Split a buf into two slices.
One is
buf[:n]
other is
buf[n:].
n maybe larger than len(buf).
Finish it just using one line code .
Is there any grace code ?
This is not elegant, nor practical, but the evaluation is on one line...
package main
import (
"fmt"
)
func main() {
buf := "abcdefg"
n := 8
// fugly one-liner
a, b, err := func() (string, string, error) {if n > len(buf) {return "", "", fmt.Errorf("out of bounds")} else {return buf[:n], buf[n:], nil}}()
if err != nil {
fmt.Println(err.Error())
} else {
fmt.Print(a + ":" + b)
}
}
Related
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]
was trying to determine if there's a way to take a given input and expected output from a file for use in go test.
main.go:
package main
import (
"fmt"
"math"
)
func main() {
var n, m, a float64
fmt.Scanln(&n, &m, &a)
a_in_n_ceil := uint64(math.Ceil(n / a))
a_in_m_ceil := uint64(math.Ceil(m / a))
a_in_n_and_m := a_in_n_ceil * a_in_m_ceil
fmt.Println(a_in_n_and_m)
}
examples:
6 6 4
4
Would it be io.readfile or something similar to grab the first line of input from the examples file and then again for the seconds line of expected output in main_test.go? Guidance is appreciated.
Use os package for file read & write
To read from file : os.ReadFile(path_to_file)
To write file : os.WriteFile("output.txt", data_in_byte_array, file_permission)
package main
import (
"fmt"
"math"
"os"
"strconv"
"strings"
)
func check(e error) {
if e != nil {
panic(e)
}
}
func ReadFromFile(path string) []float64 {
dat, err := os.ReadFile(path) // read file contents
check(err)
stringArr := strings.Split(string(dat)," ")
var numbers []float64
for _, arg := range stringArr {
if n, err := strconv.ParseFloat(arg, 64); err == nil {
numbers = append(numbers, n)
}
}
return numbers // return file contents in required format (in this case []float64)
}
func WriteFile(data string) error{
err := os.WriteFile("output.txt", []byte(data), 0644)
if err != nil {
return err
}
return nil
}
func main(){
var n, m, a float64
numbers := ReadFromFile("input.txt")
fmt.Println(numbers)
n = numbers[0]
m = numbers[1]
a = numbers[2]
a_in_n_ceil := uint64(math.Ceil(n / a))
a_in_m_ceil := uint64(math.Ceil(m / a))
a_in_n_and_m := a_in_n_ceil * a_in_m_ceil
fmt.Println(a_in_n_and_m) // print to console
err = WriteFile(fmt.Sprint(a_in_n_and_m)) // write output to a file
if err != nil {
fmt.Println("Error in file write : ", err)
}
}
input.txt
6 6 4
output.txt
4
I'm trying to read input from stdin like
3 2 1<ENTER>
and save it in a list of ints. At the moment my code looks like this:
nums = make([]int, 0)
var i int
for {
_, err := fmt.Scan(&i)
if err != nil {
if err==io.EOF { break }
log.Fatal(err)
}
nums = append(nums, i)
}
at the moment the program never leaves the for-loop. I can't find an easy way to check for a newline character in the documentation. how would i do this?
Edit:
Since I know that there will almost certainly be four numbers, I tried the following:
var i0,i1,i2,i3 int
fmt.Scanf("%d %d %d %d\n", &i0, &i1, &i2, &i3)
but this only scanned the first number and then exited the program. I'm not sure if that's because of the z-shell I'm using.
Edit:
To clarify, the program will pause and ask for the user to input a list of n numbers separated by spaces and terminated with a newline. these numbers should be stored in an array.
Ok, I decided to bring out the large bufio hammer and solve it like this:
in := bufio.NewReader(os.Stdin)
line, err := in.ReadString('\n')
if err != nil {
log.Fatal(err)
}
strs := strings.Split(line[0:len(line)-1], " ")
nums := make([]int, len(strs))
for i, str := range strs {
if nums[i], err = strconv.Atoi(str); err != nil {
log.Fatal(err)
}
}
It does seem like an awful lot of code, but it works.
