm and make it in
to a function. Can compute N!
int fact(int number){
int i = 1;
int product = 1;
Local variables re-initialized at each call
while(i <= number){
product = product * i;
i++;
}
return product;
}
15
Infinite Loop Using while
while (1)
statement;
next statement;
•Stop the program in the operating system level, for
example, Ctrl-C or Ctrl+Break in DOS.
•There is a break statement for terminating the loop
(discussed later).
•Use carefully!
16
The for Loop
The syntax is
for ( initialization ; condition; update ) {
statements;
}
Note that the for loop statement, as well as having a
condition, has an initialization and an update part also.
Another construct for doing the same thing - iteration.
17
If the initialization, condition and/or update parts are
long, place each on a separate line for clarity.
for ( initialization;
condition;
update ) {
statements;
}
next_statement;
Execution Steps:
1. initialization
2. condition expression is evaluated.
- if true,
(a) statements are executed.
(b) update expression is executed.
(c) goto step (2) again.
- if false, next statement is executed.
18
for loop versus while loop
for and while loops can always be interchanged …
but some cases are much better suited to for loops
than while loops.
i = 0;
while (i < 10) {
for(i = 0; i < 10; i++) {
printf(“%d\n”, i);
i++;
printf(“%d\n”, i);
}
}
19
Which Loop Should You Choose?
They are interchangeable, so to some extent it is
personal preference.
Use the for loop if you know how many times you
are going to do something.
Think which construct will yield the simplest,
clearest code.
20
Example
total_pay = 0.0;
for
(count_emp = 0;
/* initialization
*/
count_emp < number_emp;
/* loop repetition condition */
count_emp += 1) {
/* update
*/
printf("Hours> ");
scanf("%lf", &hours);
printf("Rate > $");
scanf("%lf", &rate);
pay = hours * rate;
printf("Pay is $%6.2f\n\n", pay);
total_pay = total_pay + pay;
}
printf("All employees processed\n");
printf("Total payroll is $%8.2f\n", total_pay);
21
Example - Factorial
int factorial(int n)
{
int i,
/* local variables */
product;
/* accumulator for product computation */
product = 1;
for (i = n;
i > 1;
--i) {
product = product * i;
}
return (product);
Note that we may
decrement or
increment the loop count
- it depends on how
we write the initialization
and condition parts
}
22
Increments and decrements other than one
Not obliged to increment / decrement the loop count
by 1.
May use any value.
23
Infinite Loop Using for
for (;;) {
statements;
}
•Stop the program in the operating system level, for
example, Ctrl-C or Ctrl+Break in DOS.
•There is a break statement for terminating the loop
(discussed later).
•Use carefully!
24
Example - Celsius / Fahrenheit table
#define CBEGIN 10
#define CLIMIT -5
#define CSTEP 5
/* Variable declarations */
int
celsius;
double fahrenheit;
/* Display the table heading */
printf("
Celsius
Fahrenheit\n");
/* Display the table */
for (celsius = CBEGIN;
celsius >= CLIMIT;
celsius -= CSTEP) {
fahrenheit = 1.8 * celsius + 32.0;
printf("%6c%3d%8c%7.2f\n", ' ', celsius,
' ', fahrenheit);
}
Practice: C-to-F-table-part.c
25
The Comma Operator and for
expr1 , expr2
• Lowest precedence of all operators.
• Left-to-right associativity.
• Value of expr2 taken as value of the whole expression.
• Example, a = 0 , b = 1;
• Example
for (i=1, factorial=1; i<=n; i++)
factorial *= i;
26
Conditional Loops
In some situations, you may not know the exact number
of loop iterations required.
It is still possible to write a loop in such circumstances.
Steps:
1. Initialize loop control variable
2. As long as exit condition has not been met
3. Continue processing
27
Loop Design
Sentinel-controlled Loops
Input a list of data values of any length, terminated by
a special (sentinel) value.
double variable; /* current input */
declarations;
initial statements;
scanf("%lf", &variable);
while (variable is not equal to sentinel){
process;
scanf("%lf", &variable);
}
final statements;
28
Example - Average Inputs
printf ( "Enter values to average, end with -1.0 \n") ;
sum = 0.0 ;
count = 0 ;
scanf ( "%lf", &next ) ;
while ( next != -1.0 ) {
sum = sum + next ;
Loop terminates when a
sentinel value (-1) is
entered at keyboard
count = count + 1;
scanf ( "%lf", &next ) ;
Comments
}
if (count > 0)
printf( "The average is %f. \n", sum / (double) count );
29
Endfile-controlled Loops
Input from a file of data and do not know how many data
items there are.
