What is the value of z when the following code finishes executing?
int x;
int y;
int z;
x = 1;
z = 1;
while (x <= 5)
{
z = z + x;
x = x + 1;
}
I know the answer is 16 but I keep running into errors when trying to compute it. I feel like I'm messing up a step in the loop in the beginning. I know I can just plug this into a java client and get the answer but I'm really trying to understand the loop sequence so as to be able to do it by hand.
To really understand it, just think like the computer. Write out what the while-loop is going to do to each of the variables on each iteration. I've omitted y because it doesn't seem to be used for anything.
x | z | (x <= 5) | z + x | x + 1
===+====+==========+=======+======
Initial: 1 | 1 | | |
===+====+==========+=======+======
Iteration 1: 1 | 1 | true | 2 | 2
Iteration 2: 2 | 2 | true | 4 | 3
Iteration 3: 3 | 4 | true | 7 | 4
Iteration 4: 4 | 7 | true | 11 | 5
Iteration 5: 5 | 11 | true | 16 | 6
Iteration 6: 6 | 16 | false | |
===+====+==========+=======+======
Final values: 6 | 16 | | |
When x equals 6, the test x <= 5 evaluates to false, so we break out of the loop. And now, at the end, we can see that z = 16.
x = 1;
z = 1;
Okay, x is 1 and z is 1.
while (x <= 5)
{
z = z + x;
x = x + 1;
}
x is less than or equal to 5, so we perform this operation. We set z to 1+1 or 2. We set x to 1+1, or 2.
x = 2
z = 2
while (x <= 5)
{
z = z + x;
x = x + 1;
}
2 is <= 5, so we loop. z = 2+2, or 4. x = 2+1, or 3.
x = 3
z = 4
while (x <= 5)
{
z = z + x;
x = x + 1;
}
x is less than or equal to 5. Now z=4+3=7, x=3+1 = 4.
x = 4
z = 7
while (x <= 5)
{
z = z + x;
x = x + 1;
}
Again, 4<=5, so z=7+4=11, x=4+1=5
x = 5
z = 11
while (x <= 5)
{
z = z + x;
x = x + 1;
}
5 is <= 5, so we loop. z=11+6=16, x=5+1=6
x = 6
z = 16
while (x <= 5)
{
z = z + x;
x = x + 1;
}
Oops, x is not <=5, so we're done. z is now 16.
Once you understand how this loop works, it's obvious. We start out at 1. Then we add 1, 2, 3, 4, and finally 5 to it. 1+1+2+3+4+5=16.
x=1
z=1
while(x is less than or equals to 5) {
z is z + x;
x is x+1
}
meaning first loop will be
z = 1 + 1 = 2
x = 1 + 1 = 2
second:
z = 2 + 2 = 4
x = 2 + 1 = 3
and so on until x is 5
When learning loops, making a chart can make it a bit easier to see what is going on (though it is a bit tedious)
z | x | while (x <= 5) | z + x | x + 1 | new z | new x
=======================================================
1 | 1 | true | 2 | 2 | 2 | 2
_______________________________________________________
2 | 2 | true | 4 | 3 | 4 | 3
_______________________________________________________
4 | 3 | true | 7 | 4 | 7 | 4
_______________________________________________________
7 | 4 | true | 11 | 5 | 11 | 5
_______________________________________________________
11 | 5 | true | 16 | 6 | 16 | 6
_______________________________________________________
16 | 6 | false | NA | NA | NA | NA
^ final value
By writing a table you can see how the value changes over time. Once you reach false, you know that your final value for z is the output.
Also, you can start to understand how the for-loop works and notice patterns
The while statement keeps running untill a condition is true, in your case the condition is x<=5, but let's go step by step.
When the code first reachs the while your condition is true because x=1, so the block inside is executed and z = 1 + 1 = 2 and x = 1 + 1 = 2, then the iteration starts again, this time z = 2 + 2 = 4 and x = 2 + 1 =3 and so on until x > 5 that renders the condition false and the program exit the while loop. At that point z will be 16.
This while loop is looping while x is less or equals with 5 and in each iteration it increase values of variables z and x.
You can use debugger to view all values step by step, or you can modify your code and print all values from variable like this:
int x;
int y;
int z;
int iteration = 1;
x = 1;
z = 1;
while (x <= 5)
{
z = z + x;
x = x + 1;
iteration++;
System.out.println("Iteration: " + iteration + " Values: x=" + x + " y=" + y + " z=" + z);
}
You can rewrite it like this, to make it clearer:
x = 1;
z = 1;
while (x <= 5)
{
z += x;
x += 1;
}
Z is incremented by x at each loop, while x is incremented by 1.
In other words :
z = 1;
for (x = 1; x <= 5; x++)
{
z += x;
}
I'm pretty confident is easier to trace by hand now, isn't?
whileloops.