Programming - Arrays & Strings(10)

Administrivia

Practice Web Page - http://www.cs.tau.ac.il/~efif/courses/Programming_Fall_04

Pointer Arithmetics

• Aprogram that finds out whether a given array appears as a sub array of another given array

  #include <stdio.h> int * find_sub_array(int * array, int size, int * sub_array, int sub_size); int main() { int array1[] = {1, 45, 67, 1001, -19, 67, 89, 1004, -867, 34, 3, -1900, 10029}; int array2[] = {34, 3, -1900}; int * position; position = find_sub_array(array1, sizeof (array1) / sizeof (int), array2, sizeof (array2) / sizeof(int)); printf("array2 appears in array1 starting from place : %d.\n", (position == NULL)? -1 : position - array1 + 1); return 0; } int * find_sub_array(int * array, int size, int * sub_array, int sub_size) { int i, j; for (i = 0; i++ < size; ++array) { for (j = 0; j < sub_size; ++j) if ( *(array + j) != sub_array[j]) break; if (j == sub_size) return array; } return NULL; }

• sub_arr.c

String

• Initializing a string with characters (and the terminating null)

  char cstring[] = {'H','e','l','l','o','\0'};

  
• Initializing a string with a string literal

  char cstring[] = "Hello, world!"

  
• A pointer to a string

  char *pstring = "Hello";

  

strlen

• Calculate the length of a string

  #include <stdio.h> #include <stdio.h> int strlen(const char * str) { const char *eos = str; while( *eos++ ) ; return(eos - str - 1); } int main() { char str[]="Hello World"; printf("Length of \"%s\" is %d\n", str, strlen(str)); return 0; }

• my_strlen.c

isspace

• isspace - checks for white-space characters. space, form-feed 75 ('\f'), newline ('\n'), carriage return ('\r'), 76 horizontal tab ('\t'), and vertical tab ('\v').
• Counting the number of words in a string

  int word_cnt (const char * s) { int cnt = 0; const char *next = s + 1; if (*s == '\0') /*empty string*/ return 0; while(*next != '\0') { if (!isspace(*s) && isspace(*next)) cnt++; s++; next++; } if (!isspace(*s)) cnt++; return cnt; }

• word_count.c

string.h

• return the length of the string s, not counting the terminating null character

  size_t strlen(const char *s);

• the programmer must ensure that s1 points to enough space for result char *strcpy(char *s1, const char *s2);
• the programmer must ensure that s1 points to enough space for result

  char *strcat(char *s1, const char *s2);

• Lexicographic comparison
  • Return value is less than, equal to, or greater than zero according comparison

    int strcmp(const char *s1, const char *s2);

  • strncmp - compares at most n characters

    int strcnmp(const char *s1, const char *s2, size_t n);

IO

• scanf - Read formatted data from stdin
  • until space

    scanf(%s, str);

• gets - Get a string from stdin
  • until until either a terminating newline or EOF

    gets(str);

strcmp

• Compare two strings

  int strcmp(const char *s1, const char *s2) { int k = 0; while(s1[k] == s2[k] && s1[k]) { k++; } return (s1[k] - s2[k]); }

• my_strcmp.c

strcpy

• Copy a string

  char *strcpy(char *s1, const char *s2) { char *head = s1; while(*s2) { *s1++ = *s2++; } return head; }

• my_strcpy.c

toupper

• Replace lower case letters with upper case

  #include <stdio.h> #define STR_SIZE 256 #define UPPER 'A' - 'a' char * toupper(char * str); int main() { char str[STR_SIZE]; gets(str); printf("%s\n", toupper(str)); return 0; } char * toupper(char * str) { char *ps = str; while(*ps) { if (*ps >= 'a' && *ps <= 'z') *ps += UPPER; ps++; } return str; }

• my_toupper.c

strstr

• foo

  include char* strstr ( char * str1, char * str2) { char *cp = str1; char *s1, *s2; if ( !*str2 ) return(str1); while (*cp) { s1 = cp; s2 = str2; while ( *s1 && *s2 && !(*s1-*s2) ) s1++, s2++; if (!*s2) return(cp); cp++; } return(NULL); } int main() { char s1[]="I am a boy"; char s2[]="am"; printf("%s", strstr(s1,s2)); return 0; }

• my_strstr.c

const char *

• const is a storage class specifier
• it says that, for a variable or array, the value(s) in it cannot be altered
• Declaration of printf()

  extern int printf(const char *, ...);

  • the function returns an int
  • it takes at least one argument - a const char *
    • it is a pointer to the first character of a string
    • printf promises it won't change the contents of that string