How to read from a file

Question

* For those having this problem, it was the problem with my command. Make sure your command to compile and run the program is correct!

I was watching a tutorial on youtube.

I did exactly the same as the op did, and I get a different output.

I am new to c++, and I am using Xcode to run the code.

here is the code: (again, it's not mine, but from a youtube tutorial video)

#include <iostream>
#include <fstream>

using namespace std;

int main()
{
    int input[100];
    int x = 0;
    int fail = 0;
    int pass = 0;

    fstream textfile;
    textfile.open("exam.txt");

    while(! textfile.eof()) {
        textfile >> input[x];
        cout << input[x] << endl;
        if(input[x] < 50) {
            fail++;
        }
        else {
            pass++;
        }
        x++;
    }
    textfile.close();
    cout << fail << "students out of " << (fail + pass) << "did not pass the exam." << endl;

    return 0;}

It's supposed to print grades.

But this is what I get when I run it on Xcode.

0
0
0
0
0
0
0
0
1606415288
32767
1606415720
32767
1606415720
32767
2003953588
1294056514
104
1
1606415312
32767
25240
1
1606415288
32767
1606415312
32767
25248
1
1606415288
32767
1606415312
32767
1606415720
32767
25264
1
1606415304
32767
1606415720
32767
1606415720
32767
6144
1
1910210376
32767
1606416728
32767
0
0
0
0
0
0
0
0
0
0
1606416544
32767
2003953588
1294056514
1606416544
32767
-1988827167
32767
0
0
1
0
1606416904
32767
0
0

It's only part of the output. It doesn't terminate.

Also, when I try to run it on linux (sshed into school linux machine),

I get this following error:

    @@?!@8  @ @@@@@?88@8@@@?? ??`?`?? ``?TT@T@DDP?tdPP@P@DDQ?tR?td??`?`  /lib64/ld-linux-x86-64.so.2GNUGNU?Yd?`˫sP???*??"b!!??  (E?L?
                                             CyIk?  ^Q#y??K@??'?[?
@3P
@ `??
@libstdc++.so.6__gmon_start___Jv_RegisterClasses_ZNSt8ios_base4InitD1Ev_ZNSt13basic_fstreamIcSt11char_traitsIcEEC1Ev_ZNSt13basic_fstreamIcSt11char_traitsIcEE5closeEv_ZNSirsERi_ZNSt13basic_fstreamIcSt11char_traitsIcEE4openEPKcSt13_Ios_Openmode__gxx_personality_v0_ZSt4cout_ZNSolsEi_ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6__ZNSt13basic_fstreamIcSt11char_traitsIcEED1Ev_ZNSolsEPFRSoS_E_ZNKSt9basic_iosIcSt11char_traitsIcEE3eofEv_ZNSt8ios_base4InitC1Evlibgcc_s.so.1_Unwind_Resumelibc.so.6__stack_chk_fail__cxa_atexit__libc_start_mainGCC_3.0GLIBC_2.4GLIBC_2.2.5CXXABI_1.3GLIBCXX_3.4 P&y
xui     ?ӯk?t)??`? `  ` ` `  `( `0 8 `  @ `                            p60ii
H `
   P `
` `h `p `x `? `H??c????H???5? ?%? @?%? h??????%? h??????%? h??????%? h?????%? h?????%? h?????%? h?????%? h?p????%? ?`????%? h   ?P????%? h
?@????%? h
          ?0????%? h
?????%z h?????%r h??????%j h?????1?I??^H??H???PTI???@H??0@H??
                                                             @????????H?H?? H??t??H?Ð????????????UH??SH??=0 uK?`H?* H???`H??H??H9?s$fDH??H? ???`H?? H9?r??? H?[]?fff.?H?=  UH??t?H??]`??]Ð?UH??SH???dH?%(H?E?1?Dž????Dž????Dž????H??????H???T???????H???????@H???????|H??0?????????Hc?H??H?H??????H??H???$?????????H????0????ƿ? `?Y?????
@H???
     ?????????H????0?????1  ???????????????????H??????H???????????d???H??????H???R???????????????????????ƿ? `??????%@H????????H???????6@H?????????
@H???`????H??????H?????????H?U?dH3%(t$?H??H??????H???????H??H???L????????H???[]?UH??H???}??u??}?u*?}???u!??!`?6????P
@?? `??!`H?????????UH?????????]?UH??}??u??U??E? ?]Ð???H?l$?L?d$?H?-? L?%? L?l$?L?t$?L?|$?H?\$?H??8L)?A??I??H??I???k???H??t1?@L??L??D??A??H??H9?u?H?\H?l$L?d$L?l$ L?t$(L?|$0H??8???Ð?????????????UH??SH?H H???t??`DH???H?H???u?H?[]Ð?H??_???H??exam.txtstudents out of did not pass the exam.D????`?????s?????????????????0p???XzRx
                              $H??? FJ
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I used this command: g++ main.cpp -o output.out Is there something wrong with my command?

Thank you so much!!!

Answer 1

You're doing some dodgey things. As mentioned already, don't test eof. You actually need to test whether your input was successful and/or whether the stream is good. You can have a catch-all when you're reading streams:

while( infile )
{
    //...
}

I mentioned also that you need to check that the value was read. It could be that an integer cannot be read because the input stream does not contain something that looks like an integer, or perhaps some other error occurred. Since you're only reading one integer at a time, you can test that operation by itself. If the stream was bad to begin with (eg the file could not be opened) it will fail straight away:

while( infile >> input[x] )
{
    //...
}

I wouldn't generally use the above approach if reading more than one value each time, because the loop condition gets cluttered. Here's an alternative:

while( infile )
{
    if( !(infile >> input[x]) ) {
        cout << "Failed to read value " << x+1 << endl;
        break;
    }

    //...
}

Let's roll with that, because it's more useful when you wanna find out what's going on. The next thing is that you have a fixed-size array with 100 values. If you read too many values, you are going to overflow. So you need to make sure this won't happen:

while( infile && x < 100 )
{
    if( !(infile >> input[x]) ) {
        cout << "Failed to read value " << x+1 << endl;
        break;
    }

    cout << input[x] << endl;

    if( input[x] < 50 )  {
        fail++;
    } else {
        pass++;
    }

    x++;
}

Now, it seems to me that you're not even using the array, so why keep it at all? You could just read into a temporary value:

    int grade;
    if( !(infile >> grade) ) break;
    // ...

But if you wanted to keep the array and didn't know how many grades you might end up reading, use a vector<int> instead of an int array. I'll show this with the simpler loop style:

std::vector<int> grades;
int grade;

while( infile >> grade )
{
    grades.push_back(grade);
    //...
}

Notice how I've used more descriptive variable names. Something like input doesn't tell you much about what it contains. That is the kind of variable name you'd get from an inexperienced programmer whose YouTube videos you should immediately boycott.

The last thing I'll suggest is using ifstream instead of fstream, because by default you don't need to open the file in read/write mode. You could supply flags to fstream::open, but why do that when you can just construct ifstream like this:

ifstream infile("exam.txt");

You also don't need to explicitly close it, because that will happen when infile's destructor is called.

Answer 2

    #include <iostream>
#include <fstream>
#include <string>
using namespace std;

void main ()
{
        string STRING;
    ifstream infile;
    infile.open ("names.txt");
        while(!infile.eof) // To get you all the lines.
        {
            getline(infile,STRING); // Saves the line in STRING.
            cout<<STRING; // Prints our STRING.
        }
    infile.close();
    system ("pause");
}

I hope, this code can help you.