1 / 36

Arrays and Pointers

Arrays and Pointers. Concepts to Learn…. Arrays C Strings Array Arguments Pointers Null Pointers Pointer Arithmetic Arrays and Pointers Static Variables Parsing C I/O Pointers to Pointers Multi-dimensional Arrays Command-line Arguments. Arrays. Arrays.

ulmer
Download Presentation

Arrays and Pointers

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Arrays and Pointers

  2. Concepts to Learn… • Arrays • C Strings • Array Arguments • Pointers • Null Pointers • Pointer Arithmetic • Arrays and Pointers • Static Variables • Parsing • C I/O • Pointers to Pointers • Multi-dimensional Arrays • Command-line Arguments

  3. Arrays Arrays • An array is a sequence of like items. • int’s, float’s, long’s, • Like any other variable, arrays must be declared before they are used. • General form: type variable-name[number_of_elements]; • The array size must be explicit at compile time – needed to reserve memory space • Array elements individually accessed with index. • General form: variable-name[index]; • Zero based subscripts • No compile-time or run-time limit checking

  4. Arrays Initialization of Arrays • Elements can be initialized when they are declared in the same way as ordinary variables. • type array_name[size] = { list of values separated by commas }; int number[3] = {0, 0, 0}; • Unspecified elements will be set to zero automatically. • Array declarations may omit the size. int counter[] = {1, 1, 1, 1}; char RGB_image[][3] = { {255,0,255}, {0,255,0}}; • Problems with C initialization of arrays • No convenient way to initialize selected elements. • No shortcut to initialize large number of elements.

  5. Local Array Example Arrays SP  int main() { int array[10]; int x; for (x = 0; x < 10; x++) { array[x] = x; } return 0; } main: 0x8040: 8031 0016 SUB.W #0x0016,SP 0x8044: 4381 0014 CLR.W 0x0014(SP) 0x8048: 90B1 000A 0014 CMP.W #0x000a,0x0014(SP) 0x804e: 340D JGE (C$DW$L$main$2$E) C$DW$L$main$2$B, C$L1: 0x8050: 411F 0014 MOV.W 0x0014(SP),R15 0x8054: 5F0F RLA.W R15 0x8056: 510F ADD.W SP,R15 0x8058: 419F 0014 0000 MOV.W 0x0014(SP),0x0000(R15) 0x805e: 5391 0014 INC.W 0x0014(SP) 0x8062: 90B1 000A 0014 CMP.W #0x000a,0x0014(SP) 0x8068: 3BF3 JL (C$L1) C$L2, C$DW$L$main$2$E: 0x806a: 430C CLR.W R12 0x806c: 5031 0016 ADD.W #0x0016,SP 0x8070: 4130 RET C_Arrays.c

  6. Global Array Example Arrays int array[10]; int x; int main() { for (x = 0; x < 10; x++) { array[x] = x; } return 0; } main: 0x806a: 4382 0214 CLR.W &x 0x806e: 90B2 000A 0214 CMP.W #0x000a,&x 0x8074: 340C JGE (C$DW$L$main$2$E) C$DW$L$main$2$B, C$L1: 0x8076: 421F 0214 MOV.W &x,R15 0x807a: 5F0F RLA.W R15 0x807c: 429F 0214 0200 MOV.W &x,0x0200(R15) 0x8082: 5392 0214 INC.W &x 0x8086: 90B2 000A 0214 CMP.W #0x000a,&x 0x808c: 3BF4 JL (C$L1) C$L2, C$DW$L$main$2$E: 0x808e: 430C CLR.W R12 0x8090: 4130 RET

  7. C Strings C Strings • A C string is an array of characters: • char outputString[16]; • C strings are terminated with a zero byte. • C strings can be initialized when defined: char outputString[] = "Text"; which is the same as: outputString[0] = 'T'; outputString[1] = 'e'; outputString[2] = 'x'; outputString[3] = 't'; outputString[4] = 0; • C has no string operators. • String functions in <string.h> library • strcpy, strlen, strcmp, strstr, … Compiler computes the size of the array (4 + 1 = 5 bytes)

