Arrays and Matrices
One-Dimensional Arrays
• An array is an indexed data structure
• All variables stored in an array are of the
same data type
• An element of an array is accessed using the
array name and an index or subscript
• The name of the array is the address of the
first element and the subscript is the offset
• In C, the subscripts always start with 0 and
increment by 1
Definition and Initialization
• An array is defined using a declaration
statement.
data type array_name[size];
– allocates memory for size elements
– subscript of first element is 0
– subscript of last element is size-1
– size must be a constant
Example
int list[10];
– allocates memory for 10 integer variables
– subscript of first element is 0
– subscript of last element is 9
– C does not perform any bounds
checking on arrays
list[0]
list[1]
list[9]
Initializing Arrays
• Arrays can be initialized at the time
they are declared.
Examples:
double taxrate[3] ={0.15, 0.25, 0.3};
char list[5] = {‘h’,’e’,’l’,’l’,’o’};
double vector[100] = {0.0}; /*assigns zero to all 100
elements*/
int s[] = {5,0,-5};
/*the size of a s is 3 */
Assigning values to an array
• for loops are often used to assign values to
an array
Example:
int list[10], i;
for(i=0; i<10; i++)
{
list[i] = i;
}
Input from a data file
• Arrays are often used to store information from a data
file
• Example
int k;
double time[10], motion[10];
FILE *sensor3;
sensor3 = fopen(“sensor3.txt”, “r”);
for(k=0; k<10; k++)
{
fscanf(sensor3, “%lf %lf”,&time[k], &motion[k]);
}
Practice!
Show the contents of the arrays defined in each of the following sets of
statements.
a)
b)
c)
d)
int x[10] = {-5, 4, 3};
char letters[] = {'a', 'b', 'c'};
double z[4];
………
z[1]=-5.5;
z[2]=z[3]=fabs(z[1]);
int k;
double time[9];
………..
for(k=0;k<=8;k++)
time[k]=(k-4)*0.1;
Practice
• Assume that the variable k and the array s have been
defined with the following statement:
int k, s[]={3,8,15,21,30,41};
• Using a hand calculation, determine the output for
each of the following sets of statements:
1. for(k=0;k<=5;k+=2)
printf(“%d %d \n”,s[k],s[k+1]);
2. for(k=0;k<=5;k++)
if(s[k]%2==0)
printf(“%d “,s[k]);
printf(“\n”);
Function Arguments
• When the information in an array is
passed to a function, two parameters
are usually used.
• One parameter specifies a particular
array.
• Other parameter specifies the number
of elements used in the array.
Passing Entire Arrays as
Arguments to Functions
• Arrays are always pass by reference
• The array name is the address of the first
element
• The maximum size of the array must be
specified at the time the array is declared.
The actual number of array elements that are
used will vary, so the actual size of the array
is usually passed as another argument to the
function
Example
int main(void)
{
/* Declare variables and functions */
FILE *exp1;
double max (double array[], int actual_size);
double x[100];
int count=0;
exp1 = fopen(“exp1.txt”, “r”);
while((fscanf(exp1, “%f”, &x[count])) == 1)
{
count++;
}
printf(“Maximum value: %f \n”, max(x, count));
fclose(exp1);
return 0;
}//end main
Selection Sort
Example:
Ascending algorithm
• Find the minimum value
• Exchanging the minimum value with the value in the
first position in the array
• Find the next minimum value begin with second
element and exchange this minimum value with the
second position in the array
• Continue until the last element
Two-Dimensional Arrays
2
3
-1
row 1
0
-3
5
row2
2
10
3
column 0
column 1
column 2
row 0
Definition and Initialization
• int x[rows][columns];
• Example, int x[4][3];
int x[4][3]={{2,3,-1},{0,-3,5},{2,6,3},{-2,10,4}};
• int x[][3]={{2,3,-1},{0,-3,5},{2,6,3},{-2,10,4}};
• If the initializing sequence is shorter than the
array, the rest of the values are initialized to
zero.
Nested ‘for’ loop
• Arrays can also be initialized with program statements.
• Two nested ‘for ‘ loops are usually required to initialize an array.
/* Declare variables */
int i, j, t[5][4];
………….
…………
/* Initialize array. */
for(i=0;i<=4;i++)
for(j=0;j<=3;j++)
t[i][j]=i;
Example
#include <stdio.h>
int main()
{
int i,j;
int data[3][4]={{1,2,3,4},{5,6,7,8},{9,10,11,12}};
for (i=0;i<3;i++)
{
for(j=0;j<4;j++)
printf("%2d ",data[i][j]);
printf("\n");
}
return 0;
}
Matrices
• A matrix is a set of number arranged in a grid
with rows and columns. A matrix is defined
using a type declaration statement.
– data type array_name[row_size][column_size];
– int matrix[3][4];
row[0]
row[1]
row[2]
in memory
row0 row1 row2
Accessing Array Elements
• int matrix[3][4];
–
–
–
–
–
matrix has 12 integer elements
matrix[0][0]
element in first row, first column
matrix[2][3]
element in last row, last column
matrix is the address of the first element
matrix[1] is the address of the second row
Practice!
int i, j, g[3][3]={{0,0,0},{1,1,1},{2,2,2}};
Give value of sum after each statements is executed:
1) sum=0;
for (i=0;i<=2; i++)
for(j=0;j<=2;j++)
sum += g[i][j];
2) sum=1;
for(i=1;i<=2;i++)
for(j=0;j<=1;j++)
sum *= g[i][j];
3) sum=0;
for(j=0;j<=2;j++)
sum -= g[2][j];
4) sum=0;
for(i=0;i<=2;i++)
sum += g[i][1];
2-Dimensional Arrays as
Arguments to Functions
Example:
void transpose(int b[NROWS][NCOLS], int bt[NCOLS][NROWS])
{
/* Declare Variables. */
int i, j;
/* Transfer values to the transpose matrix. */
for(i=0; i<NROWS; i++)
{
for(j=0; j<NCOLS; j++)
{
bt[j][i] = b[i][j];
}
}
return;
}
Character Functions
•
Character I/O
Example 1,
printf and scanf functions- print and read characters using %c specifier.
Example 2,
putchar- takes one integer argument, returns an integer value.
putchar(‘a’);
putchar(‘b’);
putchar(‘\n’);
putchar(‘c’);
same as:
putchar(97);
putchar(98);
putchar(10);
putchar(99);
• getchar function-reads character from the keyboard and returns
the integer value of the character.
Example,
int c;
….
putchar(‘\n’);
c=getchar();
putchar(c);
putchar(‘\n’);
Input
functions
Keyboard
File
Output
functions
Characters
Characters
getchar()
scanf()
putchar()
printf()
Strings
gets()
scanf()-one word
Strings
puts()
printf()
Characters
Characters
getc()
fgetc()
fscanf()
putc()
fputc()
fprintf()
Strings
fgets()
fscanf()-one word
Strings
fputs()
fprintf()
Screen
File