C Characters & Strings

C Characters & Strings • Character Review • Character Handling Library • Initialization • Standard Input/Output Library Functions • String Conversion Functions • String Handling Library

Character Review • Know your ASCII • '0' 48 dec 0x30 0011 0000 • '9' 57 dec 0x39 0011 1001 • 'A' 65 dec 0x41 0100 0001 • 'Z' 90 dec 0x5A 0101 1010 • 'a' 97 dec 0x61 0110 0001 • 'z' 122 dec 0x7A 0111 1010

Character Handling Library • Use #include <ctype.h> header file • Basic functions include: • Conversion between case • Test for upper or lower case • Test for letters (and digits and alphanumeric) • Test for control characters • Test for whitespace (and punctuation) • See example at char.c

Some Functions • int islower(char) • Is it lowercase letter? • int isupper(char) • Is it uppercase letter? • int tolower(char) • Convert to lowercase • int toupper(char) • Convert to uppercase • int iscntrl(char) • Is it a control character? (tab, alarm, backspace, newline, etc.)

More Functions • int isalpha(char) • Is it a letter? • int isalnum(char) • Is it a letter or digit? • int isdigit(char) • Is it a digit? • int isspace(char) • Is it a space, tab, or newline? • int ispunct(char) • Is it printable but not space or alphanumeric?

String Initialization • Store 6 characters + null character (null character = '\0' = 0x0) • char s[]="string"; • char s[7]="string"; • char s[]={'s','t','r','i','n','g','\0'}; • char s[7]={'s','t','r','i','n','g','\0'}; • char s[SIZE]="string"; • Where #define SIZE 7

String Initialization • Stores one ‘a’ + null characters • char s[7]={'a'}; • Stores ‘a’ + ‘b’ + null characters • char s[7]={'a', 'b'}; • Stores 7 null characters • char s[7]={0}; • char s[7]={0x0}; • char s[7]={'\0'};

String Initialization • Creates an array of pointers to two constant character strings (See init.c) char *c1[] = {"zero","one"}; printf("%s\n",c[0]); /* zero */ printf("%p\n",c[0]); /* 0x10970 */ printf("%p\n",&c[0]); /* 0xffbefa88 */ printf("%c\n",c[0][0]); /* z */ printf("%p\n",&c[0][0]); /* 0x10970*/ c[0][0]='a'; /* Segmentation fault */ c[0]=c[1]; /* assign another address */

String Initialization • Creates an array of characters char c2[2][5] = {"zero", "one"}; printf("%s\n",c2[0]); /* zero */ printf("%p\n",c2[0]); /* 0xffbefa78 */ printf("%p\n",&c2[0]); /* 0xffbefa78 */ printf("%c\n",c2[0][0]); /* z */ printf("%p\n",&c2[0][0]); /* 0xffbefa78 */ c2[0][0]='a'; /* ok */ c2[0]=c2[1]; /* incompatible types */

String Initialization • Creates an array of characters char c3[2][5] = {'z','e','r','o','\0', 'o','n','e','\0','\0'}; printf("%s\n",c3[0]); /* zero */ printf("%p\n",c3[0]); /* 0xffbefa68 */ printf("%p\n",&c3[0]); /* 0xffbefa68 */ printf("%c\n",c3[0][0]); /* z */ printf("%p\n",&c3[0][0]); /* 0xffbefa68 */ c3[0][0]='a'; /* ok */ c3[0]=c3[1]; /* incompatible types */

String Conversion Functions • Convert from strings to integers and floats • Basic functions include: • String to integer, float • Cut off non-numerical data • Display base 2-36 • Requires the #include<stdlib.h> header • General utilities library

String Conversion Functions • Converts string to an integer or double • int atoi(const char *nPtr); • double atof(const char *nPtr); • Examples int i=atoi("1234"); double d=atof("1.23"); int x=atoi('5'); /*What’s wrong with this one?*/

What’s the Output? #include <stdio.h> #include <stdlib.h> int main(){ char a[]="15", b[]="+2.7"; int c=atoi(a), i=0; double d=atof(b); printf("%x\n",*a); printf("%c\n",*b); printf("%x\n",c); printf("%f\n",d); return 0; } //See exercise1.c

String Conversion Functions • double strtod(const char *nPtr, char **endPtr); • Converts string nPtr to double • endPtr is assigned the 1st character after the converted value • char **ptr is for a pointer to an array of characters (also written as char *ptr[]) char *nextChar = NULL; char *str = "123abc "; double d=strtod(str, &nextChar); /* d=123.000000 nextChar="abc\0" */

