arrays and strings lecture 30. summary of previous lecture in the previous lecture we have covered ...
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Arrays and Strings
Lecture 30
Summary of Previous Lecture In the previous lecture we have covered
Functions Prototypes Variable Scope
Pointers Introduction to pointers Pointers and function parameters
Today’s Topics
ArraysDeclarationInitializationInput/OutputPassing arrays to functions
Today’s Topics Character Strings
Representation Declaration Index of a char in a stringString operations Common mistakes
Summary
Array An array is a way of lining up objects in
rows and columns so they are easier to count or multiply.
The rows run from left to right.
Array Row: Left to right.
This array has 3 rows
Array The columns run from top to bottom. This
array has four columns.
Array Column:
Arrays in C Program A group of contiguous memory locations used
to store a series of related values.
The array name is a pointer to the first element
All values have the same type
Individual elements of an array are accessed via an integer index: array[index]
Referencing Array Elements Element indices start at 0: array[0] is the first
element We can reference array elements by using the array’s
subscript. The first element has a subscript of 0. The last element in an array of length n has a subscript of n-1.
When we write a program, we refer to an individual array element using indexing. To index a subscript, use the array name and the subscript in a pair of square brackets:anyArray[12];
Array Example
Declaring an Array To declare an array, we need to specify its
data type, the array’s identifier and the size:type arrayName [arraySize];
The arraySize is constant Before using an array we must declare and
initialize it!
Declaration Examples
Initialization at Array Definition Arrays may be initialized with a list of
suitable values We can initialize fixed-length array
elements when we define an array. If we initialize fewer values than the length
of the array, C assigns zeroes to the remaining elements.
Array Initialization Examples
Arrays and Loops Since we can refer to individual array elements
using numbered indexes, it is very common for programmers to use for loops when processing arrays.
from Figure 8-5 in Forouzan & Gilberg, p. 462
Accessing Elements To access an array’s element, we need to provide an
integer value to identify the index we want to access. We can do this using a constant:scores[0];
We can also use a variable:for(i = 0; i < 9; i++){
scoresSum += scores[i];}
Inputting Values using a for Loop
Once we know the length of an array, we can input values using a for loop:
arrayLength = 9;
for (j = 0; j < arrayLength; j++){
scanf(“%d”, &scores[j]);}//end for
Assigning Values to Individual Elements
We can assign any value that reduces to an array’s data type:scores[5] = 42;scores[3] = 5 + 13;scores[8] = x + y;scores[0] = pow(7, 2);
Example: MonthlyRainfall
Problem: using Rainfall Table• input month• output mean rainfall for that month
month mean rainfall (in mm)0 301 402 453 954 1305 2206 2107 1858 1359 80
10 4011 45
Rainfall Table
#include <stdio.h>
int main(){ int month; int table[12] = { 30, 40, 45, 95, 130, 220, 210, 185, 135, 80, 40, 45 }; printf("Enter month: "); scanf("%d", &month); printf("Average rainfall: %d mm.\n", table[month-1]); return 0;} rainfall1.c
Example… : MonthlyRainfall (v.1)
#include <stdio.h>
int main(){ int month; int table[12] = { 30, 40, 45, 95, 130, 220, 210, 185, 135, 80, 40, 45 }; printf("Enter month: "); scanf("%d", &month); printf("Average rainfall: %d mm.\n", table[month-1]); return 0;} rainfall1.c
Example (cont.): MonthlyRainfall (v.1)
#include <stdio.h>#define NMONTHS 12
/* Store and print rainfall */
int main(){ int data[NMONTHS]; int month; for ( month=0; month < NMONTHS; month++ ) { scanf("%d", &data[month] ); } ... rainio1.c
Example: IORainfall-1 using LOOP
#include <stdio.h>#define NMONTHS 12
/* Store and print rainfall */
int main(){ int data[NMONTHS]; int month; for ( month=0; month < NMONTHS; month++ ) { scanf("%d", &data[month] ); } ... rainio1.c
Example (cont): IORainfall-1Macro: Defined NMONTHS to be 12. Later in
program NMONTHS will be used
#include <stdio.h>#define NMONTHS 12
/* Store and print rainfall */
int main(){ int data[NMONTHS]; int month; for ( month=0; month < NMONTHS; month++ ) { scanf("%d", &data[month] ); } ... rainio1.c
Example (cont): IORainfall-1
Array
1 2 3 4 5 6 7 8 9 10 11 12
0 1 2 3 4 5 6 7 8 9 10 11
Array index
Values
data
Value at this position can
be accessed through
data [6]
#include <stdio.h>#define NMONTHS 12 ... /* Print from January to December */ for ( month=0; month < NMONTHS; month++ ) { printf( "%d ", data[month] ); } printf("\n");
/* Print from December to January */ for ( month = NMONTHS - 1; month >= 0; month-- ) { printf( "%d ", data[month] ); } printf("\n"); return 0;}
rainio1.c
Example…..: IORainfall-2 (v.1)This will print all contents of the array data
at location month
Print the array in reverse
order
#include <stdio.h>#define NMONTHS 12 ... /* Print from January to December */ for ( month=0; month < NMONTHS; month++ ) { printf( "%d ", data[month] ); } printf("\n");
/* Print from December to January */ for ( month = NMONTHS - 1; month >= 0; month-- ) { printf( "%d ", data[month] ); } printf("\n"); return 0;} rainio1.c
Example…..: IORainfall-2 (v.1)
“Copying” Arrays We cannot directly copy one array to another,
even if they have the same length and share the same data type.
