Introduction to Computer Algorithmics and Programming Ceng 113

1. Introduction to Computer Algorithmics and ProgrammingCeng 113 Serap ATAY <serapatay@iyte.edu.tr> Pointers _ Part 2

2. What is wrong? • *s + 1 = ‘x’; • *(s+1) = ‘x’; • int p; .... *p=‘a’; • c = a * *b; • char ch; char *p; ... ch = ‘a’; p = &ch;

3. What is wrong? ERROR; The address operator can be used with only objects. MAX is only a logical constant. #define MAX 100 .. char *p; ... p = &MAX;

4. Pointer increment/decrement char *p; int a; ... p=p + a; p=p – 2; p += a; p= p + a + 10; ++p;

5. Sample “Swap operation” #include void swap(int *x, int *y); void main() { int a=10, b= 20; swap(&a, &b); printf(“a= %d b = %d \n”, a, b); } void swap(int *x, int *y) { int temp; temp = *x; *x=*y; *y= temp; }

6. Sample“Array operation” #include <stdio.h> int max(int *p, int n); void main() { int a[10]={10,-4,5,9,71,98,89,4,8, 10}; int m; m= max(a, 10); printf(“The greatest number=%d \n”, m); } int max(int *p, int n) { int maxval, k; maxval = p[0]; for (k=1; k <n; ++k) if (maxval < p[k]) maxval = p[k]; return maxval; } int max(int *p, int n) { int maxval, k; maxval = 0; for (k=0; k <n; ++k) { if (maxval < *p) maxval = *p; p++; } return maxval; }

7. Sample“Array operation”

8. Sample #include <stdio.h> void getname(char *s); void main() { char str[50]; getname(str); printf("%s \n", str); } void getname(char *s) { printf("Name - Last name:"); gets(s); }

9. Sample “Split String” Suppose we are given a string of the form "Last_name/First_name." We want to split this into two strings, one containing the first name and one containing the last name. We need a function to find the slash in the name. my_strchr  Finds a character in a string. Parameters: string_ptr String to look through. find Character to find. Returns: pointer to 1st occurrence of character in string or NULL for error.

10. Sample “Split String”

11. Sample “Split String” #include <stdio.h> #include <string.h> #include <stdlib.h> int main() { char line[80]; /* The input line */ char *first_ptr; /* pointer to the first name */ char *last_ptr; /* pointer to the last name */   gets(line);   last_ptr = line; /* last name is at beginning of line */   first_ptr = my_strchr(line, '/'); /* Find slash */   /* Check for an error */ if (first_ptr == NULL) { printf("Error: Unable to find slash in %s\n", line); exit (0); }   *first_ptr = '\0'; /* Zero out the slash */   ++first_ptr; /* Move to first character of name */   printf("First:%s Last:%s\n", first_ptr, last_ptr); return (0); }

12. Sample “Split String” char *my_strchr(char *string_ptr, char find) { while (*string_ptr != find) {   /* Check for end */   if (*string_ptr == '\0') return (NULL); /* not found */   ++string_ptr; } return (string_ptr); /* Found */ }

13. Sample _ String Copy // This program is copied one string to another and use pointers. # include <stdio.h> # include <string.h> void string_copy(char *source, char *dest); void main () { char str_from[80], str_to[80]; gets(str_from); str_to[0]='\0'; string_copy(str_from, str_to); puts(str_to); } void string_copy(char *source, char *dest) { while *source != '\0'); { *dest++ = *source++; } *dest = '\0'; } void string_copy(char *source, char *dest) { while *source != '\0'); { *dest = *source; dest++; source++; } *dest = '\0'; } void string_copy(char *source, char *dest) { while ((*dest++ = *source++) != '\0'); }

14. Sample – “String Compare” #include <stdio.h> #include <string.h> int str_compare(char *str1, char *str2); void main() { char s1[20], s2[20]; int result=0; printf(“Input the first string: \n”); gets(s1); printf(“Input the second string: \n”); gets(s2); result = str_compare(s1, s2); if (result == 0) printf(“The strings are same.\n”); else printf(“The strings are different. \n”); } int str_compare(char *str1, char *str2) { while (*str1 == *str2) { if (*str1 == ‘\0’) return 0; ++str1; ++str2; } return *str1 - *str2; }

15. Homework • Write an algorithm and C code to count the vowels and letters in a string. • Read a string from the screen. • Count the vowels and letters • Then print out the • number of occurrences of each of the vowels a, e, ı, i, o, ö, ü and u in the text, • the total number of letters, and • each of the vowels as an integer percentage of the letter total. • Suggested output format is: Numbers of characters: a 3 ; e 2 ; i 0 ; o 1 ; u 0 ; rest 17 Percentages of total : a 13%; e 8%; i 0%; o 4%; u 0% ; rest 73% • Use pointers for all string, comparison and calculation operations. Upload to FTP until December 14th. 2006

16. Quiz Question Write a program, which is concatenate second string to the end of the first string. For example: string_1= “Happy” string_2=“New year” After operation string_1=“HappyNew year” • Use pointers for all array operations. • Do not use “strlen” standard function.

17. String Concatenate #include <stdio.h> #include <string.h> char *string_cat(char *s1, char *s2); void main() { char str1[50], str2[50]; char *p; gets(str1); gets(str2); p = string_cat(str1, str2); printf("%s \n", p); } char *string_cat(char *s1, char *s2) { char *temp; temp = s1; while (*s1 != '\0') s1++; while ((*s1++ = *s2++)!= '\0'); return temp; }

18. Next Course • Functions