KIERAN C WOOT WOOT 2

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KIERAN C WOOT WOOT 2

  1. 1. 3/23/2014 Course material created by D. Woit 1 CPS 393 Introduction to Unix and C START OF WEEK 9 (C-3)
  2. 2. 3/23/2014 Course material created by D. Woit 2 Pointers • Pointer is a data type which allows storage of location (address) of data rather than data itself. See example1.c • int main(void) { • int Num; • int *NumPtr; • Num=5; • NumPtr = &Num; • printf(" %d n", *NumPtr ); • *NumPtr = 7; • printf(" %p %d %d n", NumPtr, *NumPtr, Num ); • return(0); • } • NumPtr is a "pointer to int" • &Num is the address of variable Num (1200 here) • *NumPtr is the value at address in NumPtr (de-reference NumPtr) • or, what NumPtr "points to"
  3. 3. 3/23/2014 Course material created by D. Woit 3 • After statements Num=5; NumPtr = &Num; memory allocation is; • 1200 1204 1208 ... 1531 - memory addresses • ------------------------ --------------- • ... | 5 | | | ... |1200 | - memory storage • ------------------------ ---------------- • Num NumPtr • After statement *NumPtr = 7; memory allocation is: • 1200 1204 1208 ... 1531 - memory addresses • ------------------------ --------------- • ... | 7 | | | ... |1200 | - memory storage • ------------------------ ---------------- • Num NumPtr
  4. 4. 3/23/2014 Course material created by D. Woit 4 Pointers and Arrays • an array name is an address (i.e., a pointer) • i.e. it is address of the 1st item of the array • Code in ptr1.c • int main(void) { • int A[5] = {1,2,3,4,5}; • int *p; • p=A; • printf("%d %d %d ", *p, *(p+1), *(p+2) ); • putchar('n'); • printf("%d %d %d ", p[0], p[1], p[2] ); • putchar('n'); • printf("%p %p %p ", p, p+1, p+2 ); • printf("nsize of int: %dn",sizeof(int)); • return(0); • }
  5. 5. 3/23/2014 Course material created by D. Woit 5 • int A[5] = {1,2,3,4,5}; • 100 104 108 112 116 ... 236 ... …. 300 • ------------------------ ----- ------ ------ • | 1 | 2 | 3 | 4 | 5 |... … | |....... |100| • ------------------------ ----- ----- ------- • A[0] A[1] A[2] A[3] A[4] A • int *p; • 100 104 108 112 116 ... 236 ... 300 • ---------------------------- -------- ----- • | 1 | 2 | 3 | 4 | 5 |... | |... |100| • -------------------------- ---- ----------- • A[0] A[1] A[2] A[3] A[4] p A • • p=A; • 100 104 108 112 116 ... 236 ... 300 • ----------------------------- ----- --- • | 1 | 2 | 3 | 4 | 5 |... |100|... |100| • ------------------------ ----- ------ ----- • A[0] A[1] A[2] A[3] A[4] p A •
  6. 6. 3/23/2014 Course material created by D. Woit 6 • Discussion of previous example in ptr1.c • printf("%d %d %d ", *p, *(p+1), *(p+2) ); • putchar('n'); • printf("%d %d %d ", p[0], p[1], p[2] ); • putchar('n'); • printf("%p %p %p ", p, p+1, p+2 ); • 1 2 3 • 1 2 3 • 100 104 108 //assuming above storage placement • why 104 ? 4 byte ints on our machine--use sizeof(int) to see. • ^ sizeof is a keyword (built-in like "for") • depends on type of p. • if int *p then p+1 is p+4bytes • if char *p then p+1 is p+1byte •
  7. 7. 3/23/2014 Course material created by D. Woit 7 Pointers cont. • Wrong usage , see ptrerr.c : • char B[] = "bcd"; • char *p = B; /*wrong initialization ptr to address of B[0] */ • /*or: char *p = &B[0]; */ • assuming &B[0] is 1000, what is the output of: • printf("%p %p %p ", p, p+1, p+2 ); • SAME THING: • char B[] = "bcd"; • char *p; • p=B; /*or p=&B[0]*/ • Note p[1] same as B[1] same as *(p+1) i.e., 'c' • p[0] same as B[0] same as *p i.e., 'b' •
  8. 8. 3/23/2014 Course material created by D. Woit 8 • //Source: cptr.c • char S[9]="Pointers"; • char *cptr; • cptr=S; # try to omit this line and compile and then run • while ( putchar(*cptr++) ); //putchar returns the char it printed • //this prints the '0' but the following • //do not: • // or, more clearly • // while ( *cptr ) putchar(*cptr++); • // or even more clearly • // while ( *cptr != '0' ) putchar(*cptr++); • // or even more clearly • // while ( *cptr != '0' ) { • // putchar(*cptr); • // cptr++; • // }
