Introduction to Computing – B142L

Arrays and strings

Academic year 2011/12



Objectives:

1. Concept of an array
2. Description of an array
3. Using an array
4. Strings as arrays of characters

Concept of an array

Programs often use a list of variables which are of the same data type and share a common list name.

For example, below is a problem which needs 3 variables which have the same float data type and share a common list name, month.

• Accept monthly revenue input for a period of 3 months
• Display the monthly average and total revenue earned in the quarter

Sample monthly revenue input values:

Variables to store the input values:


Sample output display:

Variables to store the output values:

Concept of an array

Programs often use a list of variables which are of the same data type and share a common list name.

For example, below is a problem which needs 12 variables which have the same float data type and share a common list name, month.

• Accept monthly revenue input for a period of 12 months
• Display the monthly average and total revenue earned in the quarter

Variables to store the input values:


Variables to store the output values:

Concept of an array

Which control structure could be used to collect the inputs and calculate the output values?

#include <stdio.h> int main( void ) { float month1, month2, month3, month4, month5, month6; float month7, month8, month9, month10, month11, month12; float average, total; }5100.50 5200.04 4992.37 3721.18 3299.68 4229.02 3510.54 6920.74 4291.57 3964.92 3719.98 4323.12

Arrays are data structures

Variables to store the input values:


An array is used to manage a set of variables that all have the same data type.

An array allows variables to be easily controlled using iteration.

An array to store the input values:

months[0]	months[1]	months[2]	months[3]	months[4]	months[5]	months[6]	months[7]	months[8]	months[9]	months[10]	months[11]
0	0	0	0	0	0	0	0	0	0	0	0

Description of an array

months[0]	months[1]	months[2]	months[3]	months[4]	months[5]	months[6]	months[7]	months[8]	months[9]	months[10]	months[11]
0	0	0	0	0	0	0	0	0	0	0	0

An array is used to manage a set of variables that all have the same data type.

• The array name is months
• There are 12 elements in the array
  • The range of valid elements is 1 to 12
  • For example, the first element is 1 and the last element is 12
• The array length is 12
  • The length has one value, which is 12 in this example

An array allows variables to be easily controlled using iteration.

An array to store the input values:

Description of an array

months[0]	months[1]	months[2]	months[3]	months[4]	months[5]	months[6]	months[7]	months[8]	months[9]	months[10]	months[11]
0	0	0	0	0	0	0	0	0	0	0	0

An array is used to manage a set of variables that all have the same data type.

• The array name is months
• There are 12 elements in the array
  • The range of valid elements is 1 to 12
  • For example, the first element is 1 and the last element is 12
• The array length is 12
  • The length has one value, which is 12 in this example

An array allows variables to be easily controlled using iteration.

• Individual elements of an array are directly accessed using an index value
• The first element of an array always starts at index zero
• The index value of the last element of an array is one less than the array length
• The array length is the not the same as an element index

An array to store the input values:

Using arrays

The array length can be automatically determined by the number of values in the initialization list.




Every element in the array is initialized to the same value as the single initialization value.




The array is declared, but not initialized.

Using arrays

months[0]	months[1]	months[2]	months[3]	months[4]	months[5]	months[6]	months[7]	months[8]	months[9]	months[10]	months[11]
0	0	0	0	0	0	0	0	0	0	0	0

An array is used to manage a set of variables that all have the same data type.

• The array name is months
• There are 12 elements in the array
  • The range of valid elements is 1 to 12
  • For example, the first element is 1 and the last element is 12
• The array length is 12
  • The length has one value, which is 12 in this example

An array allows variables to be easily controlled using iteration.

• Individual elements of an array are directly accessed using an index value
• The first element of an array always starts at index zero
• The index value of the last element of an array is one less than the array length
• The array length is the not the same as an element index

#include <stdio.h> int main( void ) { float months[ 12 ] = { 0 }; float average, total; int index; for ( index = 0; index < 12; index++ ) { scanf( "%f", &months[ index ] ); } }5100.50 5200.04 4992.37 3721.18 3299.68 4229.02 3510.54 6920.74 4291.57 3964.92 3719.98 4323.12

Using arrays

An array is stored as a contiguous sequence in program memory.

Amount of memory used by an array = ( array length ) x ( memory for array data type )

Using arrays

Conceptually, we think of a two dimensional array as a table:





However, arrays are stored sequentially in program memory:

Using arrays

How could we use a 2-dimensional array to store revenue for 2 years?

#include <stdio.h> int main( void ) { float months[ 12 ] = { 0 }; float average, total; int index; for ( index = 0; index < 12; index++ ) { scanf( "%f", &months[ index ] ); } }5100.50 5200.04 4992.37 3721.18 3299.68 4229.02 3510.54 6920.74 4291.57 3964.92 3719.98 4323.12 5120.50 5210.04 4932.37 3741.18 3219.68 4259.02 3580.54 6910.74 4271.57 3984.92 3709.98 4313.12

Strings as arrays of characters

A string is an array of character variables that ends with a NULL character ( '\0' ). Read the detailed reference description.

message[0]	message[1]	message[2]	( 3 to 31 )	message[32]
\0	\0	\0	\0	\0

Using strings

A string is an array of character variables that ends with a NULL character ( '\0' ). Read the detailed reference description.

