C vs. Java

Learning Outcomes

• Use a “Hello World” program to intuit C program structure.
• Do a cursory comparison of C and Java.

Hello World

#include <stdio.h>
int main(int argc, char *argv[]) {
  printf("Hello World!\n");
  return 0;
}

public class HelloWorld {
  public static void main(String[] args) {
    System.out.println("Hello world!");
  }
}

Highlights

• In C, we import libraries using #include. Here, we include stdio for printf(), which prints to stdout (here, the command-line).
• There is a main function.
  • C is function-oriented; unlike Java, this is not an object’s method.
  • The return type of main is not void; it’s an integer.
  • By convention, C programs return 0 on success. (The main rationale is it is easier to check equality to zero; we return to this soon)

Run Demo

The below instructions are mostly for reference. Refer to this section to understand details.

$ gcc hello_world.c
$ ./a.out
Hello World!

$ gcc -o hello_world hello_world.c
$ ./hello_world
Hello World!

$ gcc -d -o hello_world hello_world.c
$ gdb hello_world

C vs. Java

#tab-c-vs-java below is adapted from the C Programming vs. Java Programming table, created for Princeton University’s introductory CS sequence. Hover over the footnote for more information about each row.

More Highlights

1. Variable naming convention: In C, use snake_case⁶, NOT camelCase ⁷.

2. Command-line arguments: In our hello_world.c program, the main function can take in command-line arguments with two parameters:

• argc is an integer count of how many arguments you have. The executable itself counts as one argument. If you run something like ./hello_world my_file, argc is 2.
• argv: is a pointer to an array of the arguments themselves, as C strings. We discuss pointers, arrays, and strings in more detail next time. For now, if you run ./hello_world my_file, the first⁸ argument is the path of the program itself (./hello_world) and the second argument is the string my_file.

#include <stdio.h>
int main(int argc, char *argv[]) {
  printf("Received %d args\n", argc);
  for (int i = 0; i < argc; i++) {
    printf("arg %d: %s\n", i, argv[i]);
  }
  return 0;
}

3. Curly braces: The C language allows for some omission of curly braces for single-line statements—even for control structures like if-else and for. This is the same as in Java, but we didn’t tell you. :-)

But just because you can, doesn’t mean you should. Because subsequent lines of the control structure are considered outside of the body, omitting curly braces leads to many debugging errors⁹:

#include <stdio.h>
int main(int argc, char *argv[]) {
    int x = 0;
    if (x == 0)
        printf("x is 0\n");
    if (x != 0) // careful!
        printf("x not 0 line 1\n");
        printf("x not 0 line 2\n");
    return 0;
}