240-222 CPT: Arrays/5 1

240-222 Computer Programming Techniques240-222 Computer Programming TechniquesSemester 1, 1998Semester 1, 1998

Objective of these slides:– to introduce arrays

5. Arraysch. 6

240-222 CPT: Arrays/5 2

OutlineOutline1. What is an Array?

2. Declaring an Array

3. Accessing an Array

4. An Array Program

5. Initialising Arrays

6. Avoid 'Magic' Numbers

7. Student Polls Example

8. Functions Using Arrays

9. Two Examples

10. 2D Arrays

11. 3D Arrays

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1. What is an Array?1. What is an Array?

An array is a data structure which can hold a sequence of values.

The array c[] holds 3 integer values:

c[0]

c[1]

c[2]

-45

6

72

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2. Declaring an Array2. Declaring an Array

int c[3];

The array is comprised of 3 elements:

c[0] c[1] c[2]

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int b[100], x[27];char sent[125];

b[] holds 100 integers (in b[0] to b[99])

x[] holds 27 integers (in x[0] to x[26])

sent[] holds 125 characters (in sent[0] to sent[124])

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3. Accessing an Array3. Accessing an Array

int c[3], s, t, v;

c[1] = 3;s = 0; c[s] = 3;c[s + 1] = 2;t = 2; c[s + t] += 2;printf("%d", c[0] + c[1]);v = c[2] / 3;

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4. An Array Program 4. An Array Program Fig. 6.3Fig. 6.3

#include <stdio.h>

int main(){ int n[10], i;

for (i = 0; i < 10; i++) n[i] = 0;

printf("%s%13s\n", "Element", "Value"); for(i = 0; i < 10; i++) printf("%7d%13d\n", i, n[i]); return 0;}

notice the range

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Output:Output:

Element Value0 01 02 0: :9 0

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5. Initialising Arrays5. Initialising Arrays

int n[5] = {32, 23, 34, 1, 4};

int p[5] = {3};

int q[] = {32, 23, 34, 1, 4};

int r[3] = {1, 2, 3, 4, 5};/* WRONG */

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6. Avoid 'Magic' Numbers6. Avoid 'Magic' Numbers

/* Initialize array s to the even integers from 2 to 20 */#include <stdio.h>#define SIZE 10

int main(){ int s[SIZE], j; for (j = 0; j < SIZE; j++) s[j] = 2 + 2 * j; printf("%s%13s\n", "Element", "Value"); for (j = 0; j < SIZE; j++) printf("%7d%13d\n", j, s[j]); return 0;}

Typical rangeexpression

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Output:Output:

Element Value0 21 42 6: :9 20

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7. Student Polls Example fig. 6.77. Student Polls Example fig. 6.7

/* How many students got each mark? */#include <stdio.h>#define SIZE1 40 /* no of students */#define SIZE2 11 /* ans range (0 - 10) */

int main(){ int answer, rating; int responses[SIZE1] = {1, 2, 6, 4, 8, 5,

9, 7, 8, 10, 1, 6, 3, 8, 6, 10, 3,8, 2, 7, 6, 5, 7, 6, 8, 6, 7, 5, 6, 6, 5, 6, 7, 5, 6, 4, 8, 6, 8, 10};

int frequency[SIZE2] = {0};:

continued

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for(ans = 0; ans < SIZE1; ans++) frequency[responses[ans]]++;

printf("%s%17s\n", "Rating", "Freq");for(rating = 0; rating < SIZE2; rating++) printf("%6d%17d\n", rating,

frequency[rating]);

return 0;

}

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Consider:Consider:

for(ans = 0; ans < SIZE1; ans++) frequency[responses[ans]]++;

When responses[ans] is 1 then:frequency[1]++;

When responses[ans] is 7 then:frequency[7]++;

What happens if responses[ans] has a value >= 11 or < 0?

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8. Functions using Arrays Fig 6.138. Functions using Arrays Fig 6.13

/* A program which illustrates the differences between passing an array to a function and passing an array element */

#include <stdio.h>#define SIZE 5

void modifyarray(int b[], int size);void modifyelement(int e);

:

continued

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int main(){ int a[SIZE] = {0, 1, 2, 3, 4}; int i;

printf("The values of the originalarray are:\n");

for (i = 0; i < SIZE; i++) printf("%3d", a[i]); printf("\n");

::

continued

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modifyarray(a, SIZE);

printf("The values of the modified array are:\n");

for (i = 0; i < SIZE; i++) printf("%3d", a[i]); printf("\n");

printf("The value of a[3] is %d\n",a[3]); modifyelement(a[3]); printf("The value of a[3] is %d\n",a[3]);

return 0;}

continued

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void modifyarray(int b[], int size){ int j;

for (j = 0; j < size; j++) b[j] *= 2;}

void modifyelement(int e){ printf("Value in modifyelement

is %d\n", e *= 2);}

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Passing Arrays to FunctionsPassing Arrays to Functions

The call to the function modifyarray() is:modifyarray(a, SIZE);

C treats array arguments differently .

Any changes to the elements of a[]will be retained when the function returns.

This is call by reference.