  9. 9. 3/23/2014 Course material created by D. Woit 9 • /*Source: sample12d.c */ • /*Input: a string max length 39 */ • /*Output: the string in upper case */ • /*Purpose: convert string to upper case */ • #include <ctype.h> • #include <stdio.h> • int main(void) { • char str[40], *p; • printf("Enter a string: "); • (void) gets(str); • p=&str[0]; /*or p=str */ • while ( *p != '0' ) { /*or while(*p) */ • *p++ = toupper(*p); • } • printf("%sn",str); • exit(0); • } • Note: the body of the while stmt is exactly same as { *p = toupper(*p); p++; }
  10. 10. 3/23/2014 Course material created by D. Woit 10 • /*Source highest.c*/ • #define ELTS 5 • #include <stdio.h> • int main(void) { • int i, *high, *p; • int array[ELTS]={200,34,78,600,45}; • high=array; • p=array; • for ( i=1; i< ELTS; i++ ) { • p++; • if (*p > *high) high = p; • } • printf("the highest number is %d n", *high); • printf("at address %p n", high); • printf("at index %d of array n", high-array); • exit(0); • }
  11. 11. 3/23/2014 Course material created by D. Woit 11 Pointers and String Constants • when compiler encounters a string constant, it stores it in a string table and generates a pointer to the string (address) • char *p; • p="A String"; • printf(p); • Note that the following is BAD: • char *p; • gets(p); • printf(p); • Why bad? • p has no storage allocated to it. It might "work" sometimes • and not work other times because its storage is overwritten, • or because the value IN p does not point to an accessible • storage location.
  12. 12. 3/23/2014 Course material created by D. Woit 12 Pointers as Parameters • all parameters in C are passed by value • However, we can fake pass by reference by passing the *address*, and then use * inside function to modify the data • this explains: scanf("%d", &num ); • Example: • int A[20]; • A is the address of the array (address of A[0]) • Thus, a function call such as • sum(A) • passes the *address*; so any changes to A within sum • are retained (i.e., like call by reference.)
  13. 13. 3/23/2014 Course material created by D. Woit 13 (code in callByRef.c) • Example for call by reference: • void add1 (int *x) { • *x = *x + 1 ; • } • ... • int a=5; • add1(&a); • printf("a is %dn",a); •
  14. 14. 3/23/2014 Course material created by D. Woit 14 • /*Source: sample14.c*/ • void swap(int *arg1, int *arg2); • int main(void) { • int i=0, j=2; • printf("i is %d, j is %d n", i,j); • swap(&i,&j); • printf("i is %d, j is %d n", i,j); • exit(0); • } • • /*Function: swap • Purpose: swap values of 2 integers • Input: addresses of the 2 integers to swap • Output: none • */ • void swap(int *arg1, int *arg2 ) { • int temp; • temp = *arg1; • *arg1=*arg2; • *arg2=temp; • } • prints?
  15. 15. 3/23/2014 Course material created by D. Woit 15 • /*Source: sample15.c */ • void swap1 (int A[2] ); • int main(void) { • int A[2]={1,2}; • printf("%d %d n", A[0], A[1] ); • swap1(A); • printf("%d %d n", A[0], A[1] ); • exit(0); • } • • void swap1 (int A[2] ) { • int temp; • temp = A[0]; • A[0] = A[1]; • A[1] = temp; • } • These function prototypes are all the same: • void swap1(int A[2]); • void swap1(int A[]); • void swap1(int *A);
  16. 16. 3/23/2014 Course material created by D. Woit 16 A function may have type "pointer to type" • /*Source: sample16.c*/ • /* adds string XX to input string */ • #include <stdio.h> • #include <string.h> • char *addXX(char *S); • int main(void) { • char str1[25], *str2; • printf("input a string: "); • gets(str1); • str2=addXX(str1); • puts(str2); • // printf("concatenated string is now %sn", str2 ); • exit(0); • } • char *addXX(char *S) { • strcat(S,"XX"); • return (S); • } • Does str2 have storage allocated? No need because all processing is done on string str1. • The problem may occur if the user enters a string that is more than 23 chars long. • Why? Because original memory allocation is exceeded.
