Re: Reading Files and ASCII Code

"Tom Widmer [VC++ MVP]" <>
Thu, 07 Sep 2006 10:49:39 +0100
Dustin Ventin wrote:

I actually have two questions.

The first involves reading from a normal everyday txt file. I need to be
able to read data from a text file a variable number of characters at a time.
 I may need to read in one character at one point, or three, or five. Is
there a function that allows me to say: "read in x characters from where the
pointer is now in the file"? What about moving the pointer in the file back
a couple characters? For example, from:

This is a sample sentence.

Move back three characters to:

This is a sample sentence.

Information on these fuctions and how to use them would be invaluable.
PLEASE keep in mind that it is imperitive that whitespaces, carrage returns,
etc must ALL be included in the data returned be these functions, so that I
can save this data and print it out again, returning EXACTLY the same file as
was input.

Using IOStreams, you want something like:

std::ifstream ifs("file.txt");
//you might want to pass std::ios_base::binary as the second argument,
//depending on whether you want to see CR LF for new lines in the file.

//read 1 character:
char c;
if (!ifs.get(&c))
   //read failed
//read n characters:
std::vector<char> buffer(n);
if (![0], buffer.size()))
   //read failed
//buffer[0] - [n-1] now contain the n characters read
//seek back a couple of characters:
if (!ifs.seekg(-2, std::ios_base::cur))
   //seek failed

Second question:

I need to place the entire ASCII character library into a vector. For
example, if the ASCII character library goes 'a', 'A','b','B','c','C', my
vector will read:

MyVector[0] = 'a';
MyVector[1] = 'A';
MyVector[2] = 'b';
MyVector[3] = 'B';
MyVector[4] = 'c';
MyVector[5] = 'C';

I want to programatically write the entire library into a vector. I imagine
that this is fairly simple with a for loop and the appropriate call to the
ASCII libraby. Any ideas?

ASCII is stored in memory as bytes containing the numbers 0 to 127.
There's no need for a table as such. The 40th ascii character is simply
(char)40 (or static_cast<char>(40) using C++ style casting).


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