Reading from and Writing to Files

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Reading from and Writing to Files
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Files (3.12)

• Data stored in variables is temporary
• Files are used to permanently store large amounts
  of data
• We will learn how to write programs that can
• Create files
• Write to files
• Read from files
• This is similar to how we read from the keyboard
  and wrote to the screen

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Steps to Using Files

• There are five steps that must be taken in order
  to use files in C
• Include header files
• Define a file stream object
• Open the file
• Use the file
• Close the file

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1. Libraries

• To access files you will need to include
• ltiostreamgt
• ltfstreamgt

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2. File Stream Objects

• ifstream inputFile
• ofstream outputFile
• fstream inAndOut
• File stream objects are the ways that you refer
  to the files you are using
• Can specify which input/output file to use
• May input from more than one file
• May output to more than one file

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3. Opening Files

• inputFile.open(filename)
• Same syntax for both input and output files
• Filename is a string literal
• Example
• ifstream inputFile
• inputFile.open(input.dat)

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Check File Opened Correctly

• Before we start using the file for reading or
  writing, we should make sure that it opened
  correctly
• if(!inputInfo true)
• cout ltlt Error opening input file
• exit(1)

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true

• These two statements are equivalent
• if(!inputInfo true)
• if(!inputInfo)
• Even if you dont have true in your loop, C
  will put it there by default
• This applies to all conditional statements in
  repetition and selection structures

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4. Using File Streams

• Use input file variable wherever you use cin
• Examples
• inputFile gtgt num
• Output output file variable wherever you use cout
• Examples
• outputFile ltlt num

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20.1 Example Writing to a File

• The following program asks the user to input
  numbers and writes these numbers to a file

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Example

• includeltfstreamgt
• includeltiostreamgt
• using namespace std
• int main()
• ofstream outputFile
• int num
• outputInfo.open("out.dat")
• if (!outputInfo)
• cout ltlt " Error opening file" ltlt endl
• exit (1)
• cout ltlt "Enter a number (9999 to quit) "
• cin gtgt num
• while (num ! 9999)
• outputFile ltlt num ltlt " "
• cin gtgt num

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20.2 Reading from a File

• Write a program that will read in a sequence of
  numbers (double) from a file and calculate the
  sum. Assume that the last number is the trailer
  (-9999)

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20.3 Reading Until the EOF

• It is possible to read from a file until the end
  is reached
• while (inputFile gtgt num)
• cout ltlt num ltlt " "
• sum num

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20.4 Reading Characters

• Write a program that reads in some text from a
  file and outputs that text to the screen
• The file contains
• Hello Everyone!
• I'm a file that
• contains some text.

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Solution

• ifstream inputFile
• char letter
• inputFile.open("in.dat")
• if (!inInfo)
• cout ltlt " Error opening file" ltlt endl
• exit (1)
• while (inputFile gtgt letter)
• cout ltlt letter
• cout ltlt endl

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The Output

• HelloEveryone!I'mafilethatcontainssometext.
• Whats happened?!
• All spaces, tabs, and new lines have been
  ignored.
• This is because gtgt only reads visible characters
• How can we read all characters so that the output
  looks exactly like the input

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Solution

• ifstream inputFile
• char letter
• inputFile.open("in.dat")
• if (!inInfo)
• cout ltlt " Error opening file" ltlt endl
• exit (1)
• while (inputFile.get( letter ))
• cout ltlt letter
• cout ltlt endl

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20.5 Problem

• Consider the data file below, where - indicate
  spaces
• --12--33.4
• -d--12.3
• -2--5
• What values would be assigned to the variables
  for each of the statements below where inputFile
  is the file variable?
• int i,j
• double x,y
• char ch
• inputFile gtgt i gtgt x gtgt y
• inputFile gtgt i gtgt j
• inputFile gtgt ch gtgt i
• inputFile gtgt x gtgt y gtgt ch gtgt x

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Problem 0 ( yournumber 3)

• Read in the following file and print out the sum
  of ALL the numbers in the file and print out the
  sum of the numbers on each line, except the first
  number. The first number tells you how many more
  numbers there will be on that line.

Output Line 1 10 Line 2 6 Line 3 20 Total
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File Example 4 3 4 1 2 3 2 3 1 5 9 0 9 1 1
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Problem 1 ( yournumber 3)

• Read in the following file and print out the
  character you read and whether or not the letter
  is a vowel. At the end, print out how many of
  each vowel you found.

Output a Vowel b Not Vowel c Not Vowel A
Vowel e Vowel f Not Vowel i Vowel a 2 e
1 i 1 o 0 u 0
File Example a b c A e f i
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Problem 2 ( yournumber 3)

• Prompt the user for their first and last name and
  50 numbers. Write the first and last name to a
  file followed by the numbers, two numbers on each
  line in reverse order.

Output file Bob Smith 4 9 99 3 ... 1 9
Input Name? Bob Smith Number 1 9 Number 2
4 Number 3 3 Number 4 99 ... Number 49
9 Number 50 1