CODEWITHSUNDEEP
javaloops
Thành Đạt
Thành Đạt

Reputation: 327

Java for loop is hard to understand

May someone explain to me why the following blocks of code generate such different outputs?

public class hello 
{

    public static void main(String args[])
    {
        int a,b,c;
        for (a  = 0; a < 5; a++)
        {
            for (b = 4; b >= a; b--)
            {
                System.out.print(" ");
            }
            for (c = 0; c <= a - b; c++)
            {
                System.out.print("*");
            }
            System.out.println();
        }
    }
}

Output:

enter image description here

public class hello 
{

    public static void main(String args[])
    {
        int a,b,c;
        for (a  = 0; a < 5; a++)
        {
            for (b = 4; b >= 0; b--)
            {
                System.out.print(" ");
            }
            for (c = 0; c <= a - b; c++)
            {
                System.out.print("*");
            }
            System.out.println();
        }
    }
}

Output:

enter image description here

Shouldnt the outputs be the same since b >= a is equivalent to b >= 0 as b's value will be deducted by 1 for every loop?

Upvotes: 0

Views: 427

Answers (3)

d4u7
d4u7

Reputation: 29

Please read the comments in the code and you will se the difference! 

    int a, b, c;
    for (a = 0; a < 5; a++)
    {
        for (b = 4; b >= a; b--) // Print every time b-a + 1 underscores... since you start every time with b=4 you have for each a one space fewer
        {
            System.out.print(" ");
        }

        for (c = 0; c <= a - b; c++) // and then print (a-b)-c +1 stars (b is every time a-1)... 
                                     // (first a=0 -b=-1)+1=2 and any time it will prit 2 stars scince
        {
            System.out.print("*");
        }
        System.out.println();
    }

    for (a = 0; a < 5; a++)
    {
        for (b = 4; b >= 0; b--) // Print every time b-a + 1=4 underscores
        {
            System.out.print(" ");
        }

        for (c = 0; c <= a - b; c++) // and then print (a-b)-c +1 stars (b is every time -1)... 
                                     // first time ( a=0 -b=-1)+1 =2 , second time (a=1 - b=-1)+1=3
        {
            System.out.print("*");
        }

        System.out.println();
    }

Upvotes: 1

aUserHimself
aUserHimself

Reputation: 1597

No, the outputs cannot be the same, since every time you are executing

for (a = 0; a < 5; a++)

this gets executed as well (5 times)

for (b = 4; b >= a; b--)
    {
        System.out.print(" ");
    }

But the value of a is changing with every iteration: a will be initially 0, then 1, 2, 3 and finally 4.

Hence, the number of spaces that you are printing in the first scenario will decrease with every iteration of a.

For a = 0 we have: b = 4, b = 3, b = 2, b = 1, b = 0 (for loop stops since b=-1 is not >= a=0)

For a = 1 we have: b = 4, b = 3, b = 2, b = 1 (for loop stops since b=0 is not >= a=1)

For a = 2 we have: b = 4, b = 3, b = 2 (for loop stops since b=1 is not >= a=2)

For a = 3 we have: b = 4, b = 3 (for loop stops since b=2 is not >= a=3)

For a = 4 we have: b = 4 (for loop stops since b=3 is not >= a=4)

Upvotes: 2

Kevin Robatel
Kevin Robatel

Reputation: 8386

variable a will go from 0 to 4, so for each iteration you will have:

for (b = 4; b >= 0; b--)
    { [...]

for (b = 4; b >= 1; b--)
    { [...]

for (b = 4; b >= 2; b--)
    { [...]

Upvotes: 2

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