Map is apparently nil even though I already assigned to it

Getting Help Code Review
1

Go version: 1.12.2
Error text: panic: assignment to entry in nil map

I’m aware that assigning to just a var myMap map[string]int leads to an issue with assignment to a nil map and that you have to do myMap := make(map[string]int) but I seem to be having the same issue with a map that I’ve already assigned to. Below is the stripped down code. Beware that I’ve added a bunch of superfluous stuff as a means of troubleshooting (though I tried to rip a lot of that out):


	for frm := range input {

		go func() {
			detMap := make(map[string][]image.Rectangle)
			tmpRect := []image.Rectangle{image.Rectangle{image.Point{0, 0}, image.Point{1, 1}}}
			detMap["init"] = tmpRect
			tmpImg, _, _, _, detMap, sublError = imageFrames.Sublimate(frm)

			rects := det.DetectMultiScale(tmpImg)
			detMap[fname] = rects

			frm.Detections = detMap
			output <- frm
			return
		}()
	}

I added “tmpRect” in an attempt to make sure that detMap is actually not nil. ImageFrames.Sublimate() is below:

func Sublimate(inFrm Frame) (gocv.Mat, string, []byte, map[int]image.Rectangle, map[string][]image.Rectangle, error) {

	retMap := make(map[string][]image.Rectangle)
	retMot := make(map[int]image.Rectangle)

	retMap = inFrm.Detections
	retMot = inFrm.MotionMap

	retMat, err := gocv.NewMatFromBytes(inFrm.Wid, inFrm.Hei, inFrm.Type, inFrm.MatBytes)

	return retMat, inFrm.UserName, inFrm.UserHash, retMot, retMap, err

}

I get the same issue if I just do tmpImg, _, _, _, detMap, sublError := imageFrames.Sublimate(frm) without creating the map beforehand.

2

Hi @ashinnv
Try to send the current value of the frm to the goroutine. Example:

package main

import (
	"fmt"
	"time"
)

func main() {
	input := []int{2, 4, 6, 8, 10}

	for i := range input {
		go func(x int) {
			processValue(x)
		}(input[i])
	}
	time.Sleep(10)
}

func processValue(value int) {
	fmt.Println(value)
}

link

Or replace “input[i]” with the variable “i”. Otherwise maybe you have a concurrency problem.

package main

import (
	"fmt"
	"time"
)

func main() {
	input := []int{2, 4, 6, 8, 10}

	for i := range input {
		go func(x int) {
			processValue(x)
		}(i)
	}
	time.Sleep(10)
}

func processValue(value int) {
	fmt.Println(value)
}

link 2

1 Like
3

Which line here is that panic coming from? That should indicate which line has the map access where the map is nil.

4

Which line?

detMap[fname] = rects
5

I tried that but still ended up with the same error. I’ve re-written the function to include the lines from Sublimate(), but I’m still getting the issue. I completely removed the anonymous function, as well, so that this is now just a serial loop with just an input chan and an output chan. The new, unstripped version is below:

func detect(input chan okoframe.Frame, output chan okoframe.Frame, classifierLocation string) {

	for frm := range input {

		//go func(frm okoframe.Frame) {
		var tmpImg gocv.Mat
		var sublError error
		//tmpImg, _, _, _, detMap, sublError := okoframe.Sublimate(frm)
		//StartSublimate--------------------------------------------------------------------------------------------
		tmpImg, nmfbError := gocv.NewMatFromBytes(frm.Wid, frm.Hei, frm.Type, frm.MatBytes)
		if nmfbError != nil {
			fmt.Println("Sublimate fail:", nmfbError)
		}

		//END SUBLIMATE---------------------------------------------------------------------------------------

		fname := filepath.Base(classifierLocation)

		fmt.Println("Loading classifier from :", fname)

		det := gocv.NewCascadeClassifier()
		if !det.Load(fname) {
			fmt.Errorf("Couldn't load classifier.")
			os.Exit(1)
		}

		rects := det.DetectMultiScale(tmpImg)
		//detMap[fname] = rects
		frm.Detections[fname] = rects
		//frm.Detections = detMap

		if sublError != nil {
			fmt.Println("Big error with sublimate stuff:", sublError)
			frm.ErrList = append(frm.ErrList, sublError)
		}

		output <- frm
		//return
		//}(frm)
	}
}

This is just weird. I’m sure it’s some stupid little thing I’m completely overlooking, but still, geez.

6

SOLVED!

The issue was coming from creating the “Frame” type. I create them in a completely different process from images taken from the webcam. These are then encoded and transmitted down a pipeline. When I originally created the Frame type, I created a function that spits out a new frame to be used across the whole project for when I want to create frames, but at this time, I didn’t have the map of opencv detections included.

When I added computer vision to the project, I didn’t update the function that creates blank frames, instead only adding the struct element. This meant that following that, all new frames that were created would have a nil element where the detection map should be.

That frame with a nil detection map would be encoded and sent down the pipeline, where it caused problems in this process. Thanks for all the help and suggestions, guys.

7

Is this nil? You create retMap in the Sublimate function, but then overwrite it with whatever is in inFrm.Detections and then return that.

EDIT: Oops. I missed that you found out already. Never mind!

1 Like
8

That came from the thrashing I’d given the source in trying to figure out what was going on. I kept adding and removing stuff in an attempt to get an idea of where the issue was happening. Never considered it wasn’t even in this process at all.