Programming Language B

Transcription

1 Programming Language B Takako Nemoto (JAIST) 16 January Takako Nemoto (JAIST) 16 January 1 / 15

2 Strings and pointers #include <stdio.h> //11-1.c char str[] = "ABC"; char *ptr = "123"; printf("str = \"%s\"\n", str); printf("ptr = \"%s\"\n", ptr); Recall that strings are arrays of char-type elements. In this case, the pointer *p is initialized by the string literal "123". It points to the first element of the string. When the pointer *p is declared, it occupies storage area for both the pointer *p itself and the string literal "123". On the other hand, str[] occupies storage area only for the string "ABC". Takako Nemoto (JAIST) 16 January 2 / 15

3 Size of pointers #include <stdio.h> // c double s[] = {1,2,3; double *p = s; printf("sizeof(s) = %d\n", sizeof(s)); printf("sizeof(s[0]) = %d\n",sizeof(s[0])); printf("sizeof(p) = %d\n", sizeof(p)); printf("sizeof(*p) = %d\n", sizeof(*p)); Recall that *p is the alias for s[0]. Takako Nemoto (JAIST) 16 January 3 / 15

4 Pointers and strings #include <stdio.h> //11-2.c char s[] = "ABC"; printf("s = \"%s\"\n", s); s = "DEF"; printf("s = \"%s\"\n", s); #include <stdio.h> //11-3.c char *p = "123"; printf("p = \"%s\"\n", p); p = "456"; printf("p = \"%s\"\n", p); A string can initialize a char-type array, but cannot be assigned to it. A string can be assigned to a char *-type pointer. Takako Nemoto (JAIST) 16 January 4 / 15

5 Today s Quiz #include <stdio.h> // c char *p = "123"; printf("p = \"%s\"\n", p); printf("address of p = %p\n", p); puts("assign \"456\" to p!"); p = "456"; printf("p = \"%s\"\n", p); printf("address of p = %p\n", p); char s[] = "ABC"; printf("s = \"%s\"\n", s); printf("address of s = %p\n", &s); puts("assign \"DEF\" to s!"); Run the example and verify that the address pointed by *p is changed by the assignment. Assign "DEF" to s in the end of the program and print s and the address of s. Verify that the address of s is not changed by the assignment. Send the modified program and the screenshot of the execution result to me with the subject Quiz 13. Include your name and student ID. Takako Nemoto (JAIST) 16 January 5 / 15

6 Today s Quiz #include <stdio.h> // c char *p = "123"; printf("p = \"%s\"\n", p); printf("address of p = %p\n", p); puts("assign \"456\" to p!"); p = "456"; printf("p = \"%s\"\n", p); printf("address of p = %p\n", p); char s[] = "ABC"; printf("s = \"%s\"\n", s); printf("address of s = %p\n", &s); puts("assign \"DEF\" to s!"); Run the example and verify that the address pointed by *p is changed by the assignment. Assign "DEF" to s in the end of the program and print s and the address of s. Verify that the address of s is not changed by the assignment. Send the modified program and the screenshot of the execution result to me with the subject Quiz 13. Include your name and student ID. Takako Nemoto (JAIST) 16 January 6 / 15

7 Arrays of strings #include <stdio.h> int i; char a[][5] = {"LISP", "C", "Ada"; char *p[] = {"PAUL", "X", "MAC"; a is an array. Its elements are strings with length (at most) 5. p is an array with char *-type elements. Each element of p points to a string. for (i = 0; i < 3; i++) printf("a[%d] = \"%s\"\n", i, a[i]); for (i = 0; i < 3; i++) printf("p[%d] = \"%s\"\n", i, p[i]); Takako Nemoto (JAIST) 16 January 7 / 15

