Learn to Program with C_ Learn to Program using the Popular C Programming Language ( PDFDrive )

Chapter 1 ■ Elementary Programming Concepts
Here we have specified what the output of the program should look like. For instance, there
is a blank line between the prompt line and the line that gives the answer; we have also specified
the exact form of the answer. This is a simple example of output design. This is necessary since the
programmer cannot write the program unless he knows the precise output required.
In order to write the computer program from the algorithm, a suitable programming language
must be chosen. We can think of a program as a set of instructions, written in a programming
language, which, when executed, will produce a solution to a given problem or perform some
specified task.
The major difference between an algorithm and a program is that an algorithm can be written
using informal language without having to follow any special rules (though some conventions are
usually followed) whereas a program is written in a programming language and must follow all
the rules (the syntax rules) of the language. (Similarly, if we wish to write correct English, we must
follow the syntax rules of the English language.)
In this book, we will be showing you how to write programs in C, the programming language
developed by Ken Thompson and Dennis Ritchie of Bell Laboratories, and one of the most
popular and widely used today.
Program P1.1 is a C program that requests the user to enter the length of a side and prints the
area of the square:
Program P1.1
#include <stdio.h>
int main() {
int a, s;
printf("Enter length of side: ");
scanf("%d", &s); //store length in s
a = s * s; //calculate area; store in a
printf("\nArea of square is %d\n", a);
}
It is not too important that you understand anything about this program at this time. But you can
observe that a C program has something (a function) called main followed by opening and closing
brackets. Between the left brace { and the right brace } we have what is called the body of the
function. The statement
int a, s;
is called a declaration. The parts after // are comments that help to explain the program but have
no effect when the program is run. And * is used to denote multiplication.
All of these terms will be explained in detail in due course.
Finally, a program written in a high-level language is usually referred to as a source program
or source code.
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