How to make fmt.Scanln() read into a slice of integers - go

I have a line containing 3 numbers that I want to read from stdin with fmt.Scanln() but this code won't work:
X := make([]int, 3)
fmt.Scanln(X...)
fmt.Printf("%v\n", X)
I get this error message:
cannot use X (type []int) as type []interface {} in function argument
I don't get it.

Idiomatic Go would be:
func read(n int) ([]int, error) {
in := make([]int, n)
for i := range in {
_, err := fmt.Scan(&in[i])
if err != nil {
return in[:i], err
}
}
return in, nil
}
interface{} means nothing. Please don't use it if you don't have to.

For example,
package main
import "fmt"
func intScanln(n int) ([]int, error) {
x := make([]int, n)
y := make([]interface{}, len(x))
for i := range x {
y[i] = &x[i]
}
n, err := fmt.Scanln(y...)
x = x[:n]
return x, err
}
func main() {
x, err := intScanln(3)
if err != nil {
fmt.Println(err)
return
}
fmt.Printf("%v\n", x)
}
Input:
1 2 3
Output:
[1 2 3]

I think the the correct version should be
X := make([]int, 3)
fmt.Scanln(&X[0], &X[1], &X[2])
fmt.Printf("%v\n", X)

This error message occurs b/c there's no reasonable way to convert []int to []interface{}. Note, this is in reference to a slice. So the syntax your using is correct, but fmt.Scanln expects []interface{}. This has implications outside of pkg fmt.
The reason I've seen given for this is due to Go giving you control over memory layout so it currently has no reasonable way to do the slice conversion. This means you'll need to do the conversion manually before passing it to a function expecting the slice of a given type. For example:
package main
import (
"fmt"
)
func main() {
x := make([]int, 3)
y := make([]interface{}, 3)
y[0] = x[0]
y[1] = x[1]
y[2] = x[2]
fmt.Println(y...)
}
Or something a little more general:
x := make([]int, 3)
y := make([]interface{}, len(x))
for i, v := range x {
y[i] = v
}
fmt.Println(y...)
Regarding your specific issue, see the following:
x := make([]*int, 3)
for i := range x {
x[i] = new(int)
}
y := make([]interface{}, 3)
for i, v := range x {
y[i] = v
}
if _, err := fmt.Scanln(y...); err != nil {
fmt.Println("Scanln err: ", err)
}
for _, v := range y {
val := v.(*int)
fmt.Println(*val)
}

I saw in a comment you said the lines can have different lengths. In that case
you can implement your own fmt.Scanner:
package main
import (
"bytes"
"fmt"
)
type slice struct {
tok []int
}
func (s *slice) Scan(state fmt.ScanState, verb rune) error {
tok, err := state.Token(false, func(r rune) bool { return r != '\n' })
if err != nil { return err }
if _, _, err := state.ReadRune(); err != nil {
if len(tok) == 0 {
panic(err)
}
}
b := bytes.NewReader(tok)
for {
var d int
_, err := fmt.Fscan(b, &d)
if err != nil { break }
s.tok = append(s.tok, d)
}
return nil
}
func main() {
var s slice
fmt.Scan(&s)
fmt.Println(s.tok)
}
https://golang.org/pkg/fmt#Scanner

Related

How to compare 2 maps and export the difference in Golang?

I have 2 yaml files that I've exported to 2 different map variables. My yaml files have this example of format:
file1.yaml
a: b
c: d
e:
- f
- g
x:
- y
file2.yaml
m: n
c: d
x:
- y
- z
I've unmarshalled the yaml files into structs. My goal is to now compare the 2 maps, and only output the difference found in file2.yaml. So far I have something simple:
type ParametersYaml struct {
Parameters Params `yaml:"parameters""`
}
type Params map[string]interface{}
var pParam ParametersYaml
var sParam ParametersYaml
func Extractor(primaryYaml, secondaryYaml string) error {
primaryOpenedFile, err := fileOpener(primaryYaml)
secondaryOpenedFile, err := fileOpener(secondaryYaml)
err = yamlParser(primaryOpenedFile, &pParam)
err = yamlParser(secondaryOpenedFile, &sParam)
if err != nil {
return err
}
compare(pParam, sParam)
return err
}
func fileOpener(file string) ([]byte, error) {
f, err := ioutil.ReadFile(file)
return f, err
}
func yamlParser(fileObject []byte, param *ParametersYaml) error {
err := yaml.Unmarshal(fileObject, &param)
return err
}
func compare(pMarshalled, sMarshalled ParametersYaml) {
for s, sv := range sMarshalled.Parameters {
for p, pv := range pMarshalled.Parameters {
if s == p && sv == pv {
break
} else if s == p && sv != pv {
fmt.Printf("%s: %s,\n", s, sv)
break
} else if _, ok := pMarshalled.Parameters[s]; !ok {
fmt.Printf("%s: %s,\n", s, sv)
break
}
}
}
}
But it can't compare values with lists since the type is of interface{}.
There exists https://github.com/sters/yaml-diff which does the same thing you're intending to achieve.

