Principles of Programming I

1
Principles of Programming I

• Note Set 16

2
Semester Overview

• Functions
• Character File I/O
• Arrays
• Pointers and Dynamic Memory Allocation
• Characters and Strings
• Structures/Records
• Binary File I/O

3
Objectives

• Structures, Records and Files

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Pre-Reqs

• Review 3 steps in dealing with a file
• open a stream to the file
• use fstream, ifstream, or ofstream
• read/write to the file
• use ltlt, gtgt, get(), getline(), etc.
• close the stream to the file

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Pre-Reqs

• sizeof(dataTypeHere)

struct person char name20 int
age cout ltlt sizeof(person)
What would this display?
6
Converting Pointers

• To convert a pointer from one type to another
  type, use reinterpret_cast type cast.

General Form reinterpret_castltdataTypegt(value)
int x 10 char ptr ptr reinterpret_castltchar
gt(x)
7
Binary File I/O
Binary files contain data that is unformatted and
not necessarily stored as ASCII text
What happens if you open a MS Word file in
notepad?
8
Review
Character File I/O
int x 1234 ofstream out(stuff.dat) out ltlt x
This would write the ASCII characters 1, 2,
3, 4 to the file stuff.dat
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• Writing Binary
• Data to a File

ofstream out(file.dat, iosoutiosbinary)
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The Structure
Remember this structure
struct person char name20 int age
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Write
fileObject.write(address, size)

• fileObject the name of the file stream object
• address The starting address of the section of
  memory that is to be written to the file.
  Expected to be a char.
• size The number of bytes of memory to write.
  Must be an integer value

12
Setup
int main() person p fstream
outFile(people.dat,
iosoutiosbinary) if(outFile.fail())
cout ltlt Error opening fileltltendl return
0 getData(p, outfile)//coming up
outFile.close() return 0
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getData
void getData(person p, fstream out) char
again int flag do char buffer80 cout
ltlt "Enter the persons name " cin.getline(p.name
, 19)
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getData
//ask for, get, and validate the age
do flag 0 cout ltlt "Age " cin.getline(buff
er, 4) for (int i 0 i lt strlen(buffer)
i) if (!isdigit(bufferi)) flag
1 cout ltlt "invalid data entered" ltlt
endl break while(flag 1)
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getData
// The good stuff p.age atoi(buffer)
out.write(reinterpret_castltchar gt(p),
sizeof(person)) cout ltlt "Do you want to
enter another ltlt record? " cin gtgt
again cin.ignore(80, '\n') while
(toupper(again) 'Y')
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• Reading Binary
• Data from a File

ofstream in(file.dat, iosiniosbinary)
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Write
fileObject.read(address, size)

• fileObject the name of the file stream object
• address The starting address of the section of
  memory where the data being read from the file is
  to be stored. Expected to be a char.
• size The number of bytes of memory to read.
  Must be an integer value.

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main
int main() fstream inFile("people.dat",
iosiniosbinary)
if(inFile.fail()) cout ltlt "Error opening
file" ltlt endl return 0
readData(inFile)//coming up inFile.close()
return 0
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readData
void readData(fstream in) person p char
again in.read(reinterpret_castltchar gt(p),
sizeof(person)) while(!in.eof())
cout ltlt "Name " ltlt p.name ltlt endl cout ltlt
"Age " ltlt p.age ltlt endl cout ltlt "\nStrike
Enter to see the next record." cin.get(again)
in.read(reinterpret_castltchar gt(p),
sizeof(person)) cout ltlt endl ltlt "That's
all!" ltlt endl
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Reading Into an Array Example
int main() fstream inFile("people.dat",
iosiniosbinary) person
pArr20 int count if(inFile.fail())
cout ltlt "Error opening file" ltlt endl return
0 readData(inFile, pArr, count)//coming
up inFile.close() return 0
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Reading Into an Array Example
void readData(fstream in, person p, int
count) count 0 in.read(reinterpret_castlt
char gt(p0), sizeof(person))
while(!in.eof()) cout ltlt pcount.name ltlt
endl count in.read(reinterpret_castltchar
gt(pcount), sizeof(person))

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• Random Access
• Files

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Random Access Files

• You have the ability to set the read and write
  pointers that are in the file

write
read
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seekp seekg

• 2 functions to move the read/write position in
  the file
• seekp used w/ files opened for output
• seekg used w/ files opened for input

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Functions for Random File Access

• file.seekp(offset, mode)
• file.seekp(20L, iosbeg)
• Sets the write position to the 21st byte from the
  beginning of the file
• file.seekp(-40L, iosend)
• Sets the write postion to the 40th byte from the
  end of the file
• file.seekp(2L, ioscur)
• Sets the write position to the 3rd byte from the
  current position

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Functions for Random File Access

• file.seekg(offset, mode)
• file.seekg(20L, iosbeg)
• Sets the read position to the 21st byte from the
  beginning of the file
• file.seekg(-40L, iosend)
• Sets the read postion to the 40th byte from the
  end of the file
• file.seekg(2L, ioscur)
• Sets the read position to the 3rd byte from the
  current position

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Simple Example
int main() fstream in("letters.txt",
iosin) char ch in.seekg(5L,
iosbeg) in.get(ch) cout ltlt "Byte 5 from
beginnning " ltlt ch ltlt endl
//Contd on next slide
abcdefghijklm
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Simple Example
in.seekg(-10L, iosend) in.get(ch) cout ltlt
"Byte 10 from end " ltlt ch ltlt
endl in.seekg(3L, ioscur) in.get(ch) cout
ltlt "Byte 3 from current " ltlt ch ltlt
endl in.close() return 0
abcdefghijklm
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Random Access with Structures

• New Structure!

struct inven char desc31 int qty float
price
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Random Access with Structures
int main() fstream inventory("inven.dat",
iosoutiosbinary) inven r "", 0,
0.0f //write 5 blank records to the
inven.dat for (int i 0 i lt 5 i)
inventory.write(reinterpret_castltchar gt(r),
sizeof(inven)) inventory.close()
return 0
Weve written 5 blank records! Yay! . Now what?
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Class Exercise

• Open stream to inven.dat
• Ask the user which record they want to change
• Find that record in the file, read it, change it,
  and write it back out.

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Questions??

• ?