Standard I/O Library Functions • Use <stdio.h> library • int getchar(void); • Returns one character at a time from standard input • int putchar(int c); • Prints the character stored in “c” • char* puts(const char *s); • Prints the string followed by a newline character

Standard I/O Library Functions • Use <stdio.h> library • char* gets(char *s); • Reads one line from standard input & puts it into array “s”. Includes blank spaces. • Should not be used because of the potential for “buffer overflow” attack • char* fgets(char* s, int n, FILE* stream); • Copies characters from (input) stream stream to s, stopping when n-1 characters copied, newline copied, end-of-file reached or error occurs • Ex: char str[10]; fgets(str, sizeof(str), stdin); printf(str);

String Handling Library • Functions for manipulating strings • Requires the #include<string.h> header • Basic functions include: • Copying strings • Comparing strings • Searching strings • Tokenizing strings • Determining length of strings

Copy, Append, Compare • char *strcpy(char *s1, const char *s2); • Copies s2 into s1; the value of s1 is returned • char *strcat(char *s1, const char *s2); • Appends s2 to s1; the value of s1 is returned • char *strcmp(char *s1, const char *s2); • s1 > s2 returns positive number • s1 == s2 returns 0 • s1 < s2 returns negative number

Example Code char str[]="string"; char *ptr; strcpy(ptr, str); strcat(ptr, str); printf("%s\n",str); /*string*/ printf("%s\n",ptr); /*stringstring*/ printf("%d\n",strcmp(ptr,str)); /*115*/ printf("%d\n",strcmp(ptr,ptr)); /*0*/ printf("%d\n",strcmp(str,ptr)); /*-115*/ //See strcpy.c

Search Functions • char *strchr(const char *s, int c); • Locates & returns a pointer to the 1st occurrence of c in string s • Returns NULL if not found • char *strstr(const char *s1, const char *s2); • Locates & returns a pointer to the 1st occurrence of string s2 in string s1 • Returns NULL if not found

Example Code char str[]="string", rin[]="rin", abc[]="abc"; char i = 'i', z = 'z'; printf("%p\n",strchr(str,i)); /*0xffbefa8b*/ printf("%s\n",strchr(str,i)); /*ing*/ printf("%p\n",strchr(str,z)); /*0x0*/ printf("%p\n",strstr(str,rin)); /*0xffbefa8a*/ printf("%s\n",strstr(str,rin)); /*ring*/ printf("%p\n",strstr(str,abc)); /*0x0*/ //See strstr.c

What’s the Output? #include<stdio.h> #include<string.h> main(){ char str1[]="apple", str2[]="banana"; char ptr1[20], *ptr2=str2; strcpy(ptr1, strchr(str1,'p')); strcat(ptr1, strstr(str2, "nan")); str2[0]='N'; printf("%s\n",ptr1); printf("%s\n",ptr2); } //See exercise2.c

String Tokenizer • char *strtok(char *s1, const char *s2); • Used to break a string into tokens • A sequence of characters separated by delimiting characters (usually spaces or punctuation marks) • s1 contains string to be tokenized • s2 contains the characters used to separate the tokens

String Tokenizer #include <stdio.h> #include <string.h> int main(){ char str[]="This is a string."; char *tPtr=strtok(str," "); /*searches for the 1st non-delimiting character (space) & inserts '\0' at next delimiter character*/ while(tPtr!=NULL){ printf("%s\n",tPtr); tPtr=strtok(NULL," "); /*keeps inserting '\0' in next delimiter*/ } return 0; }//See token.c

String Tokenizer • Output from last slide This is a string. This • Problem with strtok is that it changes the original string

What’s the Output? #include <stdio.h> #include <string.h> int main(){ char str[]="abrakadabra"; char *tPtr=strtok(str,"a"); while(tPtr!=NULL){ printf("%s",tPtr); tPtr=strtok(NULL,"a"); } printf("\n%s\n",str); return 0; } //See exercise3.c

Length of a String • size_t strlen(const char s); • Returns the length of string s • Does not include the terminating null character

Example Code #include<stdio.h> #include<string.h> main(){ char str1[]="apple", *str2="banana"; char str3[6]={'a','b','\0','c','d','\0'}; printf("%d\n",strlen(str1)); /* 5 */ printf("%d\n",strlen(str2)); /* 6 */ printf("%d\n",strlen(str3)); /* 2 */ } //See strlen.c

C Characters & Strings