Instead, we can use a for loop to copy values:
for (m = 0; m < 25; m++){
array2[m] = array1[m];}//end for
Swapping Array Elements To swap (or exchange) values, we must use a
temporary variable. A common novice’s mistake is to try to assign elements to one another:
/*The following is a logic error*/numbers[3] = numbers[1];numbers[1] = numbers[3];
/*A correct approach …*/temp = numbers[3];numbers[3] = numbers[1];numbers[1] = temp;
Printing Array Elements To print an array’s contents, we would use
a for loop:for(k = 0; k < 9; k++){
printf(“%d”, scores[k]);}//end for
Range Checking Unlike some other languages, C does not
provide built-in range checking. Thus, it is possible to write code that will produce “out-of-range” errors, with unpredictable results.
Common Error (array length is 9):for(j = 1; j <= 9; j++){
scanf(“%d”, &scores[j]);}//end for
Character Strings
Topics Representation Declaration Index of a char in a string String operations Common mistakes
Representation Recall: Main memory
contiguous array of cellseach cell has an
address
0x1FFF 0x2000 0x2001 0x20020x1FFEetc
ch
Representation (cont) Recall: Variable declaration
sets aside a “box” to contain a value
Example: char ch;
ch = ‘B’;
0x1FFF 0x2000 0x2001 0x20020x1FFEetc
‘B’
Representation (cont)
Example: char name[5];
Specifies numberof cells in the array
String declarationsets aside an array of cellseach cell contains a charaddress of first cell in the array
Representation (cont) String declaration
sets aside an array of cellseach cell contains a charaddress of first cell in the array
Example: char name[5];
0x2000 0x2004
name
is 0x2000
Character Arrays vs Character Strings
A character string is a char array A character string must have the
terminating character (’\0’) The terminating character allows scanf()
and printf() to handle character strings
Character StringsDeclaration 1:
char name[5];
Declaration 2:
#define MAXLENGTH 5
char name[MAXLENGTH];
0x2000 0x2004
name
is 0x2000
String Input/Output#include <stdio.h>
#define MAXLENGTH 15
int main(){ char string1[MAXLENGTH]; char string2[MAXLENGTH];
scanf("%s %s", string1, string2); printf("%s %s\n", string1, string2);
return 0;}
No ampersand (&)!
Character String Declaration
Declaration 1:
char name[5] = “Ann”;
A n n \0
Terminating Character:• Marks the end of string• Special char: ’\0’ • aka NUL (single L)
0x2000 0x2004
name
is 0x2000
Character String Declaration
Declaration 1:
char name[5] = “Ann”;
A n n \0
0x2000 0x2004
name
is 0x2000
Could have defined this as an array:
char name[5] = {’A’,’n’,’n’,’\0’};
Character String Declaration (cont)
Declaration 1:
char name[5] = “Ann”;
Can storeat most 4 letters,because of `\0’
A n n \0
0x2000 0x2004
name
is 0x2000
Character String Declaration (cont)
Declaration 2:
char name[] = “Ann”;
Takes up an extra cell for ‘\0’
A n n \0
0x2000 0x2003
name
is 0x2000
Character String Declaration (cont)
Declaration 3:
char *name = “Ann”;
Result is “undefined”if you try to modify
this string
A n n \0
0x3000 0x3003
0x3000
name
Character String Declaration (cont)
Declaration 4:
char name[];
String with arbitrary length?
No! Will cause an error“array size missing in
`name’”
A Char in a String The size of a character string is fixed Character at position index:
string[index]first character has index 0
char name[8] = “John”;int i = 2;
printf(“Char at index %d is %c.\n”, i, name[i]);
A Char in a String (cont)
output: Char at index 2 is h.
index 0 index 4
J o h n \0
0x3995 0x399C
name
is 0x3995
A Char in a String (cont)
index 2
J o h n \0
0x3995 0x399C
name
is 0x3995
char name[8] = “John”;
name[2] = ‘X’;printf(“Name: %s\n”, name);
X
J o X n \0
0x3995 0x399C
name
is 0x3995
output: Name: JoXn
index 2
char name[8] = “John”;
name[2] = ‘X’;printf(“Name: %s\n”, name);
A Char in a String (cont)
String Operations
#include <string.h> Operations:
Assignment: strcpy()Concatenation: strcat()Comparison: strcmp()Length: strlen()
All rely on and maintain the NUL termination of the strings.
