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Chapter 10 Strings and Pointers

Chapter 10 Strings and Pointers. Introduction. String Constant Example: printf(“Hello”); “Hello” : a string constant A string constant is a series of characters surrounded by double quotes. How to declare a variable to store string values?

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Chapter 10 Strings and Pointers

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  1. Chapter 10 Strings and Pointers

  2. Introduction • String Constant • Example: printf(“Hello”); • “Hello” : a string constant • A string constant is a series of characters surrounded by double quotes. • How to declare a variable to store string values? • Represent a string using a one-dimensional array of type char char string[size]; • Question: The size of a character array is fixed, how can this variable take string constants with different lengths as values?

  3. Outline • String: • Representation of a string: \0 • Using scanf to read in string • Initilization of strings • String-Handling Functions in the Standard Library • Passing Arguments to main() using an array of strings

  4. The End-of-String Sentinel \0 • A string is • a one-dimensional array of type char. • char w[100]; • character value \0 is used to terminate a string • strings have a variable length delimited by the null character \0 but with a maximum length determined by the size of the character array • The size of the string must include the storage needed for the null character \0.

  5. The End-of-String Sentinel \0 the null character value \0 is used to terminate a string • Example: #include <stdio.h> int main(void){ char w[100]; w[0]='A'; w[1]='B'; w[2]='C'; w[3]='\0'; w[4]=‘D'; printf("%s\n", w); } #include <stdio.h> int main(void){ char w[100]; w[0]='A'; w[1]='B'; w[2]='C'; w[3]='\0'; printf("%s\n", w); } % a.out ABC % a.out ABC

  6. The End-of-String Sentinel \0 The size of the string must include the storage needed for the null character \0. • Example: #include <stdio.h> int main(void){ char w[3]; w[0]='A'; w[1]='B'; w[2]='C'; w[3]='\0'; printf("%s\n", w); } overrun the bounds of w

  7. Outline • String: • Representation of a string: \0 • Using scanf to read in string • Initilization of strings • String-Handling Functions in the Standard Library • Passing Arguments to main() using an array of strings

  8. Using scanf to reading string • Using scanf to read in a string • scanf(“%s”, w); • read in non-white space characters • positions the input stream to an initial non-white space character • read in non-white space characters • The process stops when a white space character or EOF is encountered. • a null character is placed in memory to end the string.

  9. Using scanf to reading string scanf(”%s”,w); • read in non-white space characters • positions the input stream to an initial non-white space character • read in non-white space characters • The process stops when a white space character or EOF is encountered. • a null character is placed in memory to end the string. #include <stdio.h> int main(void){ char w[10]; printf("Enter strings\n", w); scanf("%s", w); printf("%s\n", w); } % a.out Enter strings Hello Hello % a.out Enter strings Hello World Hello

  10. Outline • String: • Representation of a string: \0 • Using scanf to reading string • Initilization of strings • String-Handling Functions in the Standard Library • Passing Arguments to main() using an array of strings

  11. Initialization of Strings • Initialization of Strings • Example: initialize a string variable as “abc” • char s[] = {‘a’, ‘b’, ‘c’, ‘\0’}; • char s[]=“abc”; #include <stdio.h> int main(void){ char w[]="abc"; printf("%d\n", sizeof(w)); } % a.out 4 The size of the string must include the storage needed for the null character \0.

  12. Initialization of Strings • A pointer to char can also be initialized with a constant string. • A string constant is stored in memory by the compiler. • the pointer is assigned the address of the constant string in memory. • Example: char p* = “abc”; #include <stdio.h> int main(void){ char *p="abc"; printf("%s\n",p); } % a.out abc

  13. Initialization of Strings • Difference between • initializing an array with a constant string and • initializing a pointer with a constant string #include <stdio.h> int main(void){ char s[]="abcdefg"; char *p="abcdefg"; printf("%s\n",s); printf("%s\n",p); printf("%d\n",sizeof(p)); printf("%d\n",sizeof(s)); } % a.out abcdefg abcdefg 4 8 4 bytes is used to represent a memory address

  14. Initialization of Strings • Difference between • initializing an array with a constant string • the array contains the individual characters followed by the null character • initializing a pointer with a constant string • A string constant is stored in memory by the compiler. • the pointer is assigned the address of the constant string in memory.

