# 6.4. Tables¶

One of the things loops are good for is generating tabular data. For example, before computers were readily available, people had to calculate logarithms, sines and cosines, and other common mathematical functions by hand. To make that easier, there were books containing long tables where you could find the values of various functions. Creating these tables was slow and boring, and the result tended to be full of errors.

When computers appeared on the scene, one of the initial reactions was, “This is great! We can use the computers to generate the tables, so there will be no errors.” That turned out to be true (mostly), but shortsighted. Soon thereafter computers and calculators were so pervasive that the tables became obsolete.

Well, almost. It turns out that for some operations, computers use tables of values to get an approximate answer, and then perform computations to improve the approximation. In some cases, there have been errors in the underlying tables, most famously in the table the original Intel Pentium used to perform floating-point division.

Although a “log table” is not as useful as it once was, it still makes a good example of iteration.

The active code below outputs a sequence of values in the left column and their logarithms in the right column.

The sequence `\t`

represents a **tab** character. The sequence `\n`

represents a newline character. These sequences can be included anywhere
in a string, although in these examples the sequence is the whole
string.

A tab character causes the cursor to shift to the right until it reaches
one of the **tab stops**, which are normally every eight characters. As
we will see in a minute, tabs are useful for making columns of text line
up.

A newline character has exactly the same effect as `endl`

; it causes
the cursor to move on to the next line. Usually if a newline character
appears by itself, I use `endl`

, but if it appears as part of a
string, I use `\n`

.

The output of this program is

```
1 0
2 0.693147
3 1.09861
4 1.38629
5 1.60944
6 1.79176
7 1.94591
8 2.07944
9 2.19722
```

If these values seem odd, remember that the `log`

function uses base
\(e\). Since powers of two are so important in computer science, we
often want to find logarithms with respect to base 2. To do that, we can
use the following formula:

Changing the output statement to

```
cout << x << "\t" << log(x) / log(2.0) << endl;
```

yields

```
1 0
2 1
3 1.58496
4 2
5 2.32193
6 2.58496
7 2.80735
8 3
9 3.16993
```

We can see that 1, 2, 4 and 8 are powers of two, because their logarithms base 2 are round numbers. If we wanted to find the logarithms of other powers of two, we could modify the program like this:

If we wanted to find the logarithms of other powers of two, we could modify the program like this. Run the active code below.

Now instead of adding something to `x`

each time through the loop,
which yields an arithmetic sequence, we multiply `x`

by something,
yielding a **geometric** sequence. The result is:

```
1 0
2 1
4 2
8 3
16 4
32 5
64 6
```

Because we are using tab characters between the columns, the position of the second column does not depend on the number of digits in the first column.

Log tables may not be useful any more, but for computer scientists, knowing the powers of two is! As an exercise, modify this program so that it outputs the powers of two up to 65536 (that’s \(2^{16}\)). Print it out and memorize it.

Modify the active code below so that it outputs the power of two up to 65536, which is \(2^{16}\). If you get stuck, you can reveal the extra problem at the end for help.

Let’s write the code that prints out the powers of two.

Change

`pow(x,2)`

to`pow(3,x)`

and change`x = x + 1`

to`x = x + 2`

.-
- feedback_a:
Check the order of the

`pow`

function!

Change

`pow(x,2)`

to`pow(x,3)`

.-
This will print out the first ten perfect cubes.

Change

`pow(x,2)`

to`pow(x,3)`

and change`x = x + 1`

to`x = x + 2`

.-
Changing both the

`pow`

function and the increment in this way gives us the right answer. Change

`x < 11`

to`x < 6`

and change`pow(x,2)`

to`pow(x,3)`

.-
This will print out the first five perfect cubes, but not the first five odd perfect cubes.

Q-7: How can we modify the code below to print out a table of the first five odd numbers and their perfect cubes?

```
int main() {
int x = 1;
while (x < 11) {
cout << x << "\t" << pow(x, 2) << endl;
x = x + 1;
}
}
```