CMPUT 201: Lab 2

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CMPUT 201 Lab 2
I/O and Arrays
September 26th, 2002
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C Input / Output Objects

• In C the special objects cin, cout, and
  cerr are used for console I/O
• cin is used for reading data from the standard
  input (the keyboard)
• cout is used for sending program output to the
  standard output (the screen)
• cerr is used for sending error messages to the
  standard output

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cout Usage

• Text strings and variable values are output to
  the screen with cout
• cout statements have the following form
• cout ltlt item1 ltlt item2 ltlt item3
• The ltlt operator separates output items
• Here an item is a text string or variable name
• cout ltlt Number of files ltlt fileTotal
  ltlt endl

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cout Escape Sequences

• endl means a newline character. You can also
  use \n inside double quotes to specify a new
  line.
• This is one of several escape sequences
• Sequence Meaning
• \n new line
• \t horizontal tab
• \\ display backslash
• \ display single quote
• \ display double quote

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cout Number Formatting

• For floating point (decimal number) outputs, it
  is desirable to display a select number of digits
• Use setf function with flags to format output
• Use precision to specify number of decimal
  digits to show
• cout.setf(iosfixed)cout.setf(iosshowpoint)
  cout.precision(4)
• This tells cout to show the decimal point
  followed by a fixed number of digits, in this
  case 4.

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cerr Usage

• Same object type as cout, therefore you use the
  exact same functions
• cerr sends output data to the standard error
  stream (the screen by default)
• Useful for distinguishing between normal
  program output and error messages
• Can redirect cout so that a file contains
  program output and the monitor displays errors
  sent by cerr

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cin Usage

• Variable values are read in from the keyboard
  with cin
• cin statements have the following form
• cin gtgt studentID gtgt lastName
• This reads input from two lines and places
  that input in the variables studentID and
  lastName.
• The gtgt operator separates input items.

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cin Functions

• cin.get(char) reads a single character from
  input and stores it in char
• Returns true when successful, false when end of
  file is reached
• cin.putback(char) is useful when youve read a
  character but do not want to process it yet
• This function places the value in char at the
  beginning of the input stream so that it may be
  processed later

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Arrays in C

• Arrays are collections of objects of the same
  type stored in consecutive memory
• Arrays are declared with a specific data type
  and maximum size
• int marks10
• This creates an array of 10 integers.
• Arrays are ordinal since every element has a
  clear predecessor and successor (e.g. element 2
  is after element 1 and before element 3)

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Arrays in C

• Arrays provide a common name for collections of
  variables. Individual variables are accessed
  with indexes (or subscripts)
• Indexes range from 0 to arraySize 1, where
  arraySize represents the amount of memory
  assigned to the array
• This means element 0 is actually the first
  element and element arraySize 1 is the last

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Arrays in C

• So the marks10 arrays elements are marks0,
  marks1, marks9
• maxSoFar marks0
• This accesses the first element in the marks
  array and stores it in the variable maxSoFar.
• marks5 tempMark
• This sets the sixth element in the marks array
  to the value in tempMark.

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Structures in C

• Structures are record types consisting of data
  items of different types
• Individual items are referenced by field names
• A structure is declared by listing its name and
  then all the fields it contains
• struct Account String userId float
  quotaSize int monthlyHours

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Structures in C

• Elements are accessed by writing
  structureName.fieldName
• Account customer1customer1.monthlyHours 50
• This creates an Account record type named
  customer1 and stores the value 50 in its
  monthlyHours field.
• noHours customer1.monthlyHours
• This retrieves the value in the monthlyHours
  field and stores it in the variable noHours.

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Command Line Arguments

• Often its necessary to provide a program
  parameters (such as option flags) from the
  command line
• This is done with command line arguments
• programName arg1 arg2 arg3 ...
• The main function in a C program can use
  two special variables argc and argv to access
  any command line arguments

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Command Line Arguments

• int main( int argc, char argv ) ... if
  (argc gt 1)
• ...
• 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)