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Declaring Arrays

Declaring Arrays. Declare an array of 10 elements: int nums[10]; Best practice: #define SIZE 10 int nums[SIZE] C99: int nums[someVariable] Declare an array with an initializer list int nums[] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}; Note: Don't need a number between [ and ]

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Declaring Arrays

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  1. Declaring Arrays • Declare an array of 10 elements: int nums[10]; • Best practice: #define SIZE 10 int nums[SIZE] • C99: int nums[someVariable] • Declare an array with an initializer listint nums[] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};Note: Don't need a number between [ and ] • Access elements like primitive variablesprintf ("%d\n", nums[i]); • Declare in a function: void foo(int nums[])

  2. Content 10 20 22 33 50 66 Pointer arithmetic 2 x int i, nums[6]; for (i=0; i<6; i++) nums[i] = i*10; int *np = nums; *(np+3) = 33; nums[5] = 66; *(&np[2]) = 22; int x = &(np[2]) - &(np[0]) Address Nums Note: *(nums + 1) is not equal to *nums + 1

  3. Accessing Arrays using pointers An array is just a pointer to a contiguous block of memory • Declaring in a function:void foo(char *data) • Accessing the 10th elementprintf("%c\n", *(data + 9)); • Another waychar *ptr = data+9;printf("%c\n", *ptr); • Declare: int *x; • Be careful, there is no memory allocated • C does no memory bound checks

  4. Notes on Pointers • C Programmers often favor pointers to process arrays (less typing) • You can use all the relational operators with pointers (<, >, ==, etc.) • You can use all the arithmetic operations(++, +=, --, etc.) • The size in bytes of pointers can be obtained using the sizeof operator

  5. Declaration void foo (const int nums[] , int s) { int i=0, sum=0; for (i=0; i<s; i++) sum += nums[i]; printf("%d\n", sum); } Call int nums[] = {1, 2, 3}; foo(nums, 3); Declaration void foo(const int *nums , int s) { int sum=0, *ptr;for(ptr=nums; ptr-nums<s; ptr++) sum += *ptr; printf("%d\n", sum); } Call int nums[] = {1, 2, 3}; foo(nums, 3); Examples with functions Note: The const modifier is not required, some arrays can be changed

  6. 'a' 'b' 'c' '\0' ??? ??? Strings • In C • A string is an array of characters (C has no String type) • The entire array may not be filled • Unlike Java, strings are mutable • The string is terminated by a null ('\0') character • The hard way: char data[6]; data[0] = 'a'; data[1]= 'b'; data[2] = 'c'; data[3] = '\0'; • Declaring a string using a literal:char[] data = "abc"; • Replace the third character:data[2] = 'd';or*(data+2) = 'd'; • Note: 'a' is not "a". Question: How do they differ?

  7. 'a' 'b' '\0' Inputting Strings (scanf with %s) • Note: scanf reads till it sees white space • Example: char s[2]; scanf("%s", s); • Problem: inputting "ab" stores '\0' outside the bounds of the array • Result: possible "Segmentation Fault" • Solution: Be sure to define enough space • Another Solution: Use fgets (later topic)

  8. String Output • Print entire string: printf("%s\n", str); • Character by character (a line each)int i;for (i=0; str[i]!='\0'; i++) printf("%c\n", str[i]); • Another wayint i, len = strlen(str);for (int i=0; i<len; i++) printf("%c\n", str[i]);

  9. String Functions • Header file: string.h • Length of a string (excluding the null ('\0'))int strlen: int strlen(const s[]) • Copy from one string to anotherchar* strcpy(char toStr[], const char fromStr[]) • Compare Strings: like the Java Comparable interfaceint strcmp(const char s1[], const char s2[]) • Concatenate a stringchar* strcat(char toStr[], const char fromStr[]) • Notes: • Make sure that the destination string is big enough • There are many more string functions than these • Some (not all) systems include string functions in stdio.h • If there is no null ('\0') bizarre things can happen

  10. Header Files • Look usual places: #include <stdio.h> • Look in local folder: #include "header.h" • Put in your header files: • includes: #include • constants: #define • other preprocessing directives: #ifndef(see next slide)

  11. Problem: If more than one header refers to stdio, it will be included twice Solution: use "if not defined directive"#ifndef UNIQUENAME#define UNIQUENAME•••#endif file1.h#define MAX 5 file2.h #include "file1.h" prog.c#include file1.h#include file2.h MAX is defined twice #ifndef

  12. GDB Debugger • Compile: gcc –g main.c func.c –o main • Invoke: gdb main • Popular commands: • break point: break <line #> or break <function name> • List break points: break or list watches: display • List source: list <line #> or list <function name> • Step into: step or Step over: next or Continue: cont • Display variables and expressions: print <expression> • Create a watch: display <expression> • Run the program: run or Exit debugger: quit • Delete watch: delete display # or Delete breakpoint delete # • Current stack record: where andCaller's stack record: up • GDB Help: help or help command Note: There are cheat sheets available that can help (see class web site)

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