It seems that you want https://golang.org/pkg/fmt/#Fscanln
Something like
ok := func(err error) { if err != nil { panic(err) } }
for {
var i, j, k int
_, err := fmt.Fscanln(io.Stdin, &i, &j, &k)
ok(err)
fmt.Println(i, j, k)
}
I will suggest to use "bufio" package with the "scan()" method.
Following is the code where I'm reading two lines from "stdin" and storing the lines into an array.
Hope this helps you.
package main
import (
"fmt"
"bufio"
"os"
"strconv"
"strings"
)
func ReadInput() []string{
var lines []string
scanner := bufio.NewScanner(os.Stdin)
for scanner.Scan() {
lines = append(lines, scanner.Text())
//count, _ := strconv.Atoi(lines[0])
if len(lines) == 2 { break }
}
if err := scanner.Err(); err != nil {
fmt.Fprintln(os.Stderr, err)
}
return lines
}
func main(){
lines := ReadInput()
count ,_ := strconv.Atoi(lines[0])
num := strings.Fields(lines[1])
if count != len(num) { os.Exit(0) }
// Do whatever you want here
}
Two lines will be accepted. First line will have a count. Second line will have all the numbers. You can modify the same code as per your requirement.
Example:
3
1 5 10
How do I use the fmt.Scanf function in Go to get an integer input from the standard input?
If this can't be done using fmt.Scanf, what's the best way to read a single integer?
http://golang.org/pkg/fmt/#Scanf
All the included libraries in Go are well documented.
That being said, I believe
func main() {
var i int
_, err := fmt.Scanf("%d", &i)
}
does the trick
An alternative that can be a bit more concise is to just use fmt.Scan:
package main
import "fmt"
func main() {
var i int
fmt.Scan(&i)
fmt.Println("read number", i, "from stdin")
}
This uses reflection on the type of the argument to discover how the input should be parsed.
http://golang.org/pkg/fmt/#Scan
Here is my "Fast IO" method for reading positive integers. It could be improved with bitshifts and laying out memory in advance.
package main
import (
"io/ioutil"
"bufio"
"os"
"strconv"
)
func main() {
out := bufio.NewWriter(os.Stdout)
ints := getInts()
var T int64
T, ints = ints[0], ints[1:]
..
out.WriteString(strconv.Itoa(my_num) + "\n")
out.Flush()
}
}
func getInts() []int64 {
//assumes POSITIVE INTEGERS. Check v for '-' if you have negative.
var buf []byte
buf, _ = ioutil.ReadAll(os.Stdin)
var ints []int64
num := int64(0)
found := false
for _, v := range buf {
if '0' <= v && v <= '9' {
num = 10*num + int64(v - '0') //could use bitshifting here.
found = true
} else if found {
ints = append(ints, num)
found = false
num = 0
}
}
if found {
ints = append(ints, num)
found = false
num = 0
}
return ints
}
Golang fmt.Scan is simpler than Golang fmt.Scanf (which is simpler than Clang scanf)
If fmt.Scan errors i.e. if not nil, log & return
1 Read single variable:
import (
"fmt"
"log"
)
var i int
if _, err := fmt.Scan(&i); err != nil {
log.Print(" Scan for i failed, due to ", err)
return
}
fmt.Println(i)
2 Read multiple variables:
import (
"fmt"
"log"
)
var i, j, k int
if _, err := fmt.Scan(&i, &j, &k); err != nil {
log.Print(" Scan for i, j & k failed, due to ", err)
return
}
fmt.Println(i, j, k)
Best of luck
Example from: http://www.sortedinf.com/?q=golang-in-1-hour
You can use fmt.Scanf with a format specifier. The format specifier for the integer is %d. So you can use standard input like below.
func main() {
var someVar int
fmt.Scanf("%d", &someVar)
}
or else you can use fmt.Scan or fmt.Scanln as below.
func main() {
var someVar int
fmt.Scanln(&someVar)
}
You could also use bufio.NewReader to read an integer from the standard input.
