Reading integers from a file in C++

Question

A file contains the following data:

10 11 12 13 14 15
16 17 18 19 20 21

I have read the first line into an integer array. Now I want to read the second line of numbers into a second array. How can I accomplish the second array thing? I've used the following code to read in the first line:

while(!mystream.eof())
{
    mystream>>a[i];
    i++;
}

Hello, welcome to StackOverFlow! Please provide us with code to show us what you have tried so far and tell us where the problem is occurring so we can help you. — John Odom, Sep 10 '14 at 15:02
How did you read the first line? What does your code look like? Try doing the same thing for the second line. — interjay, Sep 10 '14 at 15:02
@AshwinRao Using `eof` to terminate a loop [is almost always wrong](http://stackoverflow.com/q/5605125/509868); your case is no exception - replace your buggy code by one suggested in answers. — anatolyg, Sep 10 '14 at 15:31

score 1 · Answer 1 · answered Sep 10 '14 at 15:16

As you haven't included any code, I guess you just need a hint. So I will show some ways you can achieve this. Feel free to ask for more details and add code.

You can read every line in a std::string and parse it. You can choose between multiple parsing options, like knowing the string format (2 digit numbers followed by a space) and using something like v[i] = (str[a] - '0') * 10 + str[a + 1] - '0'; (you need to figure what is the value of a for the i-th integer), or you can use a combination of string::find to search for spaces and string::substr to split it and something like itoa() to obtain an integer. Or, you can use sscanf(), even if it is a C function, to parse the string.

score 1 · Answer 2 · answered Sep 10 '14 at 15:23

I'd start by reading a line of text into a std::string using std::getline.

I'd then initialize a std::istringstream from that string, and parse individual ints from that line and put them into a std::vector<int>. I'd probably do this using std::istream_iterator<int>s, and initialize the vector from a pair of them.

I'd repeat that process as long as std::getline succeeded.

Reading integers from a file in C++

2 Answers2