19-Feb-02 Sudeshna Sarkar, CSE, IIT Kharagpur1 Arrays, Pointers, Strings Lecture 18 19/2/2002

19-Feb-02 Sudeshna Sarkar, CSE, IIT Kharagpur2 Pointer arithmetic and element size double a[2], * p, *q ; p=a;/*points to base of array*/ q=p+1;/*  q=&a[1] */ printf(“%d\n”, q-p);/* 1 printed */ printf(“%d\n”, (int)q - (int)p); /* 8 printed */

19-Feb-02 Sudeshna Sarkar, CSE, IIT Kharagpur3 Arrays as function arguments  In a function definition, a formal parameter that is declared as an array is actually a pointer.  When an array is passed as an argument to a function, the base address of the array is passed “call by value”. The array elements are not copied. double sum (double a[], int n) { int i; double sum; for (i=0,sum=0.0; i<n; i++) sum += a[i]; return sum; } double sum (double *a, int n) { int i; double sum; for (i=0,sum=0.0; i<n; i++) sum += a[i]; return sum; }

19-Feb-02 Sudeshna Sarkar, CSE, IIT Kharagpur4 Recursively finding the sum double sum (int a[], int n) { if ( ) return ; return + ; } n == 0 0 a[0] sum (a+1, n-1)

19-Feb-02 Sudeshna Sarkar, CSE, IIT Kharagpur5 Various ways that sum() might be called Invocation What gets computed and returned

19-Feb-02 Sudeshna Sarkar, CSE, IIT Kharagpur6 Recursive programs using arrays int sum (int a[], int size) { if (size==0) return 0; return a[0]+sum(a+1,size-1); } void reverse (int a[], int size) { if (size==0 || size==1) return; swap2 (&a[0], &a[size-1]); reverse (a+1,size-2); } could have written swap2(a,a+size-1);

19-Feb-02 Sudeshna Sarkar, CSE, IIT Kharagpur7 Recursive programs using arrays int * binsearch (int elem, int a[], int size) { if (size==0) return NULL; mid = size/2; if (elem == a[mid]) return &a[mid]; if (elem > a[mid]) { return binsearch (elem, a+mid+1, size-mid-1); } else return (elem, a, mid); }

19-Feb-02 Sudeshna Sarkar, CSE, IIT Kharagpur8 Recursive programs using arrays void remove (char c, char s[]) { char * p; if (s[0]==‘\0’) return; if (s[0] == c) { shiftleft (s); remove (c, s); } else remove (c, s+1); } void shiftleft (char s[]) { int i; for (i=0; s[i] !=‘\0’;i++) { s[i] = s[i+1]; }

19-Feb-02 Sudeshna Sarkar, CSE, IIT Kharagpur9 Strings  1-d arrays of type char  By convention, a string in C is terminated by the end-of-string sentinel \0 (null character)  char s[21] - can have variable length delimited with \0.  Max length is 20 as the size must include storage needed for the delimiter.  String constants : “hello”, “abc”  “abc” is a character array of size 4.

19-Feb-02 Sudeshna Sarkar, CSE, IIT Kharagpur10 Strings zA string constant is treated as a pointer. Its value is the base address of the string. char *p = “abc”; printf (“%s %s\n”,p,p+1); /* abc bc is printed */ abc\0 p

19-Feb-02 Sudeshna Sarkar, CSE, IIT Kharagpur11 Differences : array & pointers char *p = “abcde”; The compiler allocates space for p, puts the string constant “abcde” in memory somewhere elese, initializes p with the base address of the string constant. char s[] = “abcde”;  char s[] = {‘a’,’b’,’c’,’d’,’e’.’\0’}; The compiler allocates 6 bytes of memory for the array s which are initialized with the 6 characters. abcde\0abcde s p

