Computer Programming
Basics
LECTURE 9
Lecture 9: Outline
Character Arrays/ Character Strings
Initializing Character Strings. The null string.
Escape Characters
Displaying Character Strings
Inputting Character Strings
String processing:
Testing Strings for Equality
Comparing Strings
Copying Strings
Functions in <string.h>
String to number conversion functions
Arrays of characters
char s[] = { 'H', 'e', 'l', 'l', 'o', '!' };
To print out the contents of our array - s, we run
through each element in the array and display it using
the %c format characters or we will print all together
using %s for string.
To process the array s (copy, concatenate two strings,
etc) we need to have the actual length of the character
array in a separate variable !
Character strings
A method for dealing with character arrays without
having to worry about precisely how many characters
you have stored in them:
Placing a special character at the end of every
character string. In this manner, the function can
then determine for itself when it has reached the end
of a character string after it encounters this special
character.
In the C language, the special character that is used to
signal the end of a string is known as the null
character and is written as '\0'.
char s[] = { 'H', 'e', 'l', 'l', 'o', '!', '\0' };
Example: string length
// Function to count the number of
characters in a string
void main () {
#include <stdio.h>
char s1[] = { 's', 't', 'r', 'i', 'n', 'g',
int stringLength (char string[]){ '\0' };
int count = 0; char s2[] = { 'l', 'e', 'n', 'g', 't',
while ( string[count] != '\0' ) 'h','\0' };
++count; printf ("%d %d\n", stringLength
(s1),stringLength (s2));
return count;
}
}
Initializing character strings
Initializing a string: char s[] = "Hello!";
Is equivalent with: char s[] = { 'H', 'e', 'l', 'l', 'o', '!', '\0' };
or char s[] = { '2', '1', '3', '\0' };
The null string: A character string that contains no characters
other than the null character
char empty[]= "";
Escape characters
\a Audible alert
\b Backspace
\f Form feed
\n Newline
\r Carriage return
\t Horizontal tab
\v Vertical tab
\\ Backslash
\" Double quotation mark
\' Single quotation mark
\? Question mark
\nnn Octal character value nnn
\unnnn Universal character name
\Unnnnnnnn Universal character
name
\xnn Hexadecimal character value nn
Examples: Escape characters
printf ("\aSYSTEM SHUT DOWN IN 5 MINUTES!!\n");
printf ("\\t is the horizontal tab character.\n");
printf ("\"Hello,\" he said.\n");
Displaying character strings
Displaying a string: %s
printf ("%s\n", s1);
Inputting character strings
char string[80];
scanf ("%s", string); //NOT recommended !
The scanf function can be used with the %s format characters
to read in a string of characters up to a blank space, tab
character, or the end of the line, whichever occurs first
The string terminator (null character) is appended to string
If you type in more than 80 consecutive characters to the
preceding program without pressing the spacebar, the tab key,
or the Enter (or Return) key, scanf overflows the character
array !
Correct use of scanf for reading strings:
scanf ("%80s", string); // Safer version !
If you place a number after the % in the scanf format string,
Example: string processing
#include <stdio.h>
void concat (char result[], const char str1[], const char str2[]);
void main ()
{
const char s1[] = "Test " ;
const char s2[] = "works." ;
char s3[20];
concat (s3, s1, s2);
printf ("%s\n", s3);
}
Example: string processing
// Function to concatenate two character strings
void concat (char result[], const char str1[], const char str2[])
{
int i, j;
// copy str1 to result
for ( i = 0; str1[i] != '\0'; ++i )
result[i] = str1[i];
// copy str2 to result
for ( j = 0; str2[j] != '\0'; ++j )
result[i + j] = str2[j];
// Terminate the concatenated string with a null character
result [i + j] = '\0';
}
Testing strings for equality
bool equalStrings (const char s1[], const char s2[])
{
int i = 0;
bool areEqual;
while ( s1[i] == s2 [i] && s1[i] != '\0' && s2[i] != '\0' )
++i;
if ( s1[i] == '\0' && s2[i] == '\0' ) !!! NOT: s1==s2
areEqual = true;
else
areEqual = false;
return areEqual;
}
Alphabetically comparing strings
// Function to compare two character strings
int compareStrings (const char s1[], const char s2[])
{ int i = 0, answer;
answer=0;
while ( s1[i] == s2[i] && s1[i] != '\0'&& s2[i] != '\0' )
++i;
if (i!=strlen(s1)) !!! NOT: s1 < s2
if ( s1[i] < s2[i] ) !!! NOT: s1 > s2
answer = -1; /* s1 < s2 */
else answer = 1; /* s1 > s2 */
return answer;
}
Alphabetically comparing strings
void main ()
{
const char s1[] = "Test " ;
const char s2[] = "works." ;
int ok;
ok=compareStrings(s1, s2);
if (ok==1) printf("s1>s2");
else if (ok==-1) printf("s2>s1");
else printf("s2=s1");
}
Copying strings
void copyString(char dest[], char srs[])
{
int i;
i=0; !!! NOT: dest = srs
while ((dest[i] = srs[i]) != '\0')
i++;
}
String functions
C uses the <string.h> header for string functions
Most frequently used functions: strlen(), strcat(), strncat(), strcmp(),
strncmp(), strcpy(), and strncpy().
