错误发生在Go-lang中事件的逻辑顺序上

Question

I am having a somewhat inexplicable error in go where I encounter an error at a line which the program, in theory, should not be executing at that moment.

func exampleFunction() { 
  //cycleCount starts as 1, layerDif = 4 , composition is an int array with 5 entries  
  if cycleCount <= layerDif {
    fmt.Println("Evaluating if")
    fmt.Println("layerDif", layerDif, "cycleCount", cycleCount)
    for i := 0; i < composition[cycleCount]; i++ { //this is line 68
      //... random code
      cycleCount++
      fmt.Println("cycle++",cycleCount)
      exampleFunction() // this is line 72
    }
  } else {

    fmt.Println("Evaluating else")

    //... random code

    fmt.Println("Argyle")

    for i := 0; i < layerDif -1; i++ {
      fmt.Println("Sock", i)
      //... random code
    }

    //... random code

  }
}

This code results in an output of:

Evaluating if
layerDif 4 cycleCount 1
cycles 2
Evaluating if
layerDif 4 cycleCount 2
cycles 3
Evaluating if
layerDif 4 cycleCount 3
cycles 4
Evaluating if
layerDif 4 cycleCount 4
cycles 5
Evaluating else
panic: runtime error: index out of range

goroutine 1 [running]:
main.exampleFunction()
    /home/name/Go/Predict/src/main/Filename.go:68 +0x5c1
main.exampleFunction()
    /home/name/Go/Predict/src/main/Filename.go:72 +0x25e
main.exampleFunction()
    /home/name/Go/Predict/src/main/Filename.go:72 +0x25e
main.exampleFunction()
    /home/name/Go/Predict/src/main/Filename.go:72 +0x25e
...some other output which doesn't matter
exit status 2

I'm at somewhat of a loss for words, as the line at which the error occurs is listed as 68. However, just before this output is given, it prints "Evaluating else", an output which should be given only once line 68 is done being evaluated (The recursive function has stopped). The second mystery is why the for loop following "Evaluating else" is never executed (And should have printed a series of outputs with "Sock" in them). This is somewhat bewildering and I can't exactly pinpoint the cause of this error. Why is this occurring?

Answer 1

I was able to replicate your error with the following code:

package main

import "fmt"

var cycleCount int
var layerDif int
var composition [5]int

func main() {
    cycleCount = 1
    layerDif = 4
    composition = [5]int{0, 1, 2, 3, 4}
    exampleFunction()
}

func exampleFunction() {
    if cycleCount <= layerDif {
        fmt.Println("Evaluating if")
        fmt.Println("layerDif", layerDif, "cycleCount", cycleCount)
        for i := 0; i < composition[cycleCount]; i++ { //this is line 68
            cycleCount++
            fmt.Println("cycle++", cycleCount)
            exampleFunction() // this is line 72
        }
    } else {
        fmt.Println("Evaluating else")
        fmt.Println("Argyle")
        for i := 0; i < layerDif-1; i++ {
            fmt.Println("Sock", i)
        }
    }
}

Outputs:

Evaluating if
layerDif 4 cycleCount 1
cycle++ 2
Evaluating if
layerDif 4 cycleCount 2
cycle++ 3
Evaluating if
layerDif 4 cycleCount 3
cycle++ 4
Evaluating if
layerDif 4 cycleCount 4
cycle++ 5
Evaluating else
Argyle
Sock 0
Sock 1
Sock 2
panic: runtime error: index out of range

goroutine 1 [running]:
main.exampleFunction()
    /Users/daniel/Desktop/main.go:20 +0x38c
main.exampleFunction()
    /Users/daniel/Desktop/main.go:23 +0x21a
main.exampleFunction()
    /Users/daniel/Desktop/main.go:23 +0x21a
main.exampleFunction()
    /Users/daniel/Desktop/main.go:23 +0x21a
main.main()
    /Users/daniel/Desktop/main.go:13 +0x65
exit status 2

The problem with this code (and I guess with your code, too) is the elements of array composition. The for loop in line 68 loops from 0 to composition[cycleCount], that means:

i = 0
i = composition[CycleCount] == composition[1] == 1
// CycleCount is incremented, so:
i = composition[CycleCount] == composition[2] == 2
// and so on
i = 3
i = 4
i = 5
// at this point, else gets evaluated and you see the results of the respective for loop printed

Now, CycleCount will be incremented once more and is now 6, but composition has less than 6 elements and you get a runtime panic.

EDIT: I just realized that @matb already mentioned this in his comment. He also suggests the (in my opinion) correct solution, i.e. to check len(composition) in your for loop condition.