Exercise - Fix broken code

  • 7 minutes

It's helpful to practice identifying errors, and then fixing the code. What happens when you misspell a function name or type an incorrect symbol?

Create the code

Before you can identify the errors, create the code and have a look at it:

  1. In the terminal, run code . to open an editor.

  2. In the editor, enter or paste in this code:

    #include (iostream);
using namespace std;
int main()
{
  cout >> 'Hello World'
}

  3. Save the code file as hello.cpp, and then close the editor.

Note

To check whether the file was created successfully, in the terminal, enter the ls command for a list of files.

Identify the errors

The code you ran has errors in it. Some error messages appear as you compile the code. Each error is explained and resolved next.

  1. Run g++ and the file name to build your program:

    g++ hello.cpp

    Have a look at the errors and see how many errors you can identify.

    hello.cpp:1:10: error: #include expects "FILENAME" or <FILENAME>
#include (iostream);
          ^
hello.cpp:5:13: warning: character constant too long for its type
    cout >> 'Hello World'
            ^~~~~~~~~~~~~
hello.cpp: In function ‘int main()’:
hello.cpp:5:5: error: ‘cout’ was not declared in this scope
    cout >> 'Hello World'
    ^~~~
hello.cpp:5:5: note: ‘std::cout’ is defined in header ‘<iostream>’; did you forget to ‘#include <iostream>’?
hello.cpp:1:1:
+#include <iostream>
#include (iostream);
hello.cpp:5:5:
    cout >> 'Hello World'
    ^~~~

    Note

    The output lists many errors. Sometimes, resolving a single error might resolve other errors. Other times, you might have to resolve all the errors that are listed in the output. In most scenarios, the errors are explicitly explained and easy to understand.

Resolve the errors

You've compiled the code, which resulted in many errors. But don't worry. You can resolve each error and end up with working code.

First error: Parentheses instead of angle brackets

  1. Look at the code to identify the first error:

    hello.cpp:1:10: error: #include expects "FILENAME" or <FILENAME>
  #include (iostream)
           ^

    Here, <iostream> is expected instead of (iostream). Let's run the following code and recompile it.

  2. Run code hello.cpp to edit the code again:

    code hello.cpp

  3. Edit the first line to address the first error. Replace the parentheses around iostream with angle brackets:

    #include <iostream>;              //(iostream) is incorrect. <iostream> is correct.
using namespace std;
int main()
{
  cout >> 'Hello World'
}

Second error: An extra semicolon

  1. Run g++ hello.cpp to build your program:

    g++ hello.cpp

    You see this output, already with fewer errors, thanks to your first fix:

    PS /home/<user>> g++ hello.cpp
hello.cpp:1:20: warning: extra tokens at end of #include directive
#include <iostream>;
                     ^
hello.cpp:5:13: warning: character constant too long for its type
     cout >> 'Hello World'
             ^~~~~~~~~~~~~
hello.cpp: In function ‘int main()’:
hello.cpp:5:10: error: no match for ‘operator>>’ (operand types are ‘std::ostream’ {aka ‘std::basic_ostream<char>’} and ‘int’)
     cout >> 'Hello World'
     ~~~~~^~~~~~~~~~~~~~~~
 ...
 ...
 ...
 ...
 ...

    Note

    Despite the number of errors that were returned, resolve one error at a time, beginning at the top.

    Let's focus on the error that comes first in the recent execution.

    hello.cpp:1:20: warning: extra tokens at end of #include directive
#include <iostream>;
                    ^

    A semicolon (;) is added at the end of the filename/library iostream. You can't add a semicolon after an #include directive. Use a semicolon only at the end of a statement.

  2. Remove the semicolon from the end of #include <iostream> and see what happens.

    To edit the code again, run code hello.cpp:

    code hello.cpp

    Next, edit the code to make the correction, and then save it.

    #include <iostream>              // Removed the semicolon.
using namespace std;
int main()
{
  cout >> 'Hello World'
}

Third error: The wrong quote type for a string literal

  1. Run g++ hello.cpp to build your program.

    You should see the following output from the compilation:

    PS /home/<user>> g++ hello.cpp
hello.cpp:5:13: warning: character constant too long for its type
    cout >> 'Hello World'
            ^~~~~~~~~~~~~
hello.cpp: In function ‘int main()’:
hello.cpp:5:10: error: no match for ‘operator>>’ (operand types are ‘std::ostream’ {aka ‘std::basic_ostream<char>’} and ‘int’)
    cout >> 'Hello World'
       ~~~~~^~~~~~~~~~~~~~~~
...
...
...
...
...

    Look at the first error in the output:

    hello.cpp:5:13: warning: character constant too long for its type
    cout >> 'Hello World'
            ^~~~~~~~~~~~~

    The code uses single quotes instead of double quotes. Use single quotes only for a single character. For a string literal (character array), use double quotes.

  2. Replace the single quotes with double quotes. Ensure that your program looks like this code:

    #include <iostream>
using namespace std;
int main()
{
  cout >> "Hello World"     //Use double quotes instead of single quotes.
}

Fourth error: Wrong stream operator

  1. Run g++ hello.cpp to build your program.

    Here's what the output looks like. The error output is less as you resolve issues. Next, resolve the remaining issues.

    PS /home/<user>> g++ hello.cpp
hello.cpp: In function ‘int main()’:
hello.cpp:5:10: error: no match for ‘operator>>’ (operand types are ‘std::ostream’ {aka ‘std::basic_ostream<char>’} and ‘const char [12]’)
    cout >> "Hello World"
    ~~~~~^~~~~~~~~~~~~~~~
In file included from /usr/include/c++/8/string:53,
                from /usr/include/c++/8/bits/locale_classes.h:40,
                from /usr/include/c++/8/bits/ios_base.h:41,
                from /usr/include/c++/8/ios:42,
                from /usr/include/c++/8/ostream:38,
                from /usr/include/c++/8/iostream:39,
                from hello.cpp:1:
...
...
...
...
...

  2. In the preceding output, you can see an operator issue. Replace the >> operator with the << operator.

    #include <iostream>
using namespace std;
int main()
{
  cout << "Hello World"     //Replacing the >> operator with the << operator.
}

    Note

    Use << for the cout object.

    Use >> for the cin object. The cin object is discussed in more detail in another module.

Fifth error: Missing semicolon

  1. Run g++ hello.cpp to build your program:

    g++ hello.cpp

    Here's the output. You're almost there. There are just a few errors left to fix.

    PS /home/<user>> g++ hello.cpp
hello.cpp: In function ‘int main()’:
hello.cpp:5:34: error: expected ‘;’ before ‘}’ token
    cout << "Hello World"
                         ^
                         ;
}
~

    A semicolon is required at the end of the statement.

  2. Edit your code so that it looks like this code:

    #include <iostream>
 using namespace std;
 int main()
 {
   cout << "Hello World";     //Added a missing semicolon
 }

Are we done?

  1. Run g++ hello.cpp to build your program:

     g++ hello.cpp

    If you type ls in your terminal, you should now see a.out as one of your files. a.out is the file the g++ compiler created for you.

  2. In the terminal, run the a.out file:

     ./a.out

    And finally, you have the output you want to see:

      PS /home/<user>> ./a.out
  Hello World

    Your code has no more errors. You've debugged the code step by step and resolved all issues.

Note

The code doesn't include return 0 at the end, but no error was thrown. The reason is that the runtime assumes an implicit return type as integer for main. The return type primarily is used as an exit status. Here, returning 0 or returning nothing automatically makes the runtime assume that the code exited successfully.

Check your knowledge

1.

What is wrong with this line of code? cout << "Hello World" << endl