Using strings

A string is an array of character variables that ends with a NULL character ( '\0' ). Read the detailed reference description.

Use the strlen() function to find the length of a text string.

#include <stdio.h> #include <string.h> int main( void ) { char word1[] = "Welcome"; char word2[ 30 ] = { '\0' }; char word3[ 15 ] = { 't', 'o', 'b', '1', '4', '2', 'L' }; printf( "%d \n", strlen( word1 ) ); printf( "%d \n", strlen( word2 ) ); printf( "%d \n", strlen( word3 ) ); printf( "%d \n", strlen( "computers" ) ); }

Using strings

A string is an array of character variables that ends with a NULL character ( '\0' ). Read the detailed reference description.

Use the strncmp() function to compare 2 strings for equality.

• The return value is zero if the 2 strings are identical, otherwise it returns a non-zero value that has the same sign as the difference between the first differing pair of characters.
• Compares a specific number of characters in the string.

#include <stdio.h> #include <string.h> int main( void ) { char word1[] = "Welcome"; char word2[ 30 ] = { '\0' }; char word3[ 15 ] = { 't', 'o', 'b', '1', '4', '2', 'L' }; printf( "%d \n", strncmp( word1, word2, 6 ) ); printf( "%d \n", strncmp( word1, word3, 6 ) ); printf( "%d \n", strncmp( word1, "Welcome", 6 ) ); printf( "%d \n", strncmp( word1, "welcome", 6 ) ); printf( "%d \n", strncmp( word1, "Welcome back", 8 ) ); printf( "%d \n", strncmp( word1, "Welcome back", 4 ) ); }

Using strings

A string is an array of character variables that ends with a NULL character ( '\0' ). Read the detailed reference description.

Use the sprintf() function to store formatted output into a text string.

#include <stdio.h> #include <string.h> int main( void ) { char word1[] = "Welcome"; char word2[ 30 ] = { '\0' }; char word3[ 15 ] = { 't', 'o', ' ', 'B', '1', '4', '2', 'L' }; sprintf( word2, "%s %s", word1, word3 ); printf( "%s \n", word2 ); sprintf( word2, "%d + %d = %d", 2, 8, 10 ); printf( "%s \n", word2 ); }

Using strings

A string is an array of character variables that ends with a NULL character ( '\0' ). Read the detailed reference description.

Use the strcpy() function to copy one string to another string.

#include <stdio.h> #include <string.h> int main( void ) { char word1[] = "Welcome"; char word2[ 30 ] = { '\0' }; char word3[ 15 ] = { 't', 'o', ' ', 'B', '1', '4', '2', 'L' }; strcpy( word2, word1 ); printf( "%s \n", word2 ); strcpy( word2, word3 ); printf( "%s \n", word2 ); strcpy( word2, "computing lab" ); printf( "%s \n", word2 ); }

Using strings

A string is an array of character variables that ends with a NULL character ( '\0' ). Read the detailed reference description.

Use the strcat() function to append one string to the end of another string.

#include <stdio.h> #include <string.h> int main( void ) { char word1[] = "Welcome"; char word2[ 30 ] = { '\0' }; char word3[ 15 ] = { 't', 'o', ' ', 'B', '1', '4', '2', 'L' }; strcat( word2, word1 ); printf( "%s \n", word2 ); strcat( word2, word3 ); printf( "%s \n", word2 ); strcat( word2, "computing lab" ); printf( "%s \n", word2 ); }

Using strings

A string is an array of character variables that ends with a NULL character ( '\0' ). Read the detailed reference description.

Text in a string can be parsed into smaller units using the strtok() function.

#include <stdio.h> #include <string.h> int main( void ) { char originalText[ 128 ] = "rrrr gggg bbbb wwww"; char copyOfText[ 128 ] = { '\0' }; char delimiters[ 4 ] = " "; /* A space character separates each token in the originalText variable */ /* There is no need for a full array of characters when using tokens from parsing. */ char *nextToken = NULL; /* Make copy of original string because the strtok() function makes changes to the string when it performs the parsing. */ strcpy( copyOfText, originalText ); /* Function call to start the parsing */ nextToken = strtok( copyOfText, delimiters ); while ( nextToken != NULL ) { printf( "%s \n", nextToken ); /* This function call to continue parsing is different than the function call to start the parsing. */ nextToken = strtok( NULL, delimiters ); } }

Review questions

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Using arrays

The C program below is intended to collect the age of 40 people, stores the values in an array and displays the youngest age. At line 10, the expression on the right side of the or operator provides the condition for what?

Save the first value into the array.
Clear all values in the array.
Find the lowest age in the array.
Find the highest age in the array.

Review questions

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Using arrays

The C program below is intended to collect the ticket prices at 50 movie theatres, store them in an array and display the average ticket price. At line 7, what should replace the XX placeholders?

Review questions

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Using arrays

The C program below is intended to continue asking for an input number until an input guess matches one of the numbers stored in the array. At line 12, what should replace the XX placeholders?