Files have a special marker to indicate the end of data end-of-file (eof) marker
Condition to terminate loop can be controlled by using
the eof marker
- in C, the eof marker is denoted by the reserved
word, EOF
30
Example
sum = 0;
input_status = fscanf(inp, "%d", &score);
while (input_status != EOF) {
printf("%5d\n", score);
sum += score;
input_status = fscanf(inp, "%d", &score);
}
31
Nested Loops
How would you print the following diagram?
*****
*****
*****
Solution:
repeat 3 times
print a row of 5 *s
repeat 5 times
print *
print newline
It seems as if a loop within a loop is needed!
32
When you put one loop inside another, then you
have nested loops
Nested loops can be much more confusing than
single loops!
33
#define ROWS 3
#define COLS 5
…
row = 1;
while ( row <= ROWS ) {
/* print a row of COLS *s */
...
row = row + 1;
}
34
#define ROWS 3
#define COLS 5
row = 1;
while ( row <= ROWS ) {
/* print a row of 5 *s */
col = 1;
while (col <= COLS) {
printf("*");
col = col + 1;
Inner loop print one row
Outer loop print three rows
}
printf( "\n" );
row = row + 1;
}
35
#define ROWS 3
#define COLS 5
outer loop - print 3 rows
...
for ( row = 1; row <= ROWS ; row = row + 1 ) {
for ( col = 1 ; col <= COLS ; col = col + 1 ) {
printf(“*”);
}
printf( "\n" );
inner loop - print one row
}
36
How would you print the following diagram?
*
**
Solution:
***
for every row ( row = 1, 2, 3, 4, 5 )
print row stars
****
*****
#define ROWS 5
...
int row, col ;
for ( row = 1 ; row <= ROWS ; row = row + 1 ) {
for ( col = 1 ; col <= row ; col = col + 1) {
printf( “*” ) ;
}
printf( "\n" );
}
37
Remember the function to print rows of asterisks
between lines of output?
Now we can write a generalized function:
void print_banner( int lines, int numchar, char ch ) {
int i, j ;
for ( i = 0 ; i < lines ; i = i + 1 ) {
for ( j = 0 ; j < numchar ; j = j + 1 )
printf ( "%c", ch);
}
print (“\n”);
}
}
38
Exercise
Study and execute the following
program.
#include <stdio.h>
#define N 7
int main(void)
{
int cnt = 0, i, j, k;
for (i = 0; i <= N; ++i)
for (j = 0; j <= N; ++j)
for (k = 0; k <= N; ++k)
if (i + j + k == N) {
++cnt;
printf("%3d%3d%3d\n", i, j, k);
}
printf("\nCount: %d\n", cnt);
return (0);
}
39
do...while loop
The do…while loop is like a while loop, but it does the
condition test at the end of the loop
 all statements in loop body are executed at least
once
This can be very useful for doing things like checking
user input.
40
41
Example
Checking user input:
do {
printf(“Enter your age: “);
scanf(“%d”, &age);
} while (age < 0);
42
Example
counter = 1;
do {
printf( "%d ", counter );
} while (++counter <= 10);
Example - sum the digits of a positive number
sum = 0;
do {
sum += n % 10;
n /= 10;
}
while(n > 0);
printf("The sum of the digits is =
%d\n", sum);
43
#include <stdio.h>
while condition
void main(){
int n, sum;
while ( (printf("Enter number = ") && scanf("%d",&n) && n) != 0 )
{
sum = 0;
true (1)
do {
sum += n % 10;
true (1)
false (n==0)
true (n != 0)
n /= 10;
}
while(n > 0);
printf("The sum of the digits is = %d\n", sum);
}
}
Practice: sum-digit.c
44
Controlling Repetition
break and while
#include <stdio.h>
Causes an exit from the innermost enclosing loop.
#include <math.h>
int main(void)
{
int x;
while (1) {
scanf("%d", &x);
If (x < 0)
break;
printf ("square root = %.2f\n",
sqrt(x));
}
printf ("Bye!\n");
return (0);
}
45
Controlling Repetition …
break and for
#include <stdio.h>
Causes an exit from the innermost enclosing loop.
int main(void)
Will update be executed before leaving the for
loop?