  15. Examples: Process a string using array notation with subscripts #include <stdio.h> int main(void){ char c, name[100]; int i; printf("Enter a Message:\n"); for (i=0; (c=getchar())!='\n'; ++i) name[i]=c; name[i]='\0'; for (i=0; name[i]!='\0'; ++i) { if(isupper(name[i])) name[i]=tolower(name[i]); else if(islower(name[i])) name[i]=toupper(name[i]); } printf("\n%s\n", name); } % a.out Enter a Message: Have a Good Day! hAVE A gOOD dAY!

  16. Examples: Process a string using pointer #include <stdio.h> int main(void){ char c, name[100], *p; int i; printf("Enter a Message:\n"); for (i=0; (c=getchar())!='\n'; ++i) name[i]=c; name[i]='\0'; for (p=name; *p!='\0'; ++p) { if(isupper(*p)) *p=tolower(*p); else if(islower(*p)) *p=toupper(*p); } printf("\n%s\n", name); } % a.out Enter a Message: Have a Good Day! hAVE A gOOD dAY!

  17. Examples: Process a string using pointer #include <stdio.h> int main(void){ char s[] = "Hello World"; printf("%s\n", s); printf("%s\n", s+1); printf("%s\n", s+2); } % a.out Hello World ello World llo World

  18. Outline • String: • String-Handling Functions in the Standard Library • Passing Arguments to main() using an array of strings

  19. String-Handling Functions in the Standard Library • String-handling functions: • Function prototypes are provided by string.h • #include <string.h> • Functions: • Concatenate two strings: strcat (s1, s2); • Compare two strings: int strcmp (s1, s2); • Copy s2 to s1: strcpy (s1, s2); • Length of a string: strlen (s);

  20. String-Handling Functions in the Standard Library • char &strcat (char *s1, const char *s2); • Concatenates s1 and s2, the result is put in s1. The string s1 is returned. #include <stdio.h> #include <string.h> int main(void){ char s1[100] = "Good Day"; char s2[100] = "Hello World"; strcat(s1, s2); printf("s2=%s, s1= %s\n", s2, s1); strcat(s1, s2+6); printf("s2=%s, s1= %s\n", s2, s1); } % a.out s2=Hello World, s1= Good DayHello World s2=Hello World, s1= Good DayHello WorldWorld

  21. String-Handling Functions in the Standard Library • int strcmp (const char &s1, const char *s2); • An integer is returned that is less than, equal to, or greater tan zero, depending on whether s1 is lexicographically less than, equal to, or greater than s2 #include <stdio.h> #include <string.h> int main(void){ char s1[100] = "Good Day"; char s2[100] = "Hello World"; printf("%d\n", strcmp(s1, s2)); printf("%d\n", strcmp(s1, s1)); printf("%d\n", strcmp(s2, s1)); } % a.out -1 0 1

  22. String-Handling Functions in the Standard Library • char *strcpy (char *s1, const char *s2); • s2 is 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 value s1 is returned. #include <stdio.h> #include <string.h> int main(void){ char s1[100] = "Good Day"; char s2[100] = "Hello World"; strcpy(s1, s2); printf("s2=%s, s1= %s\n", s2, s1); strcpy(s1+1, s2); printf("s2=%s, s1= %s\n", s2, s1); strcpy(s1+1, s2+6); printf("s2=%s, s1= %s\n", s2, s1); } % a.out s2=Hello World, s1= Hello World s2=Hello World, s1= HHello World s2=Hello World, s1= HWorld

  23. String-Handling Functions in the Standard Library • unsigned strlen (const char *s); • A count of the number of characters before \0 is returned. #include <stdio.h> #include <string.h> int main(void){ char s1[100] = "Good Day"; printf("%d\n", strlen(s1)); } % a.out 8

  24. Outline • String: • String-Handling Functions in the Standard Library • Passing Arguments to main() using an array of strings

  25. Passing Arguments to main() • How main() communicates with the operating system? • int main(void) • int main( int argc, char *argv[]) • argc: the number of the command line arguments • argv: an array of strings

  26. Passing Arguments to main() #include <stdio.h> int main(int argc, char *argv[]){ int i; printf("%d \n", argc); for (i=0; i < argc; ++ i) printf("%s\n", argv[i]); } %a.out Hello World 3 a.out Hello World

  27. Summary • String: • Representing a string using an array of characters • \0 is used to terminated a string •  strings have a variable length delimited by the null character \0 but with a maximum length determined by the size of the character array • initialization of strings • String-Handling Functions in the Standard Library • Passing Arguments to main() • argc: number of arguments • argv: an array of strings

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