  17. 17. 3/23/2014 Course material created by D. Woit 17 HMWK • 1. Write a function with prototype: void swap_string1(char *A, char *B); • The function swaps the strings in A and B. It does this by looping through the string(s) and swapping individual characters. If your function needs to know the length of a string, it can use strlen from string.h, or it can just look for the '0' at the string end. • Write a main program to test swap_string1. Your main must allocate all the storage required for the strings. DO NOT copy junk from memory. • Only copy characters from the strings.
  18. 18. 3/23/2014 Course material created by D. Woit 18 Multiple Indirection • char **mp, *p, ch; • 1080 ... 1248 ... 1272 • ----------- ---------- ---------- • ... | | ... | | ... | | • ----------- ---------- ---------- • mp p ch • p=&ch; mp=&p; **mp='A'; • 1080 ... 1248 ... 1272 • ----------- ---------- -------------- • ... | 1248 | ... | 1272 | ... | A | • ----------- ---------- --------------- • mp p ch • This is same as simply ch= ‘A’
  19. 19. 3/23/2014 Course material created by D. Woit 19 Precedence of Array & Ptr operations: • Operator Precedence Associativity (see def. below) • () [] -> . Left to right • ++ -- sizeof ! ~ (type) + - * & right (ALL ARE UNARY OPERATORS) • * / % left to right • + - left (BINARY) • << >> (bitwise left and right shift) left • < <= > >= left • == != left • & (bitwise AND) left • ^ (bitwise XOR) left • | (bitwise OR) left • && left • || left • ?: right ternary operator • = += -= *= /= %= &= ^= |= <<= >>= right • , left
  20. 20. 3/23/2014 Course material created by D. Woit 20 • Associativity means if 2 operators with equal precedence are in an expression, they will be done from left-to-right if left-assoc and from right-to-left if right-assoc • Operators -> and . are used to access fields of structures • ~ is one's complement (unary) • ! is logical not: if (!x) • ++ -- are unary inc/decrement prefix or postfix: i++ or ++i • unary * unary & are ptr dereference, address of • << >> left bit shift, right bit shift • && || logical and, or • & | ^ bitwise and, or, Xor • ?: ternary operation (for conditional e.g., z = (a>b) ? a : b ; resolves to: z = a OR z = b , depending on (a>b) i.e the same as z= max(a,b)
  21. 21. 3/23/2014 Course material created by D. Woit 21 • Note: • **a means *(*a) • &(&a) will result in compiler error see sample17.c • (&&a is invalid since && is binary logical AND operator ) • &a[1] means &(a[1]) • *a[1] means *(a[1]) •
  22. 22. 3/23/2014 Course material created by D. Woit 22 Dealing with errors Compile this program with gcc –lm –o sqrt sqrt.c • //source sqrt.c • #include <stdio.h> • #include <errno.h> • #include <math.h> • #define POSITIVE 25 • #define NEGATIVE -25 •
  23. 23. 3/23/2014 Course material created by D. Woit 23 • int main(void){ • double ret; • errno = 0; • ret = sqrt(NEGATIVE); • if (errno == EDOM) /*EDOM Signifies Domain Error*/ • //printf("Domain Error : Invalid Input To Functionn"); • perror("sqrt"); • else • printf("Valid Input To Functionn"); • errno = 0; • ret = sqrt(POSITIVE); • if (errno == EDOM) • printf("Domain Error : Invalid Input To Functionn"); • else • printf("Valid Input To Functionn"); • return 0; • }
  24. 24. • If a function sets errno, it is stated in man page. e.g., • man scanf • then search for errno • then search for ERROR • You can also check man errno

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