Read several bytes, jump over N bytes, and then read several bytes again. how?

Question

I have a binary file, a.bin, which contains 768 bytes.

And I want put the bytes 16-256, 272-512, 528-768 into

char data[720]

I did somethin like

ifstream a1;
a1.open("a.bin", ios::in|ios::binary)

char tmp[256]
a1.read(tmp,256)

And then I did some loop and copy tmp to data[720] in logic. But that looks really stupid. So I want to ask

How can I read data to certain position of a char arry ?

a1.read(tmp[2],2) // not allowed, how to achieve this?
How can I jump over certain day?

a1.read(tmp16,16); I can use this to jump 16 bytes and neve use tmp16, but it looks ugly.

have a look at seekg

Sander De Dycker
– Sander De Dycker

2013-09-05 15:18:45 +00:00
Commented Sep 5, 2013 at 15:18 — Sander De Dycker
– Sander De Dycker, Commented Sep 5, 2013 at 15:18

rav_kr · Accepted Answer · 2016-07-11 21:59:31Z

11

I believe ignore is the way to go.

You go.

a1.ignore(16);                   // [1]
a1.read(data, 256-16);           // [2] 
a1.ignore(272-256);              // [3]
a1.read(&data[256-16], 512-272); // [4] 
// and so on

ignore 1st 16 bytes
you can go with data here, cause it is the address of the 1st byt of the buffer, essentially the same as &data[0]
skip next unwanted bytes
this will take and pass address of data[256-16+1] as the buffer to read into. Plain data[17] would just take a value from there, while & operator takes its address. I put 256-16 in there cause that is the number of bytes read in previous call, and we want to start reading at the next free space. Numbering from 0 this is it.

edited Jul 11, 2016 at 21:59

rav_kr

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answered Sep 5, 2013 at 15:16

luk32

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Comments

Joker_vD · Accepted Answer · 2013-09-05 15:16:01Z

8

For the first question:

a1.read(&tmp[2], 2);

reads two bytes into tmp[2] and tmp[3].

For the second question:

a1.ignore(10);

skips 10 bytes.

answered Sep 5, 2013 at 15:16

Joker_vD

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Comments

sedavidw · Accepted Answer · 2013-09-05 15:15:05Z

1

To read to a certain point in an array you need to give it the address

a1.read(tmp+2, 2)

Or you can do

a1.read(&tmp[2], 2) // the & operator is the address of operator

answered Sep 5, 2013 at 15:15

sedavidw

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Comments

zentrunix · Accepted Answer · 2013-09-06 12:27:25Z

1

To set the position to read in a stream use seekg.

http://en.cppreference.com/w/cpp/io/basic_istream/seekg

edited Sep 6, 2013 at 12:27

user283145

answered Sep 5, 2013 at 16:04

zentrunix

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