{
int i, x;
for (i=0; i<10; i++) {
printf("i = %d\t", i);
printf("x = ? ");
i=0
x = ? 10
i=1
x = ? 20
i=2
x=?4
i=3
x=?5
i=4
x=?0
After the loop, i = 4
Bye!
scanf("%d", &x);
if (x==0)
break;
}
printf("After the loop, i = %d\n", i);
printf ("Bye!\n");
return (0);
}
46
Controlling Repetition …
continue
Causes the current iteration of
a loop to stop, and begins
the next iteration.
#include <stdio.h>
#define MAX 5
int main(void)
{
int data, sum=0, k;
for (k=0; k<MAX; k++) {
scanf ("%d", &data);
if (data <= 0)
continue;
sum += data;
}
printf ("Sum of positive values is %d\n.", sum);
return (0);
}
10
20
-1
90
-5
Sum of positive values is 120.
47
How to debug and test programs
• Debugger Program
– Single-step execution (F8)
– Add Watch-window (Alt+W W)
– Breakpoints
• Debugging without a debugger
– Insert additional diagnostic calls to printf to trace the values
of critical values
#define DEBUG 0
if (DEBUG)
printf ……
48
Common Programming errors
• Off-by-One Loop errors
for (i =0; i<n; i++)
sum+= i;
for (i =0; i<=n; i++)
sum+= i;
Loop body is executed n times
Loop body is executed n+1 times
49
Common Programming errors …
Syntax of the for statement
for (initialization expression;
loop repetition condition;
update expression )
Do not put ; here
Always use braces around the loop body, whether it
contains one or many statements
50
Common Programming errors …
• Be careful when testing for inequality (!=) to
control repetition of a loop (infinite loop may
result)
• In sentinel-controlled loop:
– Provide a prompt to program’s user what value to
enter as sentinel
– Make sure that the sentinel value cannot be
confused with a normal data item
• Do not mistype equality test (==) and
assignment operator (=)
51
Arrays
Up to now have dealt with a small number of variables
and we have had a separate name for each one
- these are so-called scalar variables.
But what if we have
10 variables?
100 variables?
1000 variables?
…
53
The C programming language (like many other languages)
has a construct that allows us to deal with a collection of
many similar variables, and identify the collection with a
single name.
This construct is called an array - a structured variable
- data structure
54
Data Structures
Functions give us a way to organize programs (code).
Data structures are needed to organize data, especially:
- large amounts of data
- variable amounts of data
- sets of data where the individual pieces are
related to one another
The array is our first view of such data structures.
55
Array
– A structure of related data items
– A static entity – same size throughout
program - size cannot be altered, i.e.
number of elements in collection is fixed
at compile time.
– But there are dynamic data structures see later!
Definition: a named, ordered collection of
variables of identical type
56
Array
– Group of consecutive memory locations
– Elements of array referred to by one (same) name
– All elements must be of same type
To refer to an element of an array, must specify
– Array name, and
– Position in array - index or subscript
– either by number or arithmetic expression
Syntax:
arrayname[position number]
– First element is at position 0
– n element array named c
c[0], c[1] ... c[n – 1]
57
Name of array is c
(Note that all elements
of the array have the
same name)
c[0]
-45
c[1]
6
c[2]
0
c[3]
72
c[4]
1543
c[5]
-89
c[6]
0
c[7]
62
c[8]
-3
c[9]
1
c[10]
6453
c[11]
78
Position number of
the element within
array c - starting at 0
58
Array elements are like normal variables - manipulate
them just like simple (scalar) variables:
E.g.
c[0] = 3;
printf( "%d", c[0] );
– Perform operations on subscript.
If x equals 3, then
c[5 - 2] == c[3] == c[x]
59
Initialization of array elements
Like simple variables, array elements may be initialized
by assignment, by inputting into array element
position, or at declaration time.
E.g.
int n[5] = { 1, 2, 3, 4, 5 };
If not enough initializers are specified, the
rightmost elements become 0
E.g.
int n[5] = { 0 };
- All elements initialized to 0
60
If size of array is omitted, initializers determine it
int n[ ] = { 1, 2, 3, 4, 5 };
– Five initializers, therefore a 5 element array
The above definition is equivalent to:
int n[5] = { 1, 2, 3, 4, 5 };
61
Example
Grades of 7 students
62
How Do We Average These?
