C++ ifstream read only integers

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

Actually your code is partially broken anyway. Which resource taught you to write `while (!file.eof())`? [We need to correct it](http://stackoverflow.com/q/5605125/560648). — Lightness Races in Orbit, Apr 14 '16 at 12:40
Probably [this one](http://stackoverflow.com/questions/2084265/reading-integers-from-a-text-file-with-words) on SO. — , Apr 14 '16 at 12:42
@RawN: Unlikely. That's one of millions of obscure answers on SO exhibiting the problem, and there's a comment right underneath it saying "don't do this". I'm looking to be told which book or textbook is misteaching. — Lightness Races in Orbit, Apr 14 '16 at 12:44
@LightnessRacesinOrbit Would it be okay to fix that answer since the OP has not? — NathanOliver, Apr 14 '16 at 12:47
@LightnessRacesinOrbit I took it from [here](http://www.cplusplus.com/forum/beginner/8388/) if I recall correctly. — Christian Callau, Apr 14 '16 at 12:49
@ChristianC.Romero: Okay, best to learn from [a book](http://stackoverflow.com/q/388242/560648) (and highly upvoted SO answers) than random forum posts written by newbies ;) — Lightness Races in Orbit, Apr 14 '16 at 12:56
@ChristianC.Romero can you confirm that the `,` character in `48251182-D 6,5 6` is a decimal point ? — Jabberwocky, Apr 14 '16 at 13:00

score 5 · Answer 1 · answered Apr 14 '16 at 13:25

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.

score 2 · Answer 2 · answered Apr 14 '16 at 13:23

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...
   ...
}
...

answered Apr 14 '16 at 13:23

Jabberwocky

48,281
17
65
115

Thank you! That's exactly what I was looking for, I had trouble making stol / stoi work in codeblocks and I had to use atoi(line.c_str()); instead. Worked nicely. – Christian Callau Apr 15 '16 at 00:01

Andreas H. · Accepted Answer · 2016-04-15T07:00:36.583

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.

I misread the question and wrote code to read any 8-digit number from the file. My answer now contains also a simple solution to read numbers from the beginning of each line. — Andreas H., Apr 15 '16 at 07:02
@AndreasH. I had to process a file with the numbers not being at the beginning of each line and your first suggestion worked perfectly, thank you! — Christian Callau, Apr 17 '16 at 08:21

C++ ifstream read only integers

3 Answers3