8 Arrays of strings #include <stdio.h> int i; char a[][5] = {"LISP", "C", "Ada"; char *p[] = {"PAUL", "X", "MAC"; a is an array. Its elements are strings with length (at most) 5. p is an array with char *-type elements. Each element of p points to a string. for (i = 0; i < 3; i++) printf("a[%d] = \"%s\"\n", i, a[i]); for (i = 0; i < 3; i++) printf("p[%d] = \"%s\"\n", i, p[i]); Takako Nemoto (JAIST) 16 January 8 / 15

9 Operations for strings by pointers #include <stdio.h> //11-5.c int str_length(const char *s){ int len = 0; while(*s++) len++; return len; Recall that the null character with value 0 is inserted in the end of strings. s is incremented after checking *s. The while-loop is not executed if the value *s is 0. char str[128]; printf("input an string: "); scanf("%s", str); printf("the length of %s is %d.\n", str, str_length(str)); Takako Nemoto (JAIST) 16 January 9 / 15

10 Copying strings while(*d++ = *s++) acts 1 *s is assigned to *d; #include <stdio.h> //11-6.c char *str_copy(char *d, const char *s){ char *t = d; while(*d++ = *s++) ; return t; char str[128] = "ABC"; char tmp[128]; printf("str = \"%s\"\n", str); printf("input a string: "); scanf("%s", tmp); str_copy(str, tmp); puts("your input is copied to str."); printf("str = \"%s\"\n", str); 2 The value *d is checked; 3 if *d is not 0 (which means d does not point the end of the string), while loop is executed. It works same as while(d[i] = s[i]) i++; but works effectively since it needs no the memory for i, and it does not execute the addition for i. Takako Nemoto (JAIST) 16 January 10 / 15

11 Pointer as the return value #include <stdio.h> //ex11-3.c char *str_copy(char *d, const char *s){ char *t = d; while(*d++ = *s++) ; return t; char str[128] = "ABC"; char tmp[128]; printf("str = \"%s\"\n", str); printf("input a string: "); scanf("%s", tmp); puts("your input is copied to str."); printf("str = \"%s\"\n", str_copy(str, tmp)); Takako Nemoto (JAIST) 16 January 11 / 15

12 Bad example #include <stdio.h> //11-7.c char *str_copy(char *d, const char *s){ char *t = d; while(*d++ = *s++) ; return t; char *ptr = "1234"; char tmp[128]; printf("ptr = \"%s\"\n", ptr); printf("input a string: "); scanf("%s", tmp); str_copy(ptr, tmp); puts("your input is copied to str."); printf("ptr = \"%s\"\n", ptr); The example tries to rewrite the string literal "1234" itself, which is impossible. Furthermore, the storage pointed by *p might not enough big enough to copy the input string tmp. Takako Nemoto (JAIST) 16 January 12 / 15

13 Some more examples Many operators for strings are provided by <string.h>. Some of them are implemented easily. An example of implementation of strlen #include <stdio.h> //11-8.c size_t strlen(const char *s){ size_t len = 0; while(*s++) len++; return len; char str[128]; printf("input a string: "); scanf("%s", str); printf("the length of the string is %d.\n", strlen(str)); Takako Nemoto (JAIST) 16 January 13 / 15

14 Today s homework 1 Do Exercise 11-4 ( 11-4): Make function void put string(const char *s) to print a string without using the subscript operator [ ]. Make also the main function to check its operation. 2 Do Exercise 11-8 ( 11-8): Make a function void del digit(char *str) to delete numerals in the input string. For example, the input "A2BC19" is renewed to "ABC". Make also the main function to check its operation. 3 The following is an example of the function to concatenate two strings. Explain how it works in English (or Japanese). char *strcat(char *s1, const char *s2){ char *tmp = s1; while (*s1) s1++; while (*s1++ = *s2++) ; return tmp; Takako Nemoto (JAIST) 16 January 14 / 15

15 Today s homework 4 Read sections 12. As usual, send the programs and the text file as attached files to me via , with the title Homework Lecture 13 by the next lecture. Please make sure to include your name and student ID number. Takako Nemoto (JAIST) 16 January 15 / 15