String not returning to main function

Question

I have just started with c++ and this is for a project that I'm working on. The problem is that I am not able to return a string from the SearchFunction to main. The search function itself is working perfectly and easily finding and displaying the row its supposed to find but the string temp remains empty despite me returning a string from Search Function. As a result, the DeleteFunction is not working because its not being passed the string that it's supposed to delete.

I have tried using pointers instead of returning value but still the result is same. Please help me understand where I'm going wrong.

#include<iostream>
#include<stdio.h>
#include<string>
#include<fstream>
using namespace std;

string data,temp;

string SearchFunction(string);                  
void DeleteFunction(string);                    

int main()
{
    int choice=0,choice3=0;
    char yn1;
    string search;

    cout<<"1. Press 1 to delete."<<endl;
    cin>>choice;
    cin.clear();                                        
    cin.ignore(1000,'\n');                              

    if(choice==1)
    {   
        cout<<"Enter RegNo. of record to be deleted: ";
        getline(cin,search);                        
        search="RegNo.: "+ search;                  //Concatenate with "RegNo: " to ensure that the search is done "by RegNo".
        temp=SearchFunction(search);                

        cout<<"1. "<<temp<<"\n\n";
        cout<<temp.length()<<endl;

        cout<<"Are you sure you want to delete the above record of"<<search<<"? Y/N";
        yn1=getchar();
        cin.clear();                                        
        cin.ignore(1000,'\n');                          

        if(!(yn1=='y' || yn1=='Y' || yn1=='n' || yn1=='N'))
        {
            do
            {
                cout<<"Enter 'Y' or 'N': ";
                yn1=getchar();
            }while(!(yn1=='y' || yn1=='Y' || yn1=='n' || yn1=='N'));
        }

        if(yn1=='y' || yn1=='Y')
        {
            DeleteFunction(temp);                   //Call delete function to delete record.
        }
    }
    return 0;
}


string SearchFunction(string search)
{

    int found=0, check=0;                               //Declare and initialize both variables to 0.
    ifstream outfile;                                   //Create object for reading file.

    outfile.open("student.txt");                        //Open file.

    while(!outfile.eof())                               //Continue loop until the end of file.
    {
        found=0, check=0;                               //Initialize both variables to 0 again in anticipation of repititions.

        getline(outfile, data);                         //Input one row from file to string variable data.
        found=data.find(search, found);                 //Search for the search term in string data.
        if(found!=string::npos)                         //If search term found.
        {
            cout<<data<<endl;                           //Display row.
        }
    }
    outfile.close();

    return data;
}

void DeleteFunction(string temp)
{
    string line;
    ifstream in("student.txt");
    if( !in.is_open())
    {
        cout << "Input file failed to open\n";
    }

    ofstream out("temp.txt");

    while( getline(in,line) )
    {
        if(line != temp )
            out << line << "\n";
    }
    in.close();
    out.close();    

    remove("student.txt");
    rename("temp.txt","student.txt");
}

First, [`while (!eof())` is wrong.](http://stackoverflow.com/questions/5605125/why-is-iostreameof-inside-a-loop-condition-considered-wrong) Also, there's no reason for that returned string to be a global variable. — chris, Dec 18 '13 at 19:47
Note data is a global variable why do you have to return it ? you can just access it in main after calling the function. — Coldsteel48, Dec 18 '13 at 19:47
Why ? [when using comparison operators on string everything is done underneath using compare function](http://en.cppreference.com/w/cpp/string/basic_string/operator_cmp). — Johan, Dec 18 '13 at 19:51
@ChristopherHarris: Why should strings be compared with the `compare` method and not `operator1=`? — Thomas Matthews, Dec 18 '13 at 19:54
@ThomasMatthews: They do different things. One results in a boolean; the other in a tri-state comparison result. — Lightness Races in Orbit, Dec 18 '13 at 19:58
@chris Ok, I have changed while(!eof()). It doesn't really mater if its a global variable or not. The output is the same regardless if its a variable of main or a global one. I have tried both. — Ghias, Dec 18 '13 at 20:29
@user2957713 At first I wasn't returning it but even though the string variable is global, in main it becomes empty as is evident by data.length(); after the search is done. Something happens in between which makes it to go empty. I have tried using pointers and passing by reference but nothing gives a different output. I'm very confused now. — Ghias, Dec 18 '13 at 20:34
@user3116554, Well, as soon as you call the function again, it changes. If another function didn't expect that, kaboom. It's worse with multithreading. — chris, Dec 18 '13 at 20:43

