QBASIC (Chapter 3.1) : FOR loop (Series)

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Hope you have already practised a good amount of programs with nested if-else statements. So now we will learn how to do repetitive work in QBASIC. In the world of logic and algorithm and brain storming , the most important thing is to handle loops. Here in this chapter we will learn to form loops with the help of “FOR” keyword. Most of us think, there is not much to learn on the basic structure of FOR loop. But still I will request you to go through the basic structure of FOR loop once again. For the newcomers, it is must to focus more on this chapter, as this is the most important chapter of your programming journey. If you are good at FOR loops, then you are good at programming logics as well. Your grading as a future programmer will be decided during these for loop chapters.

We will learn total 5 things in this chapter.


First Thing to Learn :

First we will try to learn the structure of FOR loop. So starting with a simple program to print all the numbers from 1 to 10.. Obviously Solution should not be like writing PRINT() 10 times. Or to write all the numbers inside PRINT “1,2,3,4,5,6,7,8,9,10”.

So to solve this, we will introduce a FOR loop. The basic structure of FOR loop is as follows.

FOR  variable = <Start Value> to <Last Value> <Steps>
               Loop body .. 
               It may consist multiple lines.
NEXT variable



1^st part is <Start Value> à means the starting value of the variable ( Initial values before we start the FOR loop)

2^nd part is <Last Value> à Last value of the variable till when the loop will repeat itself. If it crosses the last value of the variable, then the loop will be stopped.

3^rd part is <step> à this part, we can set values for the next iteration. Means the next value of the variable. This part is optional. If you do not mention anything, the value of the variable will be incremented by 1. Keep it in mind.

4th part is Loop body à it contains few lines that would run again and again. You may write PRINT “HI” in FOR loop body. That means Multiple “HI” will be printed in the output window.




I know, this is confusing for new comers. But believe me when you will see the examples, it will be the easiest thing in the world. So directly going to the examples, then we will repeat the above 4 parts again with a reference to the example.

So now, to print 1-10 in the output window, we can write the following FOR statement.

FOR I = 1   TO   10

PRINT I

NEXT I

So let us revisit to our statements once again with a reference to the lines of above program.

As I said,

1^st part is <Start Value> à means the starting values of the variables ( Initial values before we start the FOR loop). Here we are setting 1 to the variable “i” before we enter into FOR loop.


2^nd part is <Last Value> à Loop will iterate till the last value of “i” . Here the loop will run until the value of “i” is lesser than equals to 10. Once the value of “i” crosses the last value means 10, For loop will be stopped. In this program, when the value of “i” will become 11, the loop will end. So as the program will also be ended, as there is no more line left after the FOR loop in this program.


3^rd part is <Step> à By this part we can set values for the next iteration. AS nothing is written after “10” in this program, means by default it will be 1 as said above. In 1^st run the value of i is 1, then into the next run , the value of i will be 2 and checking if it lesser than 10 or not, if TRUE, then it will print 2 and it will be incremented to 3 and so on..


4th part is Loop body à it contains few lines that would run again and again. In the above program, it will print the value of i by the line “PRINT I”.  So in output window, we will be able to see 1,2,3,…10.





Using this concept, now let us calculate the output for the following code snippets:

Code 1:

FOR I=5 TO 8

PRINT I;

NEXT I

Output:
                              5 6 7 8
Explanation:
                              i is starting from 5 and running till 8, each time incrementing by 1.





Code 2:

FOR I=4 TO 8 STEP 2

PRINT I;

NEXT I

Output:
                              4 6 8
Explanation:
                              i is starting from 4 and running till 8, each time incrementing by 2. “STEP 2” means i will be incremented by 2 every time.






Code 3:

FOR I = 4  TO 9 STEP 2

PRINT I;

NEXT I

Output:
                              4 6 8
Explanation:

i is starting from 4 and the last value is 9, each time incrementing by 2. The output will be same as code snippet 2.

Code 4:

FOR I=10 TO 5 STEP -2

PRINT I;

NEXT I

Output:
                              10  8  6
Explanation:

i is starting from 10 and the last value is 5. STEP -2 means, each time decrementing by 2. We will check the next code snippet to find what will happen in case of wrong step values.

