Declaring%20Arrays

1
Declaring Arrays

• Declare an array of 10 elements int nums10
• Best practicedefine SIZE 10 int
  numsSIZE // cannot be intSIZE nums
• C99 int numssomeVariable
• Declare an array with an initializer listint
  nums 1, 2, 3, 4, 5, 6, 7, 8, 9, 10Note
  Don't need a number between and
• Access elements like primitive variablesprintf
  ("d\n", numsi)
• Declare in a function void foo(int nums) or
  void foo(int nums)

2
Pointer arithmetic
2
x
Address

• int i, nums6
• for (i0 ilt6 i)
• numsi i10
• int np nums
• (np3) 33
• nums5 66
• (np2) 22
• int x (np2) - (np0)

1000
1004
1008
1012
1016
1020
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 functionvoid foo(char data)
• Accessing the 10th elementprintf("c\n", (data
  9))
• Another waychar ptr data9printf("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 (lt, gt, , etc.)
• You can use all the arithmetic operations(,
  , --, etc.)
• The size in bytes of pointers can be obtained
  using the sizeof operator

5
Examples with functions

• Declaration
• void foo (const int nums
• , int s)
• int i0, sum0
• for (i0 ilts i)
• sum numsi
• printf("d\n", sum)
• Call
• int nums 1, 2, 3
• foo(nums, 3)

• Declaration
• void foo(const int nums
• , int s)
• int sum0, ptrfor(ptrnums ptr-numslts
  ptr)
• sum ptr
• printf("d\n", sum)
• Call
• int nums 1, 2, 3
• foo(nums, 3)

Note The const modifier is not required, some
arrays can be changed
6
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 data6 data0 'a'
  data1 'b' data2 'c' data3 '\0'
• Declaring a string using a literal char data
  "abc"
• Replace the third character data2 'd' or
  (data2) 'd'
• Note 'a' is not "a". Question How do they
  differ?

7
Inputting Strings (scanf with s)

• Note scanf reads till it sees white space
• Example char s2 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 ifor
  (i0 stri!'\0' i) printf("c\n", stri)
• Another wayint i, len strlen(str)for (int
  i0 iltlen i) printf("c\n", stri)

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
  interface int 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 ltstdio.hgt
• Look in local folder include "header.h"
• Put in your header files
• includes include
• constants define
• other preprocessing directives ifndef(see
  next slide)

11
ifndef

• Problem If more than one header refers to stdio,
  it will be included twice
• Solution use "if not defined directive"ifndef
  UNIQUENAMEdefine UNIQUENAMEendif

• file1.hdefine MAX 5
• file2.h
• include "file1.h"
• prog.cinclude file1.hinclude file2.h
• MAX is defined twice

12
GDB Debugger

• Compile gcc g main.c func.c o main
• Invoke gdb main
• Popular commands
• break point break ltline gt or break ltfunction
  namegt
• List break points break or list watches display
• List source list ltline gt or list ltfunction
  namegt
• Step into step or Step over next or Continue
  cont
• Display variables and expressions print
  ltexpressiongt
• Create a watch display ltexpressiongt
• Run the program run or Exit debugger quit
• Delete watch delete display or Delete
  breakpoint delete
• Current stack record where and Caller's stack
  record up
• GDB Help help or help command

Note There are cheat sheets available that can
help (see class web site)