  8. Arrays Common Pitfalls with Arrays • Overrun array limits. • There is no boundary checking in C. int array[10], i; for (i = 0; i< 100; i++) // oops array[i] = 0; • Arrays must be statically declared • Compiler flags run-time declarations void SomeFunction(intnum_elements) {int temp[num_elements]; // error … }

  9. Array Arguments Passing Arrays as Arguments • C passes parameters to functions by value. • C passes the address of the 1st element of an array. #define MAX_NUMS 5 int average(int values[]) { inti, sum = 0; for (i = 0; i < MAX_NUMS; i++) sum = sum + values[i]; return (sum / MAX_NUMS); } int main() { intnums[MAX_NUMS] = { 1, 2, 3, 4, 5 }; int mean = average(nums); return 0; } 0x05f2 SP  5 15 1 SP  2 3 4 5 3

  10. Pointers K&R Ch.5

  11. Pointers Swap Function Example int main() { int a = 3; int b = 4; swap(a, b); } void swap(int a, int b) { int temp = a; a = b; b = temp; } Stack after call to swap(): 4 a 3 b swap temp 3 Return Adr main

  12. Pointers Pointers int* ptr; • char* cp; • double* dp; • int** p_ptr = &ptr; • char *strings[10]; • A pointer is an address • With pointers • functions can indirectly access variables. • functions can modify the arguments passed by the caller function. • sophisticated data structures can grow and shrink at run-time. • Arrays and pointers are closely related. • Array pointers enable us to conveniently process groups of data such as vectors, lists, and strings. • A pointer variable contains a memory address • Associated with a pointer variable is the type of value to which it points. • The asterisk (*) indicates that the following identifier is a pointer variable. • The ampersand (&) returns a pointer (address) to the following identifier.

  13. Pointers Pointer Operators • A pointer variable is declared with the asterisk operator (*) type *var; // same - whitespace doesn’t matter type* var; • Dereferencing any expression returns a value *varreturns contents of the memory location pointed to by var **varreturns contents of the memory location pointed to by the memory location pointed to by var *3 returns the contents of memory location 3 • A pointer is created with the reference operator (&) &var • Reference must be applied to a memory object • &3 is illegal as it would return a pointer to a constant

  14. * =dereference & = reference Pointers Operator Precedence and Associativity

  15. 0x05fa 0x05fc Pointers Examples of Pointers int *ptr1; int *ptr2; inti = 4; int j; ptr1 = &i; ptr2 = &j; // What will these print? print(ptr1); print(ptr2); print(*ptr1); print(*ptr2); j = *ptr1; print(j); 4 4 0x05fa 0x05fc 0x0004 ?????? 0x0004

  16. 3 4 Swap Example w/Pointers Swap Example Fixed! • Stack after call to swap() int main() { int a = 3; int b = 4; swap(&a, &b); } void swap(int* a, int* b) { int temp = *a; *a = *b; *b = temp; } 0x05fa a 0x05fc b swap temp 3 Return Adr main C_Pointers.c

  17. Pointer Arithmetic

  18. Pointer Arithmetic Pointer Arithmetic • Address calculations depend on size of elements • ints are 16-bits or 2 bytes per element. • e.g., to find 4th element (x[3]), we add 3*2 to base address • If double, we'd have to add 12 (3*4) to find address of 4th element. • C does size calculations under the covers,depending on size of item being pointed to: double x[10]; double *y = x; *(y + 3) = 13; y++; *y = 3.1415926; Allocates 40 bytes (4 per element) Same as x[3] (base address plus 12) x[1] = 3.1415926