#include <stdio.h>#include <string.h>
#define MAXLENGTH 100
int main(){ char string1[MAXLENGTH]; char string2[MAXLENGTH];
strcpy(string1, “Hello World!”); strcpy(string2, string1);
return 0;}
String Operation: Assignment
string1: <garbage>string2: <garbage>
We have previously discussed underlying array copying function
String Operation: Assignment (cont)
#include <stdio.h>#include <string.h>
#define MAXLENGTH 100
int main(){ char string1[MAXLENGTH]; char string2[MAXLENGTH];
strcpy(string1, “Hello World!”); strcpy(string2, string1);
return 0;}
string1: “Hello World!”string2: <garbage>
String Operation: Assignment#include <stdio.h>#include <string.h>
#define MAXLENGTH 100
int main(){ char string1[MAXLENGTH]; char string2[MAXLENGTH];
strcpy(string1, “Hello World!”); strcpy(string2, string1);
return 0;}
string1: “Hello World!”string2: “Hello World!”
char name1[5] = “Ann”;char name2[5] = “Dave”;
name2 = name1;
Common Mistake 1:
Incompatible types
Example:
Error: “LValue required ...”
Common Mistake 2:
Not enough space
A n n \0
0x2000 0x2003
char name[] = “Ann”;
strcpy(name, “David”);
name
is 0x2000
Common Mistake 2:
Not enough space
D a v i d \0
0x2003
char name[] = “Ann”;
strcpy(name, “David”);
0x2000
name
is 0x2000
char *name1 = “Ann”;char *name2 = “Dave”;
name2 = name1;
Caution 1:
Pointer Assignment
Example:
Caution 1:Pointer Assignment
D a v e \0
0x3990 0x3994
A n n \0
0x2000 0x2003
0x2000
name1
0x3990
name2
char *name1 = “Ann”;char *name2 = “Dave”;
Caution 1:Pointer Assignment
D a v e \0
0x3990 0x3994
A n n \0
0x2000 0x2003
0x2000
name1
0x2000
name2
name2 = name1;
String Operation: Concatenation
char string1[MAXLENGTH];char string2[MAXLENGTH];
strcpy(string1, “Goodbye”);strcpy(string2, “, Cruel ”);
strcat(string1, string2);strcat(string1, string2);strcat(string1, “World!”);
string1: “Goodbye”string2: “, Cruel “
char string1[MAXLENGTH];char string2[MAXLENGTH];
strcpy(string1, “Goodbye”);strcpy(string2, “, Cruel ”);
strcat(string1, string2);strcat(string1, string2);strcat(string1, “World!”);
string1: “Goodbye, Cruel ”string2: “, Cruel ”
String Operation: Concatenation (cont)
string1: “Goodbye, Cruel , Cruel ”string2: “, Cruel ”
String Operation: Concatenation (cont)
char string1[MAXLENGTH];char string2[MAXLENGTH];
strcpy(string1, “Goodbye”);strcpy(string2, “, Cruel ”);
strcat(string1, string2);strcat(string1, string2);strcat(string1, “World!”);
string1: “Goodbye, Cruel , Cruel World!”string2: “, Cruel ”
String Operation: Concatenation (cont)
char string1[MAXLENGTH];char string2[MAXLENGTH];
strcpy(string1, “Goodbye”);strcpy(string2, “, Cruel ”);
strcat(string1, string2);strcat(string1, string2);strcat(string1, “World!”);
Common Mistake:
char name[5];
strcpy(name, “Ann”);strcat(name, “ Smith”);
Not enough space
A n n \0
0x2000 0x2004
name
is 0x2000
Common Mistake:
char name[5];
strcpy(name, “Ann”);strcat(name, “ Smith”);
Not enough space
A n n S m i t
0x2000 0x2004
h \0name
is 0x2000
strcpy(string1, “Apple”);strcpy(string2, “Wax”);
if (strcmp(string1, string2) < 0){ printf(“%s %s\n”, string1, string2);}else{ printf(“%s %s\n”, string2, string1);}
String Operation: Comparison
Returns:
negative if string1 < string2
zero if string1 == string2
positive if string1 > string2
strcpy(string1, “Apple”);strcpy(string2, “Wax”);
if (strcmp(string1, string2) < 0){ printf(“%s %s\n”, string1, string2);}else{ printf(“%s %s\n”, string2, string1);}
String Operation: Comparison (cont)
output: Apple Wax
strcpy(string1, “Apple”);strcpy(string2, “Wax”);
if (string1 < string2){ printf(“%s %s\n”, string1, string2);}else{ printf(“%s %s\n”, string2, string1);}
Common Mistake:
Wrong Comparison
strcpy(string1, “Hi Mum”);strcpy(string2, “Hi Mum”);
if ( strcmp(string1, string2) ){ printf(“%s and %s are the same\n”,
string1, string2);}
Caution 1:
Not a Boolean
Returns zero if the strings are the same.
char string1[100];
strcpy(string1, “Apple”);
printf(“%d\n”, strlen(string1));
output: 5
Number of char-sbefore the `\0’
String Operation: Length
Common Mistake:
char name[5];
strcpy(name, “David”);
Not enough space
Don’t forget the ‘\0’
D a v i d \0
0x3990 0x3994
name
is 0x3990
Summary A string is a contiguous array of chars The string identifier is the address of the
first char in the string Individual chars are accessed using the str[index] notation
There are C library functions for copying, concatenating and comparing strings