The below program:
Prompts for an integer input
Creates a bufio.Reader to read from standard input
Reads input till it encounters a newline character '\n' (Note that this will only read a single integer. Space separated values will not work)
Removes the newline character
Converts string to int
package main
import (
"fmt"
"bufio"
"os"
"strconv"
"strings"
)
func getInt() error {
fmt.Println("Enter an integer")
userInput := bufio.NewReader(os.Stdin)
userVal, err := userInput.ReadString('\n')
if err != nil {
return err
}
input := strings.TrimSpace(userVal)
intVal, err := strconv.Atoi(input)
if err != nil {
return err
}
fmt.Printf("You entered: %d\n", intVal)
return nil
}
func main() {
getInt()
}
Why can't we just use a scanf? just like we use in C? it's working though.
package main
import "fmt"
func main() {
var i int
fmt.Scanf("%d", &i)
fmt.Println(i)
}
I'm looking for the Go equivalent of scanf().
I tried with following code:
1 package main
2
3 import (
4 "scanner"
5 "os"
6 "fmt"
7 )
8
9 func main() {
10 var s scanner.Scanner
11 s.Init(os.Stdin)
12 s.Mode = scanner.ScanInts
13 tok := s.Scan()
14 for tok != scanner.EOF {
15 fmt.Printf("%d ", tok)
16 tok = s.Scan()
17 }
18 fmt.Println()
19 }
I run it with input from a text with a line of integers.
But it always output -3 -3 ...
And how to scan a line composed of a string and some integers?
Changing the mode whenever encounter a new data type?
The Package documentation:
Package scanner
A general-purpose scanner for UTF-8
encoded text.
But it seems that the scanner is not for general use.
Updated code:
func main() {
n := scanf()
fmt.Println(n)
fmt.Println(len(n))
}
func scanf() []int {
nums := new(vector.IntVector)
reader := bufio.NewReader(os.Stdin)
str, err := reader.ReadString('\n')
for err != os.EOF {
fields := strings.Fields(str)
for _, f := range fields {
i, _ := strconv.Atoi(f)
nums.Push(i)
}
str, err = reader.ReadString('\n')
}
r := make([]int, nums.Len())
for i := 0; i < nums.Len(); i++ {
r[i] = nums.At(i)
}
return r
}
Improved version:
package main
import (
"bufio"
"os"
"io"
"fmt"
"strings"
"strconv"
"container/vector"
)
func main() {
n := fscanf(os.Stdin)
fmt.Println(len(n), n)
}
func fscanf(in io.Reader) []int {
var nums vector.IntVector
reader := bufio.NewReader(in)
str, err := reader.ReadString('\n')
for err != os.EOF {
fields := strings.Fields(str)
for _, f := range fields {
if i, err := strconv.Atoi(f); err == nil {
nums.Push(i)
}
}
str, err = reader.ReadString('\n')
}
return nums
}
Your updated code was much easier to compile without the line numbers, but it was missing the package and import statements.
Looking at your code, I noticed a few things. Here's my revised version of your code.
package main
import (
"bufio"
"fmt"
"io"
"os"
"strconv"
"strings"
"container/vector"
)
func main() {
n := scanf(os.Stdin)
fmt.Println()
fmt.Println(len(n), n)
}
func scanf(in io.Reader) []int {
var nums vector.IntVector
rd := bufio.NewReader(os.Stdin)
str, err := rd.ReadString('\n')
for err != os.EOF {
fields := strings.Fields(str)
for _, f := range fields {
if i, err := strconv.Atoi(f); err == nil {
nums.Push(i)
}
}
str, err = rd.ReadString('\n')
}
return nums
}
I might want to use any input file for scanf(), not just Stdin; scanf() takes an io.Reader as a parameter.
