Quick learning: C Fundamentals in ONE Hour

(By Dr. Leo Hu)


You can create a C program in only one hour, even though you have no knowledge of programming.

Let's begin!

1.Define a C program

A C program consists of a function called main() that looks like

int main() {

    return 0;

}

where 

return 0;

 and means that function main() has successfully completed. The word return is a reserved word (or called keyword) in programming language C.  

Attention: A reserved word, or a keyword, cannot be used to name the programmer's term. 

Note: The complete set of reserved keywords can be reviewed at https://www.c-programming-simple-steps.com/c-keywords.html.)

Attention: a statement in C shall end with a semicolon ';'. 

The keyword return is followed with an integer that has been defined by the program caller. Here the program caller is the operating system. Microsoft Windows operating systems has defined

0 - success

1 - Informational

2 - Warning

3 - Error

Therefore, statement 

return 0;

means the program has completed with success. Other operating systems have the same effect. 

Tip: To increase program source code readability, we may add spaces between items and indentations before statements. We may use spacebar to add space, and tab key to add indentations in a C program source code. It doesn't matter how many spaces or how many indentations are added in a C program source code, but we should keep in mind that we apply spaces and indentations to beautify our code and improve readability of our programs. In this simple program, we have added a space between int and main(), and an indentation before return statement. 

2.Add comments

If you want to explain something to your program reader (or remind yourself), you may add comments in your source code. 

Attention: A comment is not to be used by the computer but by your reader and yourself for understanding the code. The C language compiler, which translate a C program source code into machine code, simply omits comments in a program source code. 

For example, we may add comments in the above simple program, and it looks like

// Online C compiler to run C program online

int main() {

// Write C code here

       return 0;

}

In this program, there are two comments that start with double slashes "//". However, a comment can be enclosed between "/*" and "*/", and when a comment is written on more than one line, we must use "/*" and "*/" to enclose it.  For example, the above program can be written as below without any syntax change of C program:

/* Online C compiler

to run C program online

*/

int main() {

/* Write C code here*/

       return 0;

}

3.Add a statement to do something

Let's add a statement to display "Hello World!" on the screen. The program looks like

// Online C compiler to run C program online

int main() {

// Write C code here

printf("Hello world");

        return 0;

}

Here, statement 

printf("Hello world");

display text Hello world on the screen. printf() is a function pre-defined in another program file, <stdio.h>, called a header file, in the C Standard Library, which is commonly used in C programming. It stands for "Standard Input/Output Header". This header file defines a set of functions and macros that provide input and output capabilities for C programs (RKplus, What is stdio.h and why do we use? - Cplusplus,  2023). 

Before we can use any code pre-defined in another program file, such as function printf() in <stdio.h>, we must add that file so that the pre-defined code can be used in our program. So, the program should be rewritten as 

// Online C compiler to run C program online

#include <stdio.h>

int main()

{

// Write C code here

printf("Hello world");

return 0;

}

Here, #include, is called a preprocessor directive in C that tells the compiler to include a file in the source code program.

Hands-on: A C program to output a message

Click the link, https://www.programiz.com/c-programming/online-compiler/, to use the online C programming tool to watch the execution of the program we discussed above.  This tool has an editor in the left pane that host the program source code, and the program execution result shall be displayed in the Output in the right pane as shown in Figure 1 below.

Figure 1. Online C compiler

Note: the source code editor displays line numbers in the left side for convenience to edit the code. 

The source code of the discussed simple program has been loaded in the editor automatically, so you don't have to enter manually. 

Click the Run button in the top-right corner of the tool, you should see the display of Output in the right pane as shown in Figure 2 below:

Figure 2. An execution example of a C program 

If you are interested in verifying the use of comments, you may remove the comments lines and click the Run button again. You should see that the output is exactly the same as above. 

4.Input /Output in a C program

A program is essentially a file that ask the computer to read input, to process the input data, and to produce output, which is described as the input-process-output (IPO) model. 

In the above program, we have used a simple output statement 

    printf("Hello world");

Generally, we use pre-defined functions and utilities to create our input and output. As stated above, we need have the #include preprocessing directive such as 

#include <stdio.h>

before any code in the program. In this directive file, <stdio.h>, the default output device is the monitor, and the default input device is the keyboard. 

Now, let's add an input statement. Suppose we ask the user of the program to enter his/her first name, so that we may greet him/her in the output. What should we do?  We should do a few things in a sequence:

1) declare a variable of string type such as firstName to hold the first name input by the user

2) remind the user what to do by displaying an instruction

3) read input from the user

4) display the greeting that contains the first name

The program shall look like

/*A C program with input/output */

#include <stdio.h>

int main()

 {

  // declare variable firstName to hold the first name

  char firstName[30];


  // tell the user what to do

  printf("Enter your first name and press enter: ");


  // read input from the user

  scanf("%s", firstName);


  // display greetings with the first name

  printf("Hello %s", firstName);


  return 0;

}


Hands-on: C program with input/ouput

Click the link the online C compiler at https://www.programiz.com/c-programming/online-compiler/, copy or type the above code into the source code editor, click the Run button, you should read the instruction, type in your first name and press enter, you should see the greeting. The execution looks like below:

Figure 3. A C program with input/output

In this program, the string variable firstName is declared as a set of char, which can contain up to 30 characters. The input statement uses predefined function scanf(), which has two parameters. The first parameter (format specifier) indicates that the data type of the variable is string ("%s"), and the second parameter is the variable, firstName. Similarly, the output statement uses predefined function printf(), which also has two parameters. The first parameter tells the computer to output string "Hello" and a string variable indicated by the format factor "%s", and the second parameter is the variable, firstName, that will be printed at the position the format factor marked, that has obtained its value from the user input in scanf() earlier. 

Congratulations! You have learned how to create a very basic (and simple) C program. Now you can move on to learn more C language to hone your skills in C programming. 

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