Your first program

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Problem Solving and Programming Concepts (8th Edition)

Let’s write a program for adding the numbers from 1 to 10 (i. e. 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10).

Let’s first try to do it ourselves, manually. Be patient not to miss a single step written below. Every word below is quite important.

We shall be using a rough pad and pencil. The rough pad will be used to scribble the intermediate results that we need to remember temporarily while solving the problem. As you shall see, in stead of scribbling on the pad directly, we shall be drawing some boxes and write inside the boxes. We shall give unique names to the boxes so that we can refer those.

So, start following the steps below.

Add 1 and 2 and note the sum in a box in your rough pad. Name the box as, say, sum_till_now. Or you can choose any name you like. So, your rough pad now looks like this:

Add 3 to the value stored in sum_till_now, and write the result back in sum_till_now. Note that as the box sum_till_now can contain only one value at a time, and hence you have to erase or cross the previous value. So, now your rough pad looks like:

Now, add 4 to the value stored in sum_till_now, and write the result back in sum_till_now. After this, your rough pad would look like:

Repeat these steps till you have added 10, and then your rough pad looks as below.

You get the desired result written in sum_till_now.

Code it!

Now, let´s teach this to a computer, i. e. write a computer program for this, using the C language. The program is given below. Note that the line numbers are given just for our reference. They are not part of the program:

1.   #include <stdio.h>
2.   int main()
3.   {
4.       int sum_till_now;
5.
6.       sum_till_now = 1 + 2;
7.       sum_till_now = sum_till_now + 3;
8.       sum_till_now = sum_till_now + 4;
9.       sum_till_now = sum_till_now + 5;
10.      sum_till_now = sum_till_now + 6;
11.      sum_till_now = sum_till_now + 7;
12.      sum_till_now = sum_till_now + 8;
13.      sum_till_now = sum_till_now + 9;
14.      sum_till_now = sum_till_now + 10;
15.
16.      printf("The result is %d\n", sum_till_now);
17.  }

Let’s understand the logic.

We shall not dig into the syntax, or grammar of C, and just try to understand the logic.

A program is executed step by step, from top to bottom.

Statement number 4 is similar to your drawing a box named sum_till_now in rough pad. Computer uses its internal memory as its rough pad. sum_till_now is called a variable. Similar to a box in your rough pad, a variable can hold a value.

See statement 6. It is called an assignment statement. = is called the assignment operator. When an assignment statement is executed, whatever is in the right side of = is evaluated and put into the variable in the left side. That means, after executing statement 6, the value of sum_till_now becomes 3.

Statement 6 can be read as 1 + 2 is assigned to sum_till_now. Never confuse = with the equality operator as in common mathematics. Both are different.

Statement 7 is also an assignment statement. Evaluating the right side (i. e. sum_till_now + 3) results in 6 (remember that sum_till_now is now holding 3). So, 6 is assigned to the left side, i. e. sum_till_now.

Hence, after execution of statement 7, sum_till_now holds 6.

Carrying on, after execution of statement 14, sum_till_now holds 55, which is the desired output.

Done. But the program seems long. What if the problem were to add from 1 till 10000!