19-Feb-02 Sudeshna Sarkar, CSE, IIT Kharagpur12 A program using strings /* Count the number of words in a string */ #include int word_cnt (char s[]) { int cnt = 0, i; for (i=0; s[i] != ‘\0’) { while (isspace(s[i])) /* skip white space */ ++i; if (s[i] != ‘\0’) { /* found a word */ ++cnt; while (!isspace(s[i]) && s[i]!=‘\0’) /* skip the word */ ++i; } return cnt; }

19-Feb-02 Sudeshna Sarkar, CSE, IIT Kharagpur13 The program with pointers /* Count the number of words in a string */ #include int word_cnt (char *s) { int cnt = 0; while (*s != ‘\0’) { while (isspace(*s)) /* skip white space */ ++s; if (*s != ‘\0’) { /* found a word */ ++cnt; while (!isspace(*s) && *s!=‘\0’) /* skip the word */ ++s; } return cnt; }

19-Feb-02 Sudeshna Sarkar, CSE, IIT Kharagpur14 String-handling functions in the standard library z#define string.h zchar *strcat (char *s1, const char *s2); zTakes 2 strings as arguments, concatenates them, and puts the result in s1. Returns s1. Programmer must ensure that s1 points to enough space to hold the result. char *strcat(char *s1, const char *s2) { char *p = s1; while (*p) /* go to the end */ ++p; while (*p++ = *s2++) /* copy */ ; return s1; }

19-Feb-02 Sudeshna Sarkar, CSE, IIT Kharagpur15 Dissection of the strcat() function char *p = s1; p is being initialized, not *p. The pointer p is initialized to the pointer value s1. Thus p and s1 point to the same memory location. while (*p) ++p; /*  while (*p != ‘\0’) ++p; */ As long as the value pointed to by p is non-zero, p is incremented, causing it to point at the next character in the string. When p points to \0, the expression *p has the value 0, and control exits the while statement. while (*p++ = *s2++) ; At the beginning, p points to the null character at the end of string s1. The characters in s2 get copied one after another.

19-Feb-02 Sudeshna Sarkar, CSE, IIT Kharagpur16 zint strcmp (const char *s1, const char *s2); Two strings are passed as arguments. An integer is returned that is less than, equal to, or greater than 0, depending on whether s1 is lexicographically less than, equal to, or greater than s2. int strcmp(char *s1, char *s2) { for (;*s1!=‘\0’&&*s2!=‘\0’; s1++,s2++) { if (*s1>*s2) return 1; if (*s2>*s1) return -1; } if (*s1 != ‘\0’) return 1; if (*s2 != ‘\0’) return -1; return 0; }

19-Feb-02 Sudeshna Sarkar, CSE, IIT Kharagpur17 char *strcpy (char *s1, const char *s2); The characters is the string s2 are copied into s1 until \0 is moved. Whatever exists in s1 is overwritten. It is assumed that s1 has enough space to hold the result. The pointer s1 is returned. size_t strlen (const char *s); A count of the number of characters before \0 is returned. size_t strlen (const char *s) { size_t n; for (n=0; *s!=‘\0’; ++s) ++n; return n; } char * strcpy (char *s1, char *s2) { char *p = s1; while (*p++ = *s2++) ; return s1; }

19-Feb-02 Sudeshna Sarkar, CSE, IIT Kharagpur18 Examples of string handling functions char s1[] = “beautiful big sky country”, s2[] = “how now brown cow”; Expression Value strlen (s1) strlen (s2+8) strcmp(s1,s2) Statements What gets printed printf(“%s”,s1+10); strcpy(s1+10,s2+8); strcat(s1, “s!”); printf(“%s”, s1);

19-Feb-02 Sudeshna Sarkar, CSE, IIT Kharagpur19 Examples of string handling functions char s1[] = “beautiful big sky country”, s2[] = “how now brown cow”; Expression Value strlen (s1) 25 strlen (s2+8) 9 strcmp(s1,s2) negative integer Statements What gets printed printf(“%s”,s1+10); big sky country strcpy(s1+10,s2+8); strcat(s1, “s!”); printf(“%s”, s1); beautiful brown cows!