Use a loop to add them up:
sum = 0
for each array index
add the indexed value to sum
When finished, divide like usual
double grades[7] =
{1.2, 4.0, 3.8, 2.5, 3.3, 0.0, 3.4} ;
int i;
double sum = 0, average;
...
for (i=0; i<7; i++){
sum = sum + grades[i];
}
average = sum/7.0;
63
Declaring Arrays
• When declaring arrays, must specify
– Name of array
– Data type of array elements
– Number of elements in array
arrayType
arrayName[numberOfElements];
– Examples:
int c[10];
float myArray[999];
• Declaring multiple arrays of same type
– Format similar to regular variables
– Example:
int b[100], x[27];
64
Terminology
double rain[7];
rain is of type array of double with size 7.
rain[0], rain[1], ... rain[6] are the elements of the
array rain. Each is a variable of type double.
0,1, ... 6 are the indices of the array. Also called
subscripts.
The bounds are the lowest and highest values of the
subscripts (here: 0 and 6).
65
When you declare an array, the size must be an
integer constant!
Consider this set of declarations:
int size = 5;
double foo[3.0];
char bar[size];
Wrong!
Right!
66
Accessing Array Elements
If we have an array int foo[100], then we can
access each element using its subscript:
E.g.
foo[3] = 12;
printf(“theValue: %d\n”, foo[20]);
scanf(“%d”, &foo[66]);
for (i = 0; i < 100; i++)
foo[i] = -10;
Arrays are ideal for
using in loops!
67
When Can You Use Array Elements?
You can use an array element wherever you could use a
normal variable of the same type
- If we have an array of doubles, for example,
each element is of type double
68
Array Indices (or Indexes)
Array indexes always begin at 0 in C
There is no way to change this!
What are valid indexes in this array?
char Characters[20];
Characters[0] is the name of the first element, not
Characters[1]. The last element is Characters[19],
not Characters[20].
69
Index Rule
Rule: An array index must evaluate to an int between 0
and n-1, where n is the number of elements in the array.
No exceptions!
Example:
rain[i+3+k]
/* OK as long as 0 < i+3+k < 6 */
The index may be very simple
rain[0]
or incredibly complex
rain[(int) (3.1 * fabs(sin (2.0*PI*sqrt(29.067))))]
70
Array Bounds
These array indexes are not checked!
This is called array bounds checking.
What happens if you access element 10 in this array?
double foo[10];
Trouble!
71
What You Cannot Do With Arrays
You cannot use = to assign arrays.
You cannot use == to compare two arrays.
You cannot scanf or printf an entire array.
But, you can do these things to the individual array
elements.
72
More flexible way to declare arrays
#define
hours_size
10
int main()
{
double hours[hours_size];
Also creates an array with 10 storage locations for
array elements. Can change this to 20 elements easily
- just change the constant definition:
#define hours_size
20
73
We use for loops often to process arrays. Enables us
to access all array elements in sequence and to
process all the elements in a consistent and uniform
manner.
#define hours_size 10
double hours[hours_size];
int i;
/* Set all array elements to 0 */
for (i = 0; i < hours_size; i++)
hours[i] = 0;
/* What does this one do? */
for (i = 0; i < hours_size; i++)
hours[i] = 10 * i;
/* answer - sets array elements to 0, 10, 20, ... */
74
/* What about this one? */
hours[0] = 20;
for (i = 1; i < hours_size; i++)
hours[i] = hours[i-1] + 5;
/* answer - sets array elements to 20, 25, 30, ... */
/* print the array subscripts and element values */
printf(“%s
%s\n”, “I”, “hours[i]”);
for (i = 0; i < hours_size; i++)
printf("%d
%f\n", i, hours[i]);
Reading data into an array:
Read the values one at a time.
for (i = 0; i < hours_size; i++)
{
printf("Enter value for hours[%d]: ", i);
scanf("%lf", &hours[i]);
}
75
Write a loop that finds the sum of all values in array
hours. Also write a statement to find the average of
these values.
double sum, average;
sum = 0;
for (i = 0; i < hours_size; i++)
sum += hours[i];
/* sum = sum + hours[i];
*/
average = sum / hours_size;
76
E.g. Grading program
#include <stdio.h>
#define NUM_QUEST 10
#define NUM_CLASS_DAYS 5
typedef enum
{Monday, Tuesday, Wednesday, Thursday, Friday}
class_days;
void main () {
char answer[NUM_QUEST]= {'T','F','T','T','T','F','T','T'};
int score[NUM_CLASS_DAYS] = {0}, i;
for (i = 0; i < 10; i++) {
score[Monday] += answer[i] == 'T';
}
printf("\nMonday results = %d\n", score[Monday]);
}
77
E.g. Grading program ….
#include <stdio.h>
#define NUM_QUEST 10
What will happen if make this change?
