Thread: Read file line, search for string, get string position

Hi,

I´m pretty much a beginner in C++, although I have some basic knowledge of C.

I´m looking for a way to do this:

test.txt:

Code:

bird=animal
fish=animal
dog=animal

- read every line of this file
- search every line for "fish="
- check if it is followed by "animal"
- if so, replace "animal" with "being"

What I have so far is this:

Code:

int main(){

    string str;
    
    ofstream writefile;
    writefile.open("test.txt");
    writefile << "bird=animal\n"
            "fish=animal\n"
            "dog=animal\n"
            "";
    
    ifstream readfile;
    readfile.open("text.txt");
    getline(readfile,str);


    // what goes here?
    

    writefile.close();
    readfile.close();
    system("pause");
}

How do I get it to do the rest?

Originally Posted by laserlight

If I understand this correctly, matching lines with only contain "fish=animal". If so, it might be easier to check every line for "fish=animal" and replace the entire matching line with "fish=being". You can make use of a while loop:

Code:

while (getline(readfile, str))
{
    // ...
}

Thanks, makes sense.

But what if the line contained:
cat=animal;fish=animal;rabbit=animal;...unknown content...
I would need to look for "fish=animal", and replace it with "fish=being" without deleting the rest of the line.

Is there a beginner-friendly way of searching a line for a string, and insert/replace with another string?

Originally Posted by laserlight

To actually check for "fish=animal", you can make use of the std::string's operator==. For replacing the line... just change what is written back to the file, e.g., if you are storing lines read in a std::vector<std::string>, make "fish=animal" be the corresponding entry in the vector to be written back to file later.

So each time the while loop gets a line, I compare str with "fish=animal"? Something like this?

Code:

bool loop=true;
while (getline(readfile, str)&&loop==true)
{
    if(str=="fish=animal"){

           // how would I go about replacing the string in the file?
           // do I need to read the whole file, change a string, then replace the whole file?
           // or is there a premade way to replace only one single line?

           loop=false;    // abort reading new lines
    }

}

There is no data yet, I´m trying to understand what is possible, and I would create any files according to this.

So let´s assume I have a file like this:

Code:

cat=animal;fish=animal;rabbit=animal;
cow;horse=being;
dog;mouse;sheep;

How would the code look like for using ; as delimiter to read, append, insert, and replace strings in this file?

I tried to find a good tutorial on file handling, but they are either too short (only simple read,write) or way too complex for me to understand yet.
Some code examples for possible operations would be much appreciated!

Originally Posted by laserlight

Well, then you need to be clear as to the file format. Like, here's another example:

Code:

catfish=animal;fish=animals

Got it, so in this case I would need to read from line beginning to first delimiter ;
then analyze the string and do something with it. So only "fish=animal" would be used.

Originally Posted by laserlight

std::string has find and replace member functions that may be applicable.

Do you have a basic code for this, or know a complete, but easy to understand tut on this topic?

Well there are a few ways to do this. Your goal is to create a function that takes into every possible situation that the format of the data can be in. I would start with your first example and assume the data is perfectly regular such as,

bird=animal
fish=animal
dog=animal

In this case the delimiter is a newline, also called a linefeed (lf), which can be written as \n.

However reading the data in a loop using getline takes care of this for you.

So in your example the common delimiter is the '=' character.

So you need to use the string function find_first_of, this return the position of the character you are looking for.

Like,

Code:

int i = 0;
while (getline(readfile, str))
{
     i = str.find_first_of("=");
     if(str.substr(0, i ) == "bird" && str.substr(i, str.length() - i) == "animal"){
    //Do Stuff
    }
  }

Originally Posted by laserlight

Out of curiosity, but why do you want to do this?

Non scholae, sed vitae discimus, right?
I´m trying to learn C++, no particular reason. Why this specifically? I tend to get stuck on small things. And filehandling always comes in handy.

Originally Posted by laserlight

That's an example of file content. We can infer the file format, but then you can just as easily say "But what if the line contained..."
See what I mean? I inferred the file format from what you described in post #1, but apparently I was wrong. So, going by my assumptions, I can reply to your "But what if the line contained" with: "the file content does not follow the file format, so stop parsing an inform the user of an error".

I´m trying to learn the the code for:
- searching a given string inside a line
- appending a string to the line
- deleting a string within a line
- inserting a string into a line

Originally Posted by laserlight

It depends on what you are trying to do. For example, maybe we shall store each string delimited by the ';' as a separate entry in a std::vector:

Code:

std::vector<std::string> tokens;
std::string token;
while (getline(readfile, token, ';'))
{
    tokens.push_back(token);
}

Now, we manipulate the std::vector, and eventually write the tokens back to file.

Ok, so I put this into a file:

Code:

    ifstream readfile;
    readfile.open("text.txt");
    vector<string> tokens;
    string token;
    int i=0;
    while (getline(readfile, token, ';'))
    {
        tokens.push_back(token);
        i++;
    }
    readfile.close();
    
    for(int j=0; j<i;j++){
    cout << tokens[j] << endl;
    }

But I get no output...

@jim_0:
I get output "=animal" with:

Code:

str.substr(i+1, 9)

but with

Code:

str.substr(i+2, 9)

I get nothing again.

I tried this:

Code:

    fstream file("test.txt",ios::in|ios::out|ios::ate);
    vector<string> tokens;
    string token;
    int i=0;
    while (getline(file, token, ';'))
    {
        tokens.push_back(token);
        i++;
        
        int k = token.find_first_of("=");
        if(k>=1){
        cout << token.substr(0, k ) << endl <<
        token.substr(k+1) << endl;
        }
        else{
            cout << token.substr(0) << endl;
            k = token.find_first_of(";");
            file.seekp(k);
            file.write("=animal",7);
            // I tried: file.insert("=animal",7); but getting an error
        }
    }
    file.close();
    
    for(int j=0; j<i;j++){
    cout << tokens[j] << endl;
    }

But no output.