Problem of "uchar"

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I want to calculate average value in a rectangle without Invalid value(0).
But outputs of my program have problem. num_valid = 0, sumValue = 0 and outputValue = -nan.
Why???
Please help me.

        int num_valid=0;
        int sumValue=0;
        for(int row = selectRect.y; row < selectRect.height; row++)  //selectRect is a rectangle
        {
            for(int col=selectRect.x; col<selectRect.width; col++)
            {
                uchar value = outputImage[RIGHT].at<uchar>(row, col);  //outputImage[RIGHT] is a grayImage
                if(value != 0)
                {
                    num_valid++;
                    sumValue += value;
                }
            }
        }
        if(num_valid > selectRect.width * selectRect.height / 2){
            printf("invalid\n");
        }
        else{
            float outputValue = sumValue / num_valid;
            printf("Out: %d / %d = %f \n", sumValue, num_valid, outputValue);
        }