Instructions

Test suite

Solution

Calculate the number of grains of wheat on a chessboard given that the number on each square doubles.

There once was a wise servant who saved the life of a prince. The king promised to pay whatever the servant could dream up. Knowing that the king loved chess, the servant told the king he would like to have grains of wheat. One grain on the first square of a chess board. Two grains on the next. Four on the third, and so on.

There are 64 squares on a chessboard.

Write code that shows:

- how many grains were on each square, and
- the total number of grains

Did you get the tests passing and the code clean? If you want to, these are some additional things you could try:

- Optimize for speed.
- Optimize for readability.

Then please share your thoughts in a comment on the submission. Did this experiment make the code better? Worse? Did you learn anything from it?

To run the tests run the command `go test`

from within the exercise directory.

If the test suite contains benchmarks, you can run these with the `--bench`

and `--benchmem`

flags:

```
go test -v --bench . --benchmem
```

Keep in mind that each reviewer will run benchmarks on a different machine, with different specs, so the results from these benchmark tests may vary.

For more detailed information about the Go track, including how to get help if you're having trouble, please visit the exercism.io Go language page.

JavaRanch Cattle Drive, exercise 6 http://www.javaranch.com/grains.jsp

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

```
package grains
// Source: exercism/problem-specifications
// Commit: f079c2d grains: Move input (square) to input object (#1191)
// Problem Specifications Version: 1.1.0
// returns the number of grains on the square
var squareTests = []struct {
description string
input int
expectedVal uint64
expectError bool
}{
{
description: "1",
input: 1,
expectedVal: 1,
},
{
description: "2",
input: 2,
expectedVal: 2,
},
{
description: "3",
input: 3,
expectedVal: 4,
},
{
description: "4",
input: 4,
expectedVal: 8,
},
{
description: "16",
input: 16,
expectedVal: 32768,
},
{
description: "32",
input: 32,
expectedVal: 2147483648,
},
{
description: "64",
input: 64,
expectedVal: 9223372036854775808,
},
{
description: "square 0 returns an error",
input: 0,
expectError: true,
},
{
description: "negative square returns an error",
input: -1,
expectError: true,
},
{
description: "square greater than 64 returns an error",
input: 65,
expectError: true,
},
}
```

```
package grains
import (
"testing"
)
func TestSquare(t *testing.T) {
for _, test := range squareTests {
actualVal, actualErr := Square(test.input)
// check actualVal only if no error expected
if !test.expectError && actualVal != test.expectedVal {
t.Fatalf("FAIL: %s\nSquare(%d) expected %d, Actual %d", test.description, test.input, test.expectedVal, actualVal)
}
// if we expect an error and there isn't one
if test.expectError && actualErr == nil {
t.Fatalf("FAIL: %s\nSquare(%d) expected an error, but error is nil", test.description, test.input)
}
// if we don't expect an error and there is one
if !test.expectError && actualErr != nil {
var _ error = actualErr
t.Fatalf("FAIL: %s\nSquare(%d) expected no error, but error is: %s", test.description, test.input, actualErr)
}
t.Logf("PASS: %s", test.description)
}
}
func TestTotal(t *testing.T) {
var expected uint64 = 18446744073709551615
if actual := Total(); actual != expected {
t.Errorf("Total() expected %d, Actual %d", expected, actual)
}
}
func BenchmarkSquare(b *testing.B) {
for i := 0; i < b.N; i++ {
for _, test := range squareTests {
Square(test.input)
}
}
}
func BenchmarkTotal(b *testing.B) {
for i := 0; i < b.N; i++ {
Total()
}
}
```

```
package grains
import (
"errors"
)
// Square returns the count of grains in the given field.
func Square(input int) (uint64, error) {
if input < 1 || input > 64 {
return 0, errors.New("input must be between 1 and 64")
}
return 1 << uint(input-1), nil
}
// Total returns the sum of all grains.
func Total() uint64 {
return 1<<64 - 1
}
```

A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

- What compromises have been made?
- Are there new concepts here that you could read more about to improve your understanding?

## Community comments

hi, Please help me understand this solution!

Hi, Shahid4183!

Please, sorry for my English, I speak English a little.

The count of grains in the field with number N is Math.pow(2, N-1). The operator "<<" shifts the bit to the right. This is the same as multiplying by 2. It means that Math.pow(2, N-1) is the same as 1 << N-1. It is the first part.

The sum of all grains is the same as sum of Math.pow(2, 0), Math.pow(2, 1), Math.pow(2, 2) ... Math.pow(2, 63). The sum of the powers of 2 from 0 to N-1 is the same as the Nth power of 2 minus 1. It means that the sum of all grains is the same as Math.pow(2, 64) - 1.

Great! Thanks a lot for your quick response!