#define NUM_CLASS_DAYS 5
typedef enum
{Monday, Tuesday, Wednesday, Thursday, Friday}
class_days;
void main () {
char answer[NUM_QUEST]= {'T','F','T','T','T','F','T','T'};
int score[NUM_CLASS_DAYS] = {0}, i;
for (i = 0; i < 10; i++) {
score[Friday + 1] += answer[i] == 'T';
}
printf("\nMonday results = %d\n", score[Friday + 1]);
}
78
Using array elements as function arguments
You can pass individual array elements to a function
just like other variables.
Passing array elements:
– Passed by call-by-value
– Pass subscripted name (e.g. myArray[3]) to
function.
– Array element must correspond to formal
parameter in type.
79
If you have declared a function with prototype
void my_fun(int, double);
and arrays
int id[10];
double hours[10];
then you can write
my_fun(id[3], hours[9]);
Array element id[3] (type int variable) is the first actual
argument and array element hours[9] (type double
variable) is the second actual argument.
80
Individual array elements can be used as parameters, just
like other simple variables.
Examples:
printf( "Last two are %f, %f", rain[5], rain[6] );
draw_house( color[i], x[i], y[i], windows[i] );
scanf( "%lf", &rain[0] );
swap( &rain[i], &rain[i+1] );
81
Array arguments
You can also pass entire arrays to functions as
arguments.
Passing arrays:
– To pass an array argument to a function, specify the
name of the array without any brackets :
int myArray[24];
myFunction( myArray, 24 );
• Array size usually passed to function.
– Arrays are passed by call-by-reference
– Name of array is the address of first element.
 Function knows where the array is stored
 May modify original memory locations.
82
An array is never copied (no call by value)
The array name is always treated as a pointer parameter.
The & and * operators are not used.
Programming issue: in C, arrays do not contain
information about their size, so the size often needs to
be passed as an additional parameter.
83
Example: function print_int_array displays all
elements of a type int array.
Number of elements in array is a parameter
since we do not know the size of the actual
argument array when function is called.
void print_int_array(int list[], /* array to print */
int n)
/* number of elements */
{
int i;
/* subscript and loop control variable */
for (i = 0; i < n; i++)
printf("%
%d", i, list[i]);
}
There is no actual array with the name list. The array
that is processed is determined by the first actual
argument in a function call.
84
The statement
print_int_array(id, 10);
displays all values in array id (array id corresponds
to the first formal parameter - int list[]). Each
reference to list[i] processes the element in actual
array id with subscript i.
Notice there are no brackets after the array name in the
function call, but brackets are required to identify the
formal parameter as an array (int list[]).
The function prototype is:
void print_int_array(int[], int);
First parameter is an int array
85
#define ARRAY_SIZE 200
double average( int a[ARRAY_SIZE] ) {
int i, total = 0;
for ( i = 0 ; i < ARRAY_SIZE ; i = i + 1 )
total = total + a[i];
return ((double) total / (double) ARRAY_SIZE);
}
int x[ARRAY_SIZE];
...
x_avg = average ( x );
86
void vectorSum( int a[3], int b[3], int vsum[3] )
{
int i;
for ( i = 0 ; i < 3 ; i = i + 1 )
Note:
vsum[i] = a[i] + b[i];
No *
}
No &
int main(void)
{
int x[3] = {1,2,3}, y[3] = {4,5,6}, z[3];
vectorSum( x , y , z );
printf( "%d %d %d", z[0], z[1], z[2] );
}
87
Function to add two arrays and store the result in a third
array.
void add_arrays(double a[],
double b[],
double sum[],
int n)
{
int i;
/*
/*
/*
/*
input - first array */
input - second array */
output - array of sums*/
input- number of elements*/
for (i = 0; i < n; i++)
sum[i] = a[i] + b[i];
}
Notice that the assignment statement in the function
changes the value stored in the element of array sum with
subscript i. This is an example of a function that
changes an argument value. The array is considered a
function output argument, not an input argument.
88
Multi-Dimensional Arrays
Arrays as Data Structures
Review: An array is an ordered collection of values of
identical type Name the collection; number the elements
Arrays are the natural choice for organizing a large
number of values, all of identical type.
In the mathematical sense, a (one-dimensional) array
represents a vector.
90
Beyond Simple Arrays
Sometimes the collection of values has some additional
regular pattern or structure.
One common such structure is the matrix or table
In C, we can express this as a two-dimensional array.