Read line of numbers in Go

I have the following input, where on the first line is N - count of numbers, and on the second line N numbers, separated by space:
5
2 1 0 3 4
In Python I can read numbers without specifying its count (N):
_ = input()
numbers = list(map(int, input().split()))
How can I do the same in Go? Or I have to know exactly how many numbers are?
You can iterate through a file line-by-line using bufio, and the strings module can split a string into a slice. So that gets us something like:
package main
import (
"bufio"
"fmt"
"os"
"strconv"
"strings"
)
func main() {
readFile, err := os.Open("data.txt")
defer readFile.Close()
if err != nil {
fmt.Println(err)
}
fileScanner := bufio.NewScanner(readFile)
fileScanner.Split(bufio.ScanLines)
for fileScanner.Scan() {
// get next line from the file
line := fileScanner.Text()
// split it into a list of space-delimited tokens
chars := strings.Split(line, " ")
// create an slice of ints the same length as
// the chars slice
ints := make([]int, len(chars))
for i, s := range chars {
// convert string to int
val, err := strconv.Atoi(s)
if err != nil {
panic(err)
}
// update the corresponding position in the
// ints slice
ints[i] = val
}
fmt.Printf("%v\n", ints)
}
}
Which given your sample data will output:
[5]
[2 1 0 3 4]
Since you know the delimiter and you only have 2 lines, this is also a more compact solution:
package main
import (
"fmt"
"os"
"regexp"
"strconv"
"strings"
)
func main() {
parts, err := readRaw("data.txt")
if err != nil {
panic(err)
}
n, nums, err := toNumbers(parts)
if err != nil {
panic(err)
}
fmt.Printf("%d: %v\n", n, nums)
}
// readRaw reads the file in input and returns the numbers inside as a slice of strings
func readRaw(fn string) ([]string, error) {
b, err := os.ReadFile(fn)
if err != nil {
return nil, err
}
return regexp.MustCompile(`\s`).Split(strings.TrimSpace(string(b)), -1), nil
}
// toNumbers plays with the input string to return the data as a slice of int
func toNumbers(parts []string) (int, []int, error) {
n, err := strconv.Atoi(parts[0])
if err != nil {
return 0, nil, err
}
nums := make([]int, 0)
for _, p := range parts[1:] {
num, err := strconv.Atoi(p)
if err != nil {
return n, nums, err
}
nums = append(nums, num)
}
return n, nums, nil
}
The output out be:
5: [2 1 0 3 4]

Slice automatically be sorted?

While I want to create my own pipeline to practice with goroutines, there's something particularly weird.
I use the random perm function to generate some int numbers, randomly obviously, I write them to IO writer and then read them form IO reader, cuz its binary source so I print them and they are sorted!!
Here's the code:
func RandomSource(tally int) chan int {
out := make(chan int)
sli := rand.Perm(tally)
fmt.Println(sli)
go func() {
for num := range sli {
out <- num
}
close(out)
}()
return out
}
func ReaderSource(reader io.Reader) chan int {
out := make(chan int)
go func() {
buffer := make([]byte, 8)
for ; ; {
n, err := reader.Read(buffer)
if n > 0 {
v := int(binary.BigEndian.Uint64(buffer))
out <- v
}
if err != nil {
break
}
}
close(out)
}()
return out
}
func WriterSink(writer io.Writer, in chan int) {
for v := range in {
buffer := make([]byte, 8)
binary.BigEndian.PutUint64(
buffer, uint64(v))
writer.Write(buffer)
}
}
func main() {
fileName := "small.in"
file, err := os.Create(fileName)
if err != nil {
log.Fatal(err)
}
defer file.Close()
p := RandomSource(500)
WriterSink(file, p)
file, err = os.Open(fileName)
if err != nil {
log.Fatal(err)
}
defer file.Close()
p = ReaderSource(file)
for v := range p {
fmt.Println(v)
}
}
range returns an index as the first value for an array or slice, which always goes from 0 up to len - 1. Use for _, num := range sli { if you want to iterate over the values themselves rather than the set of indices.