Code 5:

FOR I=20 TO 10 STEP -1

PRINT I;

NEXT I

Output:
                              20 19 18 17 16 15 14 13 12 11 10
Explanation:

i is starting from 20 and the last value is 10. STEP -2 means, each time decrementing by 2. We will check the next code snippet to find what will happen in case of wrong step values.

Code 6:

FOR I=10 TO 5 STEP 2

PRINT I;

NEXT I

Output:

Explanation:

Nothing will be printed as i is starting from 10 and the last value will be 5 , and we are telling i to be incremented by 2 each time. Computer will be confused and will not print anything.

Code 7:

FOR I=1 TO 10 STEP 0

PRINT I;

NEXT I

Output:
                              1  1  1  1  1  1  1  1  1  1  1  1  1  ……….. infinite times
Explanation:

Here the value of i will be starting from 1. As the value of i is not increasing, it can not reach to 10. So this loop will be an infinite loop again.

Code 8:

FOR I=1 TO 10

PRINT I;

NEXT I

PRINT “ATLAST THE VALUE IS “ ; I

Output:
                              1  2  3  4  5  6  7  8  9  10 ATLAST THE VALUE IS 11
Explanation:

Concentrate on the last line. After the loop the value of i is 11.. It has to be.. Because to put an end of the loop, the value of i should reach to 11. So that it will cross 10 and the loop will end. So if we try to print the value of i outside of the loop, it will show 11. 

Second Thing to Learn :
In all above program , numbers are getting printed in the same line. What if I want to print them in separate lines.

Code 9:
FOR I = 1 TO 10

PRINT I

NEXT I

Output:
1
2
3
4
5
6
7
8
9
10


Value will be printed in different line normally. PRINT statement always print something and then change the line. But if you put a semicolon after the PRINT statement, Line would not be changed. We did the same for the code snippet 1-8. That’s why, elements were printed in the same line for program 1-8. In program 9 we did not put the semicolon after PRINT, so numbers were printed in different lines.



Third Thing to Learn :
Now if anyone asks to print 10 stars (*), what would you write? It would be the same code, but printing * instead of i ..

Code 10:

FOR I=1 TO 10

PRINT “*”;

NEXT I

Output:

* * * * * * * * * *

Explanation:

Simply, loop is iterating 10 times and is printing 10 stars.

Code 11:

FOR I=1 TO 10

PRINT “#”;

NEXT I

Output:

# # # # # # # # # #

Explanation:

Simply the loop is iterating 10 times and printing 10 hashes (#).

Code 12:

K=100;

FOR I=1 TO 10

PRINT K;

NEXT I

Output:

100 100 100 100 100 100 100 100 100 100

Explanation:

Simply the loop is iterating 10 times and printing the value of K 10 times. Means the loop will print the value 100 ten times.

Code 13:

K=100;

FOR I=1 TO 10

PRINT K;

K=K+5

NEXT I

Output:

100 105 110 115 120 125 130 135 140 145

Explanation:

Simply the loop is iterating 10 times and printing the value of K 10 times. At first, the loop will print the value 100 and the value of K will be incremented to 105. Then it will print 105 and the value will be incremented to 110 and so on.

So we have learnt 2 things from code snippet 13:

1.      We can use one variable to control the loop and another value to print the elements in output window.

2.      We can print 10 terms or n terms of a series. Means we can print a series without giving the limit.

If you look closely, we have not put any limit like FOR I = 100 TO 145 in the last program. Even we did not know that the 10^th term of the series will be 145. We are printing 10 terms only by the following FOR loop.

K=100;

FOR I=1 TO 10

PRINT K;

K=K+5

NEXT I

Number of terms are being controlled by the variable i whereas the values are being hold by the variable k.

So please follow following 2 code snippets. It will clarify what I want to mean.

Code 14:

REM “PRINTING THE SERIES 7  11  15  19  23  …..  <=50”

FOR I=7 TO 50 STEP 4

PRINT I;

NEXT I

Explanation:

The above series will surely be terminated by 50. It may end at 47 or 48 or 49 , I have not calculated the exact value.. But it will be surely on or before 50.