Higher-dimensional arrays (3-D, 4-D, …) are possible,
but we will not use them in this course.
91
Two-Dimensional Arrays
An ordered collection of values of identical type -
- Name the collection; number the elements
Sounds familiar? Just like 1-D arrays, but a different
numbering scheme.
Rows
Columns
92
Example: scores for 7 students on 4 homeworks
int score[7][4];
Number of rows
Number of columns
First dimension specifies number of rows
Second dimension specifies number of columns
Note that element indexes run from 0 - 6 and 0 - 3
93
2-D Arrays: Terminology
Syntax:
type name[# rows][# columns]
E.g.
int score[7][4];
says that score is a two-dimensional array of int of size
7 by 4
score[0][0], score[0][1], .... , score[6][3] are
the elements of the array
94
Declaring a 2-D Array
#define MAX_STUDENTS 7
#define MAX_HWKS 4
...
int score [MAX_STUDENTS][MAX_HWKS];
95
An Alternate View
int score[7][4];
We could also view each row as an element:
“score is an array of size 7”
With this view, each element (i.e. row) is a 1-D array of
type “array of size 4 of int”
We can either think of scores as a 7 by 4 array of scores
... or as an array of 7 one dimensional arrays of size 4
96
Accessing Array Elements
We access the elements of the array just like we did
for 1-D arrays, but now with 2 subscripts:
E.g.
scores[5][1];
As before, each element of the array behaves exactly
like a regular variable.
97
Reading in an Array
Use nested for loops
E.g. read in assignment scores for 7 students:
int i, j, scores[7][4];
for (i=0; i<7; j++
{
for (j=0; j<4; i++)
{
scanf(“%d”, &scores[i][j]);
}
}
Read row by row
98
Printing the Array
Again, use for loops
for (i=0; i<7; j++)
{
for (j=0; j<4; i++)
{
printf(“%d”, &scores[i][j]);
}
}
99
Bookkeeping
As with 1-D arrays, often we only use part of the space
available in a 2-D array.
Declared size of the array specifies its maximum capacity.
The current size (# of rows and columns currently in use)
needs to be kept track of in separate variables
100
Reading in Data
Problem: Read in data for student assignments
Input data format: The number of students, then the
number of assignments, followed by the data per
student.
A nested loop is the right program structure for reading
in the data details
int score [MAX_STUDENTS][MAX_HWKS] ;
int nstudents, nhwks, i, j ;
101
Reading a 2-D Array
/* Read the number of students and assignments,then
loop to read detailed data */
scanf ("%d %d", &nstudents, &nhwks);
if (nstudents <= MAX_STUDENTS && nhws <= MAX_HWKS)
{
for ( i = 0 ; i < nstudents ; i = i + 1 )
for ( j = 0 ; j < nhwks ; j = j + 1 )
scanf("%d", &score [i][j]) ;
}
Partially-used array
102
Printing a 2-D Array
if (nstudents <= MAX_STUDENTS && nhwks <= MAX_HWKS)
{
for ( i = 0 ; i < nstudents ; i = i + 1 )
{
for ( j = 0 ; j < nhwks ; j = j + 1 )
printf("%d", score [ i ][ j ]);
printf("\n") ;
}
}
103
2-D Arrays as Parameters
Same as 1-D arrays (almost!):
- Individual array elements can be either value or
pointer parameters.
- Entire arrays are always passed as pointer
parameters - never copied
- Don’t use & and * with entire array parameters
Difference:
- Must always specify the size of each dimension
except for the first one - no empty brackets,
except the first
104
2-D Array As Parameter
A function to read into array a the grade information for
the given number of students and assignments
void read_2D( int a[MAX_STUDENTS][MAX_HWS],
int nstudents, int nhws )
{
...
}
105
Array Function Arguments
int main(void)
{
int score[MAX_STUDENTS][MAX_HWS];
int nstudents, nhws;
scanf (“%d %d”, &nstudents, &nhws);
if ( nstudents <= MAX_STUDENTS && nhws <= MAX_HWS)
read_2D (score, nstudents, nhws) ;
...
}
106
Example - Images
An image is normally stored
as a 2D array - a rectangular
grid.
Image may be moved across
screen - animation - by repositioning black and white
squares of image in the grid.
107
Representation of Arrays
A computer’s memory is a one-dimensional array of
cells.
How is a 2-D array stored?
Answer: In C, the array rows are stored sequentially:
row 0, 1, 2, …
We think:
Actually:
108
Representation
Each row of the array is laid out sequentially in memory,
one after the other.
109