COMP102 Lab 7

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COMP102 Lab 7
2
File I/O

• The file operations always follow the following
  sequences
• Open the file
• Read/write the file contents
• Close the file
• REMEMBER to close the file. Otherwise, the
  contents may be lost!

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eof()

• A function which return 1/true if the ifstream
  HAS reached the end of the file
• ifstream will NOT know it has reached the end
  untill it has tried to read something from the
  file

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Example

• Suppose there are 2 characters in the file a.txt
• What will bethe output of the following code
  segment?

ifstream fin char c fin.open("a.txt") while
(!fin.eof()) fin.get(c) cout ltlt
c cout ltlt "end" ltlt endl fin.close()
ab
a.txt
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Example

• The output will be something like this
• There is an extra space between ab and end
• The extra space is actually the EOF character
• Why??

ab end
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Example

• Lets add some code to display the value of eof()

ifstream fin char c fin.open("a.txt") while
(!fin.eof()) cout ltlt "BEFORE get() eof()"
ltlt fin.eof() ltlt endl fin.get(c) cout ltlt
"c" ltlt c ltlt endl cout ltlt " AFTER get()
eof()" ltlt fin.eof() ltlt endl cout ltlt "end" ltlt
endl fin.close()
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Example

• Output

BEFORE get() eof()0 ca AFTER get()
eof()0 BEFORE get() eof()0 cb AFTER get()
eof()0 BEFORE get() eof()0 c  AFTER get()
eof()1 end
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Explanation

• Before each iteration, the condition in while
  loop will be evaluated
• The loop body will be executed only when the
  condition is true, i.e. eof() 0 or false
• While a character is successfully read from the
  file stream, the value of eof() will not change,
  even it is the last character of the file
• ifstream will change the return value of eof()
  only when no more character can be read. In other
  words, an attempt to get a character is required

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Explanation

• Therefore, the character collected from get() in
  the loop body may NOT be valid
• get() will return -1 if no more character can be
  read from the file stream
• There are a number of ways to verify if the
  character collected from get() is valid or not
• Add an extra eof() check after get()
• Use the return value of get()

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Solution 1 Extra eof() Check
ifstream fin char c fin.open("a.txt") while
(!fin.eof()) fin.get(c) if
(!fin.eof()) cout ltlt c cout ltlt "end" ltlt
endl fin.close()
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Solution 2 Return Value of get()
ifstream fin char c fin.open("a.txt") while
(!fin.eof()) if (fin.get(c)) cout ltlt
c cout ltlt "end" ltlt endl fin.close()