strcat (s1, s2)
Concatenates the character string s2 to the end of s1, placing a null
character at the end of the final string. The function also returns s1.
strcmp (s1, s2)
Compares strings s1 and s2 and returns a value less than zero if s1 is
less than s2, equal to zero if s1 is equal to s2, and greater than zero if
s1 is greater than s2.
strcpy (s1, s2)
Copies the string s2 to s1, also returning s1.
strlen (s)
Returns the number of characters in s, excluding the null character.
String functions
strncat (s1, s2, n)
Copies s2 to the end of s1 until either the null character is reached or
n characters have been copied, whichever occurs first. Returns s1.
strncmp (s1, s2, n)
Performs the same function as strcmp, except that at most n
characters from the strings are compared.
strncpy (s1, s2, n)
Copies s2 to s1 until either the null character is reached or n
characters have been copied, whichever occurs first. Returns s1.
String functions
strchr (s, c)
Searches the string s for the last occurrence of the character c. If
found, a pointer to the character in s is returned; otherwise, the null
pointer is returned.
strstr (s1, s2)
Searches the string s1 for the first occurrence of the string s2. If
found, a pointer to the start of where s2 is located inside s1 is
returned; otherwise, if s2 is not located inside s1, the null pointer is
returned.
getchar() and putchar()
To read a character: getchar()
To display a character: putchar()
#include <stdio.h> #include <stdio.h>
void main() void main()
{ {
char ch; char ch;
while ((ch = getchar()) != '*') while ((ch = getchar()) != EOF)
putchar(ch); putchar(ch);
}
}
getchar() and putchar()
/* Read characters from input over several lines until EOF. Count lines and
characters.*/
#include <stdio.h>
void main()
{
int nl, nc;
char c;
nl = 0;
nc = 0;
while ((c = getchar()) != EOF) {
nc++;
if (c == '\n')
nl++;
}
printf("Number of lines in input: %d\n", nl);
printf("Number of characters in input: %d\n", nc);
}
gets() and puts()
To read a string: gets(string)
To display a string: puts(string)
#include<stdio.h>
#include<string.h>
void main(){
char string1[25],string2[25];
printf("Enter first string ");
gets_s(string1);
printf("Enter second string ");
gets_s(string2);
printf("Our strings are\n");
puts(string1);
puts(string2);
}
String to number conversions
Storing a number as a string means storing the digit characters
Example: the number 213 can be stored in a character string array as the
digits '2', '1', '3', '\0'. Storing 213 in numeric form means storing it as an
int.
Ex:
1. char s[] = { '2', '1', '3', '\0' };
printf("sirul meu =%s\n",s); // sirul meu =213
2. int a[3]={65,66,67};
printf("%c\n",a[0]);//A
printf("%c\n",a[1]);//B
printf("%c\n",a[2]);//C
ASCII character table
String to number
conversion functions
<stdlib.h>
atoi(s) converts string s to a type int value and returns it. The
function converts characters until it encounters something that is
not part of an integer.
atof() converts a string to a type double value and returns it
atol() converts a string to a type long value and returns it
printf ("%d\n", atoi("245"));
printf ("%d\n", atoi("100") + 25);
printf ("%d\n", atoi("13x5"));