Code 15:

REM “PRINTING THE SERIES 7  11  15  19  23  …..  10 TERMS”

                        K=7

FOR I=1 TO 10

PRINT K;

K=K+4

NEXT I

Explanation:

If we try to print only 10 terms of this series, then we can calculate the 10^th term on paper and then put the limit at FOR loop condition. But it will be wiser to use the logic of variable K that we have used above. Here the FOR loop will be iterating 10 times with the help of variable i . But in output window, the value of k will be printed and k will be updated accordingly to print the desired series.

Again we are taking 1 more program examples as below:



Code 16:

REM “PRINTING THE SERIES 5  15  45  … 6 TERMS ”

                        K=5

FOR I=1 TO 6 

PRINT K;

K=K*3

NEXT I

Fourth Thing to Learn :
Sometimes we are confused with the following types of program where some numbers are with positive sign and some are with negative sign.

printing the series 7  -11  15  -19  …  upto 10 terms
printing the series 7  -11  15  -19  …  <=50


I will always suggest to write the code for the series first by omitting the positive and negative signs. After completing the series without any sign, then only think how can we bring that signs alternatively.



Code 17:

REM “printing the series 7 -11 15 -19  …  upto 10 terms”

K=7

FOR I=1 to 10

            IF I MOD 2 =0 THEN

                        PRINT -K;

            ELSE

                        PRINT K

            END IF

            K=K+4

NEXT I

Explanation:

If you look closely, if we omit positive and negative signs, it is similar with the code snippet number 15. So for that reason FOR loop statements are same for both code 15 and 17. Extra thing is the IF-ELSE part inside the FOR loop body. On the basis of some condition, sometimes it is positive, sometimes it is negative. Here the condition is , number is negative in each 2^nd , 4^th, 6^th term. Means k will be negative when i is an even number.

Code 18:

REM “ PRINTING THE SERIES 7 -11 15 -19 .. UPTO 10 TERMS”

A=1

FOR I = 7 TO 50 STEP 4

            IF A MOD 2 =0 THEN

                        PRINT -I;

            ELSE

                        PRINT I;

            END IF

            A = A + 1

NEXT I

Explanation:

If you look closely, if we omit positive and negative signs, the series is similar with the code snippet number 14. So for that reason, FOR loop line is almost same for both code 14 and 18. Extra thing is the IF-ELSE part inside the FOR loop body. On the basis of some condition, sometimes it is positive, sometimes it is negative. Here the condition is , number is positive in each 2^nd , 4^th, 6^th term. So to count the terms, we have introduced an extra variable named “a” which is just counting the number of terms. Means “i” will be positive when “a” is an even number.

Fifth Thing to Learn :
The last part of this chapter is to print the sum of the series. It is really easy to calculate the sum of any series. I am saying if we can print the series , then we can surely calculate the sum by 3 simple statements.

Statement 1: introduce a variable named “sum” with value zero before we start the FOR loop. Inside this new variable “sum” , we are going to collect the sum.

Statement 2: Inside the FOR loop body we will not print the variable , rather we will put the printable value into “sum”.

Statement 3: After the loop , print the variable “sum”



Let us take code snippet number 15 and 17..


Code 15 :: calculating SUM:

REM “printing the SUM of the series 7  11  15  19  23  …..  <=50”

SUM=0 

FOR I = 7 TO 50 STEP 4

SUM = SUM + I  

PRINT “THE SUM IS“ ; SUM

Code 17 :: calculating SUM :

REM “printing the sum of the series 7 -11 15 -19  …  upto 10 terms”

SUM=0

K=7

FOR I=1 to 10

            IF I MOD 2 =0 THEN

                        PRINT -K;

            ELSE

                        PRINT K

            END IF

            K=K+4

NEXT I 

PRINT “THE SUM IS “ ; SUM

I can guarantee , if you can print the series anyhow, then by applying these simple 3 statements you can find the sum of that series. These 3 steps will be applicable to all the series programs. If not, you can mail me one example. So to print the sum of any series, first focus on printing the same series.



Now you have to do some hands-on before moving to the next chapter. As I previously said, these FOR loop chapters will decide your strength in this programming language. You can start solving the exercise “For Series” from the above said Facebook group. In case of any confusion, you may contact me on the abovesaid mail or whatsapp number.  Best of luck..