Logic behind nested 'for' and other loops

int main () 
{
    int row,col;
    for (row=1; row<=3; row++)
    {
        for (col=1; col<=row; col++)
        {
		printf ("%d", row);
	}
        printf ("\n");
    }
    getch();
}

Firstly,
row=1, then condition will be checked, if condition is true it will enter the block
2ndly,
if condition is true control moves to the inner for loop where col=1 and again condition will be checked, if condition true it will entered inside

and value of row will be printed then col value will be incremented col++ and again condition will be checked if it false the control again move to upper for loop and incremented value of row, row++ and step moves till the upper condition get false.

The inner loop executes completely for every iteration of the outer loop. For example, lets say we want to count the percentage of girls in each class in a school.

The outer loop would iterate over each class. For Each iteration(in this case each class), the inner loop would count the number of girls in that class and divide it by the number of students and store the result. The outer loop would then move on to the next class.

Hope this helps.

-Debdatta Basu

Logic of loop 1)

for (row=1; row<=3; row++)

Until value of variable row is lowest or equal to 3, loop.

Logic of loop 2)

for (col=1; col<=row; col++)

Until value of variable col is lowest or equal to value of current row, loop.


Why your result is: 1,22,333? Becuase inside column-loop, you print a row no.

Iteration no.	Value of row	Value of col	Comment
1	1	1	Print row no. 1; col is equal row, the condition is true, so jump to the next row
2	2	1	Print row no. 2
3	2	2	Print row no. 2; col is equal to row, jump to the next row
4	3	1	Print row no. 3
5	3	2	Print row no. 3
6	3	3	Print row no. 3; col is equal to row, exit form inside and outside loop

To "print" two-dimensional array, for example 3 x 3 cells, use:

int row,col;
for (row=1; row<=3; row++)
  {
      for (col=1; col<=3; col++)
      {
          printf ("%d", col);
      }
      printf ("\n");
  }
  getch();

int main ()
{
    int row,col;
    for (row=1; row<=3; row++)
    {
        for (col=1; col<=row; col++)
        {
        printf ("%d", row);
    }
        printf ("\n");
    }
    getch();
}

Given this code, and having in mind that execution is sequential (unless you establish interruptions, timers, threads or events) then the code will go into the first loop and execute the second loop 3 times.

The idea is that you will go inside the second loop for the first row (row = 1) and you will execute the second loop an amount of rows (as row is changing the first time you will do it only one time). Once the execution of the second loop will be finished you will start again with the first one, calling again the second one... you will leave this looping once the first loop is completed for the last time.

then you will call getch() which will stop the program.

Good luck!

Any book on C++, will have a chapter on looping.

But generally to give you a basic understanding of for loops and nested for loops:-

The for loop has 3 parts:
1) Declare and initialize a variable : Ex: int i = 0;
2) Test the variable : For any condition you want. Ex: i< 4
3) Perform some operation: Usually the variable is incremented or decremented. Ex: i++;

1)The for loop (not nested) can be used to for displaying values, testing an array of values etc.

Ex :

int main()
 {
   int a[4] = {1, 2, 3,4 };  
   for (int i=0; i < 4; i++)
   {
 cout << a[i] << endl;
   }
 }

output:
1
2
3
4

2) But when you are dealing with 2 - d array, That is, which consists of both row and column, the nested for loop can be used.

The example you have illustrated itself is a very good example. But still, To print 5 rows and 6 columns of "*", the for loop would be:

main()
 {
  int row,col;
  for (row=1; row<=5; row++)
    {
        for (col=1; col<=6; col++)
        {
		printf ("*", col);
	       }
        printf ("\n");
    }
    getch(); 

output:

******
******
******
******
******

Here, the variable "row" in the outer loop is initialized to 0
row = 1 This is Row 1
Then the loop goes into inner loop, where the loop is run until the condition col<=6 is reached -
col = 1;
print *
col is incremented and checked if it is less than /equal to 6,
if yes print *

This is done until col = 6, thus displaying 6 columns. Then the loop moves to the outer loop again, increments row, checks if the value is less than 5 and the process continues until row = 5. Then stops.

Try this same example with different values for row and column and run your program in debug. You will understand what is going on exactly.

at first, please forget about outer loop.
step1-then, please trace the inner loop.
then pay attention that for every, row=1 & row=2 &row=3 runs step1