C for beginners: Introduction

The first C program

Armed with the knowledge about the types of variables, constants and keywords the next logical step is to combine them to form instructions. However, instead of this, we would write our first C program now. Once we have done that, we would see in detail the instructions that it made use of.

Before we begin with our first C program, remember the following rules that are applicable to all C programs:

  1. Each instruction in a C program is written as a separate statement. Therefore, a complete C program would comprise of a series of statements.
  2. The statements in a program must appear in the same order in which we wish them to be executed; unless of course the logic of the problem demands a deliberate “jump” or transfer of control to a statement which is out of sequence.
  3. Blank spaces may be inserted between two words to improve the readability of the statement. However, no blank spaces are allowed within a variable, constant or keyword.
  4. All statements are entered in lower case letters.
  5. C has no specific rules for the position at which a statement is to be written. That’s why it is often called a free-form language.
  6. Every C statement must end with a “;”. Thus, ; acts as a statement terminator.

Let us now write down our first C program. It would simply calculate simple interest for a set of values representing principle, number of years and rate of interest.

 * Calculation of simple interest
 * Author: gekay - Date: 25/05/2004
 * Modified by: Sarah - Date: 2020-10-02

#include <stdio.h>

int main() {
  int principle, noy;
  float rate, si;

  principle = 1000;
  noy = 3;
  rate = 8.5;

  // Formula for simple interest
  si = principle * noy * rate / 100;

  printf("%f\n", si);

  return 0; // 0 represents success

Now a few useful tips about the program:

  • There are two types of comments:
    • Multiline (/* */)
    • Single line (“//”)

    Comments should be used for explain parts of your code as they will be ignored by the compiler

  • The multiline comments not neccesarily needs to be multiline, you can create them at any place in your code (as long as it contents is enclosed in “/* */”), for example:
    /* Formula */ si = principle * noy * rate / 100;
    si = principle * noy * rate / 100; /* Formula */
    si = principle * noy * rate / /* Formula */ 100;
  • Sometimes, it is not so obvious as to what a particular statement in a program accomplishes. At such times, it is worthwhile mentioning the purpose of the statement (or a set of statements) using a comment. For example:
    // Formula for simple interest
    si = principle * noy * rate / 100;
  • Often, programmers seem to ignore writing of comments. But when a team is building up big software, well commented code is almost essential for other team members to understand it.
  • Although a lot of comments are probably not necessary in this program, it is usually the case of programmers that tend to use too few comments rather than too many. An adequate number of comments can save hours of misery and suffering when you later try to figure out what the program does.
  • The normal language rules do not apply to text written within /* */. Thus, we can type this text is lower case, capital or a combination.
  • Comments cannot be nested. For example:
    /* Calculation of simple interest
      Author: sam - date 01/01/01

    Is invalid.

  • int main() is a collective name given to a set of statements. This name has to be main, it cannot be anything else. All statements that belong to main are enclosed within a pair of braces { } as shown below:

    int main() {
    statement 1;
    statement 2;
    statement 3;
  • Technically speaking, main is a function. Every function has a pair of parenthesis ( ) associated with it. We would discuss functions and their working in great detail later.

  • Any variable used in the program must be declared before using it. For example:

    int principle, noy;
    float rate, si;
  • Any C statement always ends with a “;”. For example:

    float r, si;
    r = 8.5;
  • In the statement:

    si = principle * noy * rate / 100;

    * and / are arithmetic operators. The arithmetic operators available in C are +, -, * and /. C is very rich in operators. There are about 45 operators available in C. Surprisingly there is no operator for exponentiation… A slip, which can be forgiven considering the fact that C has been developed by an individual and not by a committee.

  • Once the value si is calculated it needs to be displayed on the screen. Unlike other languages, C does not contain any instruction to display output on the screen. All printed output is achieved by using readymade library functions. One such function is printf(). We have used it to display on the screen the value contained in si.

    The general form of printf function is:

    printf("<format string>", <list of variables>);

    “format string” can contain:

    • %f for printing real values
    • %d for printing integer values
    • %c for printing character values

    In addition to format specifiers like %f, %d and %c the format string may also contain any other characters. These characters are printed as they are when the printf function is called.

    Following are some examples of the usage of the printf function:

    printf("%f", si);
    printf("%d %d %f %f", principle, noy, rate, si);
    printf("Simple interest = Rs. %f", si);
    printf("Principle = %d\nRate = %f\n", principle, rate);

    The output of the last statement would look like this:

    Principle = 1000
    Rate = 8.5

    What is “\n“doing in this statement? It is called newline and it takes the cursor to the next line. Therefore, you get the output split over two lines. “\n” is one of the several Escape Sequences available in C.

    printf can not only print values of variables, it can also print the result of an expression. An expression is nothing but a valid combination of constants, variables and operators. Thus, 3, 3 + 2, c and a + b * c - d all are valid expressions. The results of these expressions can be printed as shown below:

    printf("%d %d %d %d", 3, 3 + 2, c, a + b * c - d);

    Note that 3 and c also represent valid expressions.