  19. 2 5 2 3 6 // y = a[0]+1 y=2, ip=0x05f2 // a[0] = a[0]+1 y=2, ip=0x05f2 0x05f4 // y = ++a[0] y=3, ip=0x05f2 // y = a[0], ip++ y=3, ip=0x05f4 // y=a[1], a[1]++ y=5, ip=0x05f4 Pointer Arithmetic Incrementing Pointers • A pointer increments according to its type. • The unary operators * and & bind more tightly than arithmetic operators. 0 3 int y = 0; int a[5] = {1, 5, 9, 13, 17}; int* ip = &a[0]; y = *ip + 1; *ip += 1; y = ++*ip; y = *ip++; y = (*ip)++; 1 5 9 13 17 0x05f2

  20. Pointer Arithmetic *ip++ • Alternate forms of pointer arithmetic // strcpy: copy s to d; version 1 void strcpy(char* d, char* s) { while ((*d = *s) != ‘\0’) { d++; s++; } } // strcpy: copy s to d; version 2 void strcpy(char* d, char* s) { while ((*d++ = *s++) != ‘\0’); } • The value of *s++ is the character that s pointed to before s was incremented; the postfix ++ does not change s until after this character has been fetched.

  21. Example // #include "msp430.h" void main(void) { char dog[] = {1, 2, 3, 4, 5, 6}; intcat[] = {1, 2, 3, 4, 5, 6); long pig[] = {1, 2, 3, 4, 5, 6); char* apple = dog; int* orange = cat; long* banana = pig; lcd_init(); // initlcd printLcd(*(++apple + 2)); printLcd(&orange[5] - cat); printLcd((int)&orange[5] - (int)cat); printLcd((pig + 2) - banana); printLcd((int)(pig + 2) - (int)&banana[0]); } 1. What is output?

  22. Pointers and Arrays

  23. h e l l o \0 a: w o r l d \0 p: Arrays and Pointers • Array names are NOT pointer variables • char a[6]; • Requests memory for 6 characters, to be known by the name “a”. • “a” is not a variable and known only at compile time (ie. defined in the compiler symbol table). • char *p; • Requests memory for a single pointer variable, to be known by the name “p”. • “p” can point to any char (or contiguous array of chars). • Example: char a[] = "hello"; char *p = "world";

  24. Arrays and Pointers Arrays and Pointers • An array name (at compile time) is essentially a pointer to the first element in an array. • Can change the value (contents) of a pointer. char word[10]; char *cptr; cptr = word; // points to word[0] • Each of the following lines evaluate the same: cptr word &word[0] address of word[0] (cptr + n) (word + n) &word[n] address of word[n] *cptr *word word[0] value of word[0] *(cptr + n) *(word + n) word[n] value of word[n]

  25. Static Variables Static Variables • Static variables are “global local” variables • May be declared inside a C function • Like local variables, a static variable cannot be referred to outside the function • However, unlike a local variable, the value continues to exist even after the function exits • The value comes back into scope when the function is called again (value retained) • A function can pass out a point to a static variable and de-reference the value static int sum = 0;

  26. Static Variables Static Array • Pointer to static array // month_name: return name of n-th month char* month_name(int n) { static char* name[ ] = { "Illegal month", "January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December" }; return ( n < 1 || n > 12 ) ? name[0] : name[n]; } // end month_name

  27. Example • Show different ways to access the character 'o' in the array x without using brackets. char x[] = "Hello"; char* y = x; char** z = &y; Want x[4]: *(x+4) *(y+4) Z[0][4] *(z[0]+4) *(*z+4) *(*(&y)+4)

  28. Pointers and More…

  29. Parsing String Parsing • What is the value of X? char str[] = "Point X = 100"; Walk the array until token or NULL terminator. (Token must be unique) • Find token: char* ptr = str; int number = 0; while (*ptr && (*ptr++ != 'X')); • Find beginning of number: while (*ptr && !isdigit(*ptr)) ++ptr; • Convert to decimal: while (isdigit(*ptr)) number = number * 10 + (*ptr++ - '0'); isdigit returns 0 or 1 • Proceed with parsing of line. Return result in number

