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Chapter 9 - Arrays and Pointers

Chapter 9 - Arrays and Pointers. CS 124. Learning Objectives…. After completing this section, you should be able to Explain what an interrupt is. Write an Interrupt Service Routine ( ISR’s). Use the Watchdog as a limited feature timer. Discuss the computer energy consumption issues.

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Chapter 9 - Arrays and Pointers

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  1. Chapter 9- Arrays and Pointers

  2. CS 124 3.1 - The C Language

  3. Learning Objectives… After completing this section, you should be able to • Explain what an interrupt is. • Write an Interrupt Service Routine (ISR’s). • Use the Watchdog as a limited feature timer. • Discuss the computer energy consumption issues. • Use the correct processor clock for a given problem. • Use low power mode to make the process sleep. • Use timer interrupts for PWM, timing, debouncing, etc. • Explain the problem with mechanical switches. • Apply Pulse Width Modulation (PWM) to a variety of digital to analog problems. Interrupts

  4. 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 Chapter 9 - Pointers and Arrays

  5. Arrays Arrays • An array is a sequence of like items. • int’s, float’s, long’s, objects, etc • 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 Chapter 9 - Pointers and Arrays

  6. 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}}; • Global variables • Zeroed before main is called (***NOT WITH CCS***) • Problems with C initialization of arrays • No convenient way to initialize selected elements. • No shortcut to initialize large number of elements. Chapter 9 - Pointers and Arrays

  7. Arrays Local Array Example 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 Chapter 9 - Pointers and Arrays

  8. Arrays Global Array Example 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 Chapter 9 - Pointers and Arrays

  9. 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) Chapter 9 - Pointers and Arrays

  10. Arrays Common Pitfalls with Arrays • Overrun array limits. • There is no boundary checking in C. int array[10], i; for (i = 0; i <= 10; i++) // oops array[i] = 0; • Arrays must be statically declared • Compiler flags run-time declarations void SomeFunction(int num_elements) { int temp[num_elements]; // error … } Chapter 9 - Pointers and Arrays

  11. 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 Chapter 9 - Pointers and Arrays

  12. Quiz 3.2.1 How much, where, and when is memory allocated for the variable sum? How much, where, and when is memory allocated for nums? How much, where, and when is memory allocated for the variable data? How big is the activation record for the function average? // quiz 9.1 #include "msp430.h" #include <stdlib.h> #include "RBX430-1.h" #include "RBX430_lcd.h" #define MAX_NUMS 5 intaverage(intdata[]) { inti, sum = 0; for (i = 0; i < MAX_NUMS; i++) sum = sum + data[i]; return (sum / MAX_NUMS); } intnums[MAX_NUMS] = {1, 2, 3, 4, 5}; intmain() { RBX430_init(_1MHZ); lcd_init(); lcd_printf("%d", average(nums)); return 0; } Chapter 9 - Pointers and Arrays

  13. Pointers

  14. 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 Chapter 9 - Pointers and Arrays

  15. 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. Chapter 9 - Pointers and Arrays

  16. 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 Chapter 9 - Pointers and Arrays

  17. * =dereference & = reference Pointers Operator Precedence and Associativity Chapter 9 - Pointers and Arrays

  18. 0x05fa 0x05fc Pointers Examples of Pointers int *ptr1; int *ptr2; int i = 4; int j; ptr1 = &i; ptr2 = &j; // What will these print? printf("\n%04x", ptr1); printf("\n%04x", ptr2); printf("\n%04x", *ptr1); printf("\n%04x", *ptr2); j = *ptr1; printf("\n%04x", j); 4 4 0x05fa 0x05fc 0x0004 ?????? 0x0004 Chapter 9 - Pointers and Arrays

  19. 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 Chapter 9 - Pointers and Arrays

  20. Null Pointers Null Pointers • Sometimes we want a pointer that points to nothing. • Used for invalid pointer error returns • Used to catch errors • NULLis a predefined macro that contains a value that non-null pointer should never hold, usually NULL=0. int *p; p = NULL; // p is a null pointer Chapter 9 - Pointers and Arrays

  21. Quiz 3.2.2 1. What are the values on the stack just before main exits? // quiz 9.2 #include "msp430.h" #include <stdlib.h> #include "RBX430-1.h" #include "RBX430_lcd.h" void main(void) { int *ptr1; int**ptr2; inti = 10; RBX430_init(_1MHZ); // initboard lcd_init(); // initlcd ptr1 = &i; ptr2 = &ptr1; lcd_printf("\n%04x", ptr1); lcd_printf("\n%04x", ptr2); lcd_printf("\n%04x", *ptr1); lcd_printf("\n%04x", *ptr2); lcd_printf("\n%04x", **ptr2); } 2. What is the output? ptr1 ptr2 i ret Note: %04x = 4 digit hex Chapter 9 - Pointers and Arrays

  22. Pointer Arithmetic

  23. 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 Chapter 9 - Pointers and Arrays

  24. 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 Chapter 9 - Pointers and Arrays

  25. Pointer Arithmetic *ip++ • Form used by “experienced” C programmers // 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. Chapter 9 - Pointers and Arrays

  26. Quiz 3.3.3 // quiz 9.3 #include "msp430.h" #include <stdlib.h> #include "RBX430-1.h" #include "RBX430_lcd.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; RBX430_init(_1MHZ); // initboard lcd_init(); // initlcd lcd_printf("\n%d", *(++apple + 2)); lcd_printf("\n%d", &orange[5] - cat); lcd_printf("\n%d", (int)&orange[5] - (int)cat); lcd_printf("\n%d", (pig + 2) - banana); lcd_printf("\n%d", (int)(pig + 2) - (int)&banana[0]); } 1. What is output? Chapter 9 - Pointers and Arrays

  27. Pointers and Arrays

  28. 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"; Chapter 9 - Pointers and Arrays

  29. 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] Chapter 9 - Pointers and Arrays

  30. 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; Chapter 9 - Pointers and Arrays

  31. 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 Chapter 9 - Pointers and Arrays

  32. Quiz 3.3.4 • Show 3 different ways to access the character 'o' in the array x without using brackets. char x[] = "Hello"; char* y = x; char** z = &y; Chapter 9 - Pointers and Arrays

  33. Pointers and More…

  34. 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 Chapter 9 - Pointers and Arrays

  35. 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" • % "%%" Chapter 9 - Pointers and Arrays

  36. 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'; Chapter 9 - Pointers and Arrays

  37. 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 Chapter 9 - Pointers and Arrays

  38. 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]) {…} Chapter 9 - Pointers and Arrays

  39. These are OK main Function The main Function Return code for system • 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. # of command line arguments Chapter 9 - Pointers and Arrays

  40. main Function C main to Assembly 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 Chapter 9 - Pointers and Arrays

  41. Quiz 3.3.5 List the output of the following echo C program as called by: >>echo Good Morning America intmain(intargc, char* argv[ ]) { while (--argc > 0) { printf("%s%s", *++argv, (argc > 1) ? " " : ""); } printf("\n"); return 0; } Chapter 9 - Pointers and Arrays

  42. Chapter 9 - Pointers and Arrays

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