Image.At returns nil

I'm having an issue. Here is my code:
package main
import (
"math/rand"
"image/draw"
"image/png"
"image/color"
"strconv"
"os"
"time"
"fmt"
)
func genSites(width, height int) ([][]int) {
rand.Seed(time.Now().Unix())
l, err := strconv.Atoi(os.Args[len(os.Args)-2])
if err != nil {
panic(err)
}
sites := make([][]int, l)
for i := range sites {
sites[i] = make([]int, 2)
sites[i][0] = rand.Intn(width)
sites[i][1] = rand.Intn(height)
}
return sites
}
func main() {
inputF, err := os.Open(os.Args[len(os.Args)-3])
if err != nil {
panic(err)
}
defer inputF.Close()
inputR, err := png.Decode(inputF)
if err != nil {
panic(err)
}
input := inputR.(draw.Image)
minx, miny := input.Bounds().Min.X, input.Bounds().Min.Y
maxx, maxy := input.Bounds().Max.X-1, input.Bounds().Max.Y-1
sites := genSites(maxx-minx, maxy-miny)
siteColors := make([][]color.Color, len(sites))
//todo: figure out something better than this
for i := range siteColors {
siteColors[i] = make([]color.Color, (maxx-minx)*(maxy-miny))
}
siteBelongs := make([][]int, maxx - minx)
for x := range siteBelongs {
siteBelongs[x] = make([]int, maxy - miny)
for y := range siteBelongs[x] {
dmin := (maxx-minx)*(maxx-minx) + (maxy-miny)*(maxy-miny)
var smin int
for i, s := range sites {
d := (s[0]-x)*(s[0]-x) + (s[1]-y)*(s[1]-y)
if d > dmin {
smin = i
dmin = d
}
}
siteBelongs[x][y] = smin
siteColors[smin] = append(siteColors[smin], input.At(x+minx, y+miny))
}
}
siteAvgColors := make([]color.Color, len(sites))
for i := range siteAvgColors {
var sR, sG, sB, sA int
for _, val := range siteColors[i] {
fmt.Println(val)
r, g, b, a := val.RGBA()
sR += int(r)
sG += int(g)
sB += int(b)
sA += int(a)
}
siteAvgColors[i] = color.RGBA{
uint8(sR/len(siteColors[i])),
uint8(sG/len(siteColors[i])),
uint8(sB/len(siteColors[i])),
uint8(sA/len(siteColors[i]))}
}
for x := range siteBelongs {
for y := range siteBelongs[x] {
input.Set(minx + x, miny + y, siteAvgColors[siteBelongs[x][y]])
}
}
output, err := os.Create(os.Args[len(os.Args)-1])
if err != nil {
panic(err)
}
defer output.Close()
err = png.Encode(output, input)
if err != nil {
panic(err)
}
}
The error is this:
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x4b46e1]
goroutine 1 [running]:
main.main()
/home/victor/programs/goprograms/src/v/imagerandvornoi/main.go:71 +0x7a1
Line 71 is the one that says r, g, b, a := val.RGBA(). That val is inserted at line 63, or siteColors[smin] = append(siteColors[smin], input.At(x+minx, y+miny)), which means that input.At is returning nil. Why? How can I fix this?
This has to do with the semantics of the make() builtin within Go. make is special in that it can take two or three arguments:
make(Type, length) or make(Type, length, capacity). For the former, the length and capacity are set to the same value. If you're going to be assigning values to the slice using append, you're going to want to use the latter form. This is because the two argument form (make(Type, 10)) builds a slice with 10 elements in it already with the zero value, when you use append() it becomes the 11th item.
You're hitting a nil derefernece here because you're using the two-argument version of make with append, and so the first item in the slice is nil.

Reading a user input and splitting it into two float64 numbers

I have a function to read a single float64 from stdin:
func readFloat() float64 {
scanner := bufio.NewScanner(os.Stdin)
for {
scanner.Scan()
in := scanner.Text()
n, err := strconv.ParseFloat(in, 64)
if err == nil {
return n
} else {
fmt.Println("ERROR:", err)
fmt.Print("\nPlease enter a valid number: ")
}
}
}
I would like to modify this to read two floating point numbers for e.g.
func main() {
fmt.Print("\nEnter x, y coordinates for point1: ")
x1, y1 := readFloat()
Problem I am facing is splitting scanner.Text(). There is a function scanner.Split() but cannot understand how to use it.
Any possible solutions would be helpful.
I would probably go with fmt.Sscanf here
package main
import (
"fmt"
)
func main() {
testCases := []string{"1,2", "0.1,0.2", "1.234,2.234"}
var a, b float64
for _, s := range testCases {
_, err := fmt.Sscanf(s, "%f,%f", &a, &b)
if err != nil {
panic(err)
}
fmt.Printf("Got %f, %f\n", a, b)
}
}
output:
Got 1.000000, 2.000000
Got 0.100000, 0.200000
Got 1.234000, 2.234000
https://play.golang.org/p/7ATyjlkPhnD
Use strings.Split:
package main
import (
"bufio"
"fmt"
"os"
"strconv"
"strings"
)
func readFloat() (float64, float64) {
scanner := bufio.NewScanner(os.Stdin)
for {
scanner.Scan()
in := scanner.Text()
parts := strings.Split(in, ",")
x, err := strconv.ParseFloat(parts[0], 64)
if err != nil {
fmt.Println("ERROR:", err)
fmt.Print("\nPlease enter a valid number: ")
}
y, err := strconv.ParseFloat(parts[1], 64)
if err != nil {
fmt.Println("ERROR:", err)
fmt.Print("\nPlease enter a valid number: ")
}
return x, y
}
}
func main() {
fmt.Print("\nEnter x, y coordinates for point1: ")
x1, y1 := readFloat()
fmt.Println(x1, y1)
}
Using it:
$ go run main.go
Enter x, y coordinates for point1: 1.2,3.4
1.2 3.4

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