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Output:Output:

The values of the original array are:0 1 2 3 4

The values of the modified array are:0 2 4 6 8

The value of a[3] is 6Value in modifyelement is 12The value of a[3] is 6

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9. Two Examples9. Two Examples

9.1 Reverse Input Text

9.2 Capitalise Input Text

9.3 Reverse & Capitalise

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9.1. Reverse Input Text9.1. Reverse Input Text/* Reverse lines */#include <stdio.h>#include <string.h>#define SIZE 200

void rev_print(char ln[]);

int main(){ char line[SIZE];

while (gets(line) != NULL) rev_print(line); return 0;}

continued

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void rev_print(char ln[])/* Find the length of the line and then use a for-loop to print it backwards */{ int i;

for(i=strlen(ln)-1; i >=0; i--) putchar(ln[i]); putchar('\n');}

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Use:Use:

$ revhello worlddlrow ollehabbaabba$

$ rev < test > rev_test

output fromprogram

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9.2. Capitalise Input Text9.2. Capitalise Input Text/* Make lines uppercase */#include <stdio.h>#include <ctype.h>#define SIZE 200

void upper(char ln[]);

int main(){ char line[SIZE]; while (gets(line) != NULL) { upper(line); puts(line); } return 0;}

continued

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void upper(char ln[])/* Convert every lowercase char to uppercase */{ int count = 0;

while (ln[count] != '\0') { if (islower(ln[count])) ln[count] = toupper(ln[count]); count++; }}

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Use:Use:

$ uphello worldHELLO WORLD4 abbas4 ABBAS$

$ up < test > up_test

output fromprogram

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9.3. Reverse & Capitalise9.3. Reverse & Capitalise

$ rev < rev.c | up > rev-up

$ cat rev-up/* DESREVER TI TNIRP DNA ENIL A DAER YLDETAEPER */

>H.OIDTS< EDULCNI#>H.GNIRTS< EDULCNI#

4201 EZIS ENIFED#

;)][NL RAHC(TNIRP_VER DIOV::}

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10. Two-dimensional Arrays10. Two-dimensional Arrays

10.1. Declaring 2D Arrays

10.2. 2D Array Initialisation

10.3. Using a 2D Array

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10.1. Declaring 2D Arrays10.1. Declaring 2D Arrays

int b[3][4]

The first subscript is the number of rows (3), the second subscript is the number of columns (4).

Row 0

Row 1

Row 2

Col 0 Col 1 Col 2 Col 3

b[0][0] b[0][1] b[0][2] b[0][3]

b[1][0] b[1][1] b[1][2] b[1][3]

b[2][0] b[2][1] b[2][2] b[2][3]

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10.2. 2D Array Initialisation10.2. 2D Array Initialisation

Many alternatives:

int a[2][3] = {{1,2,3}, {4,5,6}}; – Indexing is by rows,

so a[0] row initialised with {1,2,3}.

int a[2][3] = {1,2,3,4,5,6};

int a[][3] = {{1,2,3},{4,5,6}};

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Initialisation of partsInitialisation of parts

int b[2][2] = {{1}, {3,4}};– b[0][1] is assigned 0

b[0][0] = 1

b[1][0] = 3, b[1][1] = 4

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Initialisation with codeInitialisation with code

int a[3][4];:

for (row = 0; row < 3; row++) for (col = 0; col < 4; col++) a[row][col] = row * col;

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10.3. Using a 2D Array10.3. Using a 2D Array

#include <stdio.h>

int sum(int a[][]);

int main(){ int scores[3][5] = {0}; int tot;

: /* scores gets some values */ tot = sum(scores); printf("Total score is %d\n", tot); return 0;}

continued

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int sum(int a[][5])/* sum all the elements of the array */{ int i, j, sum = 0;

for (i = 0; i < 3; ++i) for (j = 0; j < 5; ++j) sum += a[i][j]; return sum;}

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11. Three-dimensional Arrays11. Three-dimensional Arrays

11.1. Declaring 3D Arrays

11.2. 3D Array Initialisation

11.3. Using a 3D Array

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11.1. Declaraing 3D Arrays11.1. Declaraing 3D Arrays

int a[2][2][3] = {{{1,1,0}, {2,0,0}},{{3,0,0}, {4,4,0}} };

The first inner brackets refer to the first subscript. The next inner brackets refer to the second

subscript. The three values inside each of the inner brackets

refer to final subscript.

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a[0][1][2] = 2a[0][1][2] = 2

X

0

1

Y0 1

Z

0

1

2

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11.2. 3D Array Initialisation11.2. 3D Array Initialisation

int a[2][2][3] = {0};/* everything to 0 */

Using code:int a[3][4][5];

:for (row = 0; row < 3; row++) for (col = 0; col < 4; col++) for (dep = 0; dep < 5; dep++) a[row][col][dep] = row*col*dep;

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11.3. Using a 3D Array11.3. Using a 3D Array

#include <stdio.h>

int sum_coords(int a[][][]);

int main(){ int orbit[10][20][30] = {0}; int points; : /* initialise orbit array */ points = sum_coords(orbit); printf(" Orbit points sum: %d\n",

points); return 0;}

continued

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int sum_coords(int a[][20][30])/* sum the number of points in the array */{ int i, j, k, sum = 0; for (i = 0; i < 10; ++i) for (j = 0; j < 20; ++j) for (k = 0; k < 30; ++k) sum += a[i][j][k]; return sum;}