You wrote: nums := new(vector.IntVector), where type IntVector []int. This allocates an integer slice reference named nums and initializes it to zero, then the new() function allocates an integer slice reference and initializes it to zero, and then assigns it to nums. I wrote: var nums vector.IntVector, which avoids the redundancy by simply allocating an integer slice reference named nums and initializing it to zero.
You didn't check the err value for strconv.Atoi(), which meant invalid input was converted to a zero value; I skip it.
To copy from the vector to a new slice and return the slice, you wrote:
r := make([]int, nums.Len())
for i := 0; i < nums.Len(); i++ {
r[i] = nums.At(i)
}
return r
First, I simply replaced that with an equivalent, the IntVector.Data() method: return nums.Data(). Then, I took advantage of the fact that type IntVector []int and avoided the allocation and copy by replacing that by: return nums.
Although it can be used for other things, the scanner package is designed to scan Go program text. Ints (-123), Chars('c'), Strings("str"), etc. are Go language token types.
package main
import (
"fmt"
"os"
"scanner"
"strconv"
)
func main() {
var s scanner.Scanner
s.Init(os.Stdin)
s.Error = func(s *scanner.Scanner, msg string) { fmt.Println("scan error", msg) }
s.Mode = scanner.ScanInts | scanner.ScanStrings | scanner.ScanRawStrings
for tok := s.Scan(); tok != scanner.EOF; tok = s.Scan() {
txt := s.TokenText()
fmt.Print("token:", tok, "text:", txt)
switch tok {
case scanner.Int:
si, err := strconv.Atoi64(txt)
if err == nil {
fmt.Print(" integer: ", si)
}
case scanner.String, scanner.RawString:
fmt.Print(" string: ", txt)
default:
if tok >= 0 {
fmt.Print(" unicode: ", "rune = ", tok)
} else {
fmt.Print(" ERROR")
}
}
fmt.Println()
}
}
This example always reads in a line at a time and returns the entire line as a string. If you want to parse out specific values from it you could.
package main
import (
"fmt"
"bufio"
"os"
"strings"
)
func main() {
value := Input("Please enter a value: ")
trimmed := strings.TrimSpace(value)
fmt.Printf("Hello %s!\n", trimmed)
}
func Input(str string) string {
print(str)
reader := bufio.NewReader(os.Stdin)
input, _ := reader.ReadString('\n')
return input
}
In a comment to one of my answers, you said:
From the Language Specification: "When
memory is allocated to store a value,
either through a declaration or make()
or new() call, and no explicit
initialization is provided, the memory
is given a default initialization".
Then what's the point of new()?
If we run:
package main
import ("fmt")
func main() {
var i int
var j *int
fmt.Println("i (a value) = ", i, "; j (a pointer) = ", j)
j = new(int)
fmt.Println("i (a value) = ", i, "; j (a pointer) = ", j, "; *j (a value) = ", *j)
}
The declaration var i int allocates memory to store an integer value and initializes the value to zero. The declaration var j *int allocates memory to store a pointer to an integer value and initializes the pointer to zero (a nil pointer); no memory is allocated to store an integer value. We see program output similar to:
i (a value) = 0 ; j (a pointer) = <nil>
The built-in function new takes a type T and returns a value of type *T. The memory is initialized to zero values. The statement j = new(int) allocates memory to store an integer value and initializes the value to zero, then it stores a pointer to this integer value in j. We see program output similar to:
i (a value) = 0 ; j (a pointer) = 0x7fcf913a90f0 ; *j (a value) = 0
The latest release of Go (2010-05-27) has added two functions to the fmt package: Scan() and Scanln(). They don't take any pattern string. like in C, but checks the type of the arguments instead.
package main
import (
"fmt"
"os"
"container/vector"
)
func main() {
numbers := new(vector.IntVector)
var number int
n, err := fmt.Scan(os.Stdin, &number)
for n == 1 && err == nil {
numbers.Push(number)
n, err = fmt.Scan(os.Stdin, &number)
}
fmt.Printf("%v\n", numbers.Data())
}