CMPUT 201: Lab 3

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CMPUT 201 Lab 3
C-style Strings and Debugging
October 3rd, 2002
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Command Line Arguments

• Often its necessary to provide a program
  parameters (such as names of files to use) from
  the command line
• This is done with command line arguments
• programName arg1 arg2 arg3 ...
• A modified version of the main function uses
  two special variables argc and argv to access
  command line arguments

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Command Line Arguments

• int main( int argc, char argv ) ... if
  (argc gt 1) cout ltlt Program name ltlt
  argv0 ltlt endl
• argc contains the number of parameters, which
  includes the program name (so in a program called
  with one option, argc 2)
• argv contains an array of strings that holds
  the arguments (so argv0 is the program name,
  argv1 the first parameter and so on)

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C-style Strings

• In C there is no built-in string class strings
  are implemented simply as character arrays
• These arrays contain the characters in the
  string followed by a null byte that is
  represented as \0
• \0 is an escape sequence like \n (the
  newline character) that marks the end of the
  string
• C string manipulation functions depend on the
  presence of \0 since this tells them when to
  stop reading the string

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C-style Strings

• So a strings size is always one more than the
  number of text characters it contains
• char str hello
• This string is 6 bytes long and is stored as
• str0 str1 str2 str3 str4 str5 h
  e l l o \0
• The above declaration is equivalent to
• char str6 hello
• since this string is 6 bytes long

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C String Pitfalls

• You can not use a C string variable the same
  way as other variables in assignment or
  comparison expressions
• You can use the assignment operator after a
  string only in initialization
• // this is acceptablechar myString20 alpha
  beta
• // this is illegalmyString gamma
• You can not use the comparison operator to
  test if two C strings are equal

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Built-in String Functions

• The following functions are included in the
  library cstring
• Function Description
• strlen(str) Returns the length of str
  without \0
• strcmp(str1, Compares the two strings
  alphabetically str2) Returns
  zero (0) if they are equal, non-
  zero otherwise
• strcpy(dest, Puts the contents of source
  in dest source) source remains
  unchanged

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Built-in String Functions

• Function Description
• strcat(target, Concatenates target and
  source and source) places the
  result in target
• strstr(ref, Returns the position of the
  first search) occurrence of search
  in ref
• To use any of these functions, your program
  must include the cstring library
• include ltcstringgt // should be at the top of
  your program

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Debugging

• Many interactive debugging tools exist for
  Unix we will use Data Display Debugger (DDD)
• You must compile all the modules (the .cpp
  files) with the g flag in order for DDD to
  understand how to debug your application
• To debug an application, type
• ddd tokenize1
• The debugger window then opens with a display
  of the module containing the applications main
  function