Raydel Miranda · Accepted Answer · 2013-12-18T20:57:18.833

1

You have stop reading the file after you found the row you are looking for. Maybe you want to change the function to:

string SearchFunction(string search)
{

    int found=0, check=0;                               //Declare and initialize both variables to 0.
    ifstream outfile;                                   //Create object for reading file.

    outfile.open("student.txt");                        //Open file.

    // Also check if found!!!
    while(!outfile.eof() && !found)                     //Continue loop until the end of file.
    {
        found=0, check=0;                               //Initialize both variables to 0 again in anticipation of repititions.

        getline(outfile, data);                         //Input one row from file to string variable data.
        found=data.find(search, found);                 //Search for the search term in string data.
        if(found!=string::npos)                         //If search term found.
        {
            cout<<data<<endl;                           //Display row.
        }
    }
    outfile.close();

    return data;
}

edited Dec 18 '13 at 20:57

answered Dec 18 '13 at 20:10

Raydel Miranda

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if you found it why to keep running in while ? why just not to return after cout inside the while ? – Coldsteel48 Dec 18 '13 at 20:52
No no, that's what you are doing. And it's wrong. Sorry I meant: "You must **not** keep running in while" That's why you have to check in while statement if found become true. See the code I posted. – Raydel Miranda Dec 18 '13 at 20:56
Not me :) , and yes you are right I confused the statement of While, still Imo it is better to return inside the if inside the while , and then outside the while you can return string if you didnt find anything , thus you can handle not found at all . – Coldsteel48 Dec 18 '13 at 20:59
@user2957713 I'm agree with you, but since he said he has just started with c++, I wanted give him a response with the less change to the code as possible. Move the return statement, or even add another might not be so easy to understand for him. – Raydel Miranda Dec 18 '13 at 21:07
I am getting what you all are saying. So I should return in the if in the while and then return again at the end if I haven't found anything so that I can handle the no data found situation in main through the empty returned string. Right? – Ghias Dec 18 '13 at 21:13
Yes , if you add the return inside `if` in the `while` you will return data with found string otherwise you will return an empty string after while is done Note: if you want to do it this way the `while` can stay original as you did it without the `$$ !found` . – Coldsteel48 Dec 18 '13 at 21:16
1

@user3116554 also please use data and temp as local variables. Globals are bad. It won't change anything in this part of code -> but it is a good initiation of bad habits that may result in future unwanted behavior – Coldsteel48 Dec 18 '13 at 21:18
@user2957713 Yes, I will use local variables. I turned global in desperation when nothing was seemingly working. – Ghias Dec 18 '13 at 21:23
@Raydel Miranda Thanks bro. I have got it. My code is working perfectly now. – Ghias Dec 18 '13 at 21:24
@user3116554 I hope the above answer helped you , if so accept it if you don't mind and have a nice c plus plusing :) – Coldsteel48 Dec 18 '13 at 21:25

score 1 · Answer 2 · answered Dec 18 '13 at 20:10

1

You need to break out of the while loop when you've found your data. A simple way is to just return at that point.

Don't use globals unless you have some very good reason. Globals used as scratch-pad variables, as above, are just Evil™.

answered Dec 18 '13 at 20:10

Cheers and hth. - Alf

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String not returning to main function

2 Answers2