C++ ifstream read only integers - c++

Question

I have a txt file with numbers with this format 12345678-A plus some random numbers and text in between. I need to read the file and save only the 8 digit integrers to an array. How do I do it?
Current code I have, works if there are numbers only:
const int MAX = 1000;
ifstream file("file.txt");
int data;
int index = 0;
int bigdata[MAX];
while (!file.eof() && index < MAX)
{
file >> data;
if (data > 20000000 && data < 90000000)
{
bigdata[index] = data;
index++;
}
}
Sample of input text:
48251182-D 6,5 6
49315945-F 7 3
45647536-I 3,5 3
45652122-H 7 6,5
77751157-L 2 2,5
75106729-S 2 5
77789857-B 4 3 3,5 3
59932967-V 4 8,5
39533235-Q 8 8,5
45013275-A 5 2
48053435-Y 6 8
48015522-N 3,5 4
48015515-T
48118362-B 7,5 3,5
39931759-Q 5,5 3
39941188-D 3,5 1,5
39143874-I 3,5 4
48281181-O 6,5 6

Answer 1

If all you need is to strip out the first number in each line then you can use the stream's operator >> to read in the integer part and then use std::getline to consume the rest of the line. Using
std::vector<int> data;
ifstream fin("file.txt");
int number;
std::string eater;
while (fin >> number) // get first 8 digit number. stops at the '-'
{
data.push_back(number);
std::getline(fin, eater); // consume the rest of the line
//now we are on the next line
}
// now data has all of the numbers that start each line.

Answer 2

You need this:
...
#include <string>
...
string line;
while (getline(file, line)) // read the whole line
{
int data = stol(line); // get number at start of line (stops
// automatically at the '-' sign
// data is the 8 digit number
// process data here...
...
}
...

Answer 3

One solution to this problem would be to read the stream character by character and search for 8-digit numbers:
static const int MAX = 1000;
int index = 0;
int bigdata[MAX];
int value = 0;
int digits = 0;
for (auto c = file.get(); c != EOF 0; c = file.get())
{
if (c >= '0' && c <= '9')
{
value = (digits ? (value * 10) : 0) + (c - '0');
++digits;
if (digits == 8)
{
if (index < MAX)
bigdata[index++] = value;
digits = 0;
}
} else
{
digits = 0;
}
}
The code reads byte by byte and builds an integer number if decimal digits are read. If the digit count reaches 8 the number is stored and the integer number buffer is reset. The integer number buffer is reset immediatly if the character read is not a decimal digit.
istream.get() returns EOF if reading past the end of the file so no need for a call to the .eof() member function.
Please note that this code stores all 8-digit numbers. You have to add another test if you need only numbers in the range 20000000..90000000.
If your lines are always starting with an 8-digit number followed by a minus then you could use this simple solution which I would recommend because of improved readability.
std::string buffer;
std::vector<int> target;
while (std::getline(file, buffer))
target.push_back(atoi(buffer.c_str()));
But this solution does not check for the presence of a valid number and simply stores 0 for each line not starting with a number.