  30. C I/O Formatted Input/Output • The printf function outputs formatted values to the stdout stream using putc printf( const char *format, ... ); printf("\nX = %d, Y = %d", x, y); Output y as an ASCII string • The function scanf is similar to printf, providing many of the same conversion facilities in the opposite direction: scanf( const char *format, ... ); scanf("%s = %f", name, &cost); name and cost MUST be pointers • Decimal "%d" or "%i" • String "%s" • Character "%c“ • Hexadecimal "%x“ • Unsigned decimal "%u • Floating point "%f" • Scientific notation "%e" • Pointer "%p" • % "%%"

  31. linesptr[0] defghi linesptr[1] lmnopqrstuvwxyz linesptr[2] abc … Pointers to Pointers Pointers to Pointers • Since pointers are variables themselves, they can be stored in arrays just as other variables can. Example: char* lines[8]; char** linesptr = lines; char* lines[8]; //char** linesptr = &lines[0]; //char** linesptr = &*lines; char** linesptr = lines; linesptr[0][0] = 'c';

  32. linesptr[0] linesptr[1] linesptr[2] … Pointers to Pointers Pointers to Pointers • Since pointers are variables themselves, they can be stored in arrays just as other variables can. Example: char* lines[8]; char** linesptr = lines; defghi lmnopqrstuvwxyz abc

  33. Multi-dimensional Arrays Multi-dimensional Arrays • Multi-dimensional arrays declared with multiple indexes • Pointers to pointers… • daytab[i][j] /* [row][col] */ • daytab[i,j] /* WRONG! */ • Array elements are stored by rows • The rightmost subscript varies the fastest • Array name “points” to 1st element • Multi-dimensional arrays passed to a function • by value (address of 1st element in 1st row, 1st column… • must declare the number of elements for every subscript except the first • func(intdaytab[ ][13]) {…}

  34. These are OK main Function The main Function Return code for system # of command line arguments • The main function • must be present in every C program. • is “called” by the operating system. • exits the program with a return statement. • The prototype of main is pre-declared as: int main (intargc, char *argv[]); • The definition doesn’t have to match. int main() { ... }main() { ... } • The return statement can be omitted.

  35. C main to Assembly main Function main: 0x8040: 8031 000A SUB.W #0x000a,SP 0x8044: 4D81 0002 MOV.W R13,0x0002(SP) 0x8048: 4C81 0000 MOV.W R12,0x0000(SP) 0x804c: 40B1 0007 0004 MOV.W #0x0007,0x0004(SP) 0x8052: 40B1 0005 0006 MOV.W #0x0005,0x0006(SP) 0x8058: 411C 0004 MOV.W 0x0004(SP),R12 0x805c: 411D 0006 MOV.W 0x0006(SP),R13 0x8060: 12B0 80DA CALL #__mpyi 0x8064: 4C81 0008 MOV.W R12,0x0008(SP) 0x8068: 430C CLR.W R12 0x806a: 5031 000A ADD.W #0x000a,SP 0x806e: 4130 RET __mpyi: 0x80da: 430E CLR.W R14 mpyi_add_loop: 0x80dc: C312 CLRC 0x80de: 100C RRC R12 0x80e0: 2801 JLO shift_test_mpyi 0x80e2: 5D0E ADD.W R13,R14 shift_test_mpyi: 0x80e4: 5D0D RLA.W R13 0x80e6: 930C TST.W R12 0x80e8: 23F9 JNE mpyi_add_loop 0x80ea: 4E0C MOV.W R14,R12 0x80ec: 4130 RET int main(intargc, char** argv) { unsigned int x = 7; unsigned int y = 5; unsigned int z; z = x * y; return 0; } x05f4 SP argc (r12) x05f6 argv (r13) x05f8 x x05fa y x05fc z x05fe ret adr x0600 Stack

  36. Example char* constmenuText[] = { "MSPEXP430F5438", " 1. App1", " 2. App2", " 3. App3", " 4. App4", " 5. App5", " 6. App6", " 7. App7", }; // menuText[0] = addr of "MSP...", *menuText[0]='M'.

More Related