File IO

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• Programming

• File I/O

2
Using Input/Output Files (Review)

• A computer file
• is stored secondary storage devices (hard drive,
  CD)
• can be used to provide input data or receive
  output data, or both
• must be opened before using

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Using Input/Output Files

• stream - a sequence of characters
• where is it from (input)?
• where does it go (output)?
• interactive (iostream)
• ? cin - input stream associated with keyboard
• ? cout - output stream associated with display
• file (fstream)
• ? ifstream - defines new input stream (normally
  associated with a file)
• ? ofstream - defines new output stream (normally
  associated with a file)

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Example

• When you type I am Peter on keyboard, we say
  that I am Peter is an input stream (of
  characters).
• A C program can be considered simply as a long
  stream of characters, an input stream from a
  file.
• e.g. include ltiostream.hgt void main() cout ltlt
  Hello Mary! ltlt endl return 0

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File-Related Functions

• include ltfstreamgt
• xxx.open(fname)
• connects stream xxx to the external file fname
• xxx.get(ch)
• Gets the next character from the input stream xxx
  and places it in the character variable ch
• xxx.put(ch)
• Puts the character ch into the output stream xxx
• xxx.eof()
• tests for end-of-file condition
• xxx.close()
• disconnects the stream and closes the file

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ltlt and gtgt Example 1

• You can read and write integers, doubles, chars,
  etc. from files just like cin gtgt and cout ltlt
• include ltiostreamgt
• include ltfstreamgt
• using namespace std
• void main()
• ifstream fin
• int A4, r
• fin.open("file1.dat") // read data file of four
  integers
• for(r0 rlt4 r) // into array
• fin gtgt Ar
• fin.close()
• ofstream fout
• fout.open("file2.dat") // write data file
• for(r3 rgt0 r--) // with numbers reversed
• fout ltlt Ar ltlt ' '
• fout.close()

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File I/O Example 1

• file1.dat
• 1 2 3 4(eof)
• file2.dat
• 4 3 2 1 (eof)

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File I/O Example 2

• // Copies indata.dat to outdata.dat
• // and counts the number of lines.
• // Prints file to screen too.
• include ltiostreamgt
• include ltfstreamgt
• using namespace std
• void main()
• ifstream ins
• ofstream outs
• int count0
• char next
• ins.open("indata.dat") // open the input file
• outs.open("outdata.dat") // open the output file

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File I/O Example 2

• while(true) // loop for each line
• while(true) // loop to read each char on line
• ins.get(next)
• if(ins.eof() next '\n')
• break
• cout ltlt next // echo next char on display
• outs ltlt next // copy next char to file
• count
• cout ltlt endl // echo '\n' on display
• if(ins.eof())
• break
• outs ltlt endl // copy '\n' to file
• ins.close()
• outs.close()
• cout ltlt "Number of lines copied " ltlt count ltlt
  endl

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File I/O Example 2

• indata.dat
• a b c
• top10 methods to count spaces
• 1 3(eof)
• outdata.dat
• a b c
• top10 methods to count spaces
• 1 3(eof)
• Output to screen
• a b c
• top10 methods to count spaces
• 1 3
• Number of lines copied 4

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File I/O Example 3

• // Counts the number of blanks on each line of
  indata.dat.
• // Outputs each line, and number of blanks on
  each line.
• include ltiostreamgt
• include ltfstreamgt
• using namespace std
• void main()
• ifstream ins
• int count
• char next
• ins.open("indata.dat") // open the file

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File I/O Example 3

• while(true) // loop to read each line
• count 0
• while(true) // loop to count spaces on
  line
• ins.get(next) // read next character
• if(ins.eof() next '\n') // line done?
• break // go to next line if line done
• cout ltlt next // otherwise output character
• if(next' ') // increment if blank
• count
• cout ltlt endl
• cout ltlt "Blanks " ltlt count ltlt endl
• if(ins.eof()) // done?
• break
• ins.close()

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File I/O Example 3

• indata.dat
• a b c
• top10 methods to count spaces
• 1 3(eof)
• Output to screen
• a b c
• Blanks 2
• top10 methods to count spaces
• Blanks 4
• Blanks 0
• 1 3
• Blanks 3