search string in textfile [closed] - c++

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Say you have a text file like this
all you want from the text is the "size value"
but repeats more that thousand time with different value
"field_pic_flag : 0
bottom_field_flag : 0
idr_pic_id : 0
Found NAL at offset ,size 28813 !! Found NAL"
"field_pic_flag : 0
bottom_field_flag : 0
idr_pic_id : 0
Found NAL at offset ,size 210 !! Found NAL"
Results
i just want a code to write a text file with this format as shown below
size 28813
size 210
and so on

Answer 1

If using C/C++ is not mandatory, then grep is probably your friend :
grep "size [0-9]*" -o yourfile.txt > all_sizes.txt
And if you only need the 50 first results, head it is :
head -n 50 all_sizes > result.txt
(Now, this assumes you're using some kind of Unix, or OS X ...)

Answer 2

I saw that the question is tagged as C/C++. If you're allowed to use C++ 11, than you can easily use the new introduced regex library.
In the code bellow the data is stored directly in a string, but you can easily read it from a file. The results is saved in the result.txt file, been displayed on the screen at the same time.
#include <iostream>
#include <string>
#include <regex> // The new library introduced in C++ 11
#include <fstream>
int main()
{
const int n = 50;
int i = 0;
std::ofstream outputFile;
outputFile.open("result.txt");
// You can read this string from your input file
std::string s("\"field_pic_flag : 0\
bottom_field_flag : 0\
idr_pic_id : 0\
Found NAL at offset, size 28813 !!Found NAL\"\
\"field_pic_flag : 0\
bottom_field_flag : 0\
idr_pic_id : 0\
Found NAL at offset, size 210 !!Found NAL\"");
std::smatch m;
std::regex e("size [0-9]*");
while (std::regex_search(s, m, e) && i++<n) {
for (auto x : m) {
std::cout << x << " ";
outputFile << x << " ";
}
std::cout << std::endl;
outputFile << std::endl;
s = m.suffix().str();
}
outputFile.close();
return 0;
}
A classical solution will require a little more effort.