Python / MDAnalysis SelectionError:选择失败：‘无法将字符串转换为浮动

Question

我是Python和MDAnalysis的新手。我正在尝试使用MDAnalysis进行分析。但是，我遇到了以下错误：

SelectionError: Selection failed: could not convert string to float 'cold'.

我想选择"BF4“、"EMI”和"TMA“的分子。选择取决于分子的z位置。我有cold和cnew，但是我不能将每个分子的z坐标的值与这两个值进行比较，因为它们被认为是字符串而不是浮点。

你能帮帮我吗？非常感谢

  with MDAnalysis.Writer("tube.xtc",tube.n_atoms) as W: 
     t=50 
     with open('lineCNT16.gro','w') as f: 
        for ts in universe.trajectory: 
           W.write(tube) 
           tube = sum(sorted(tube, key=lambda x: x.position[2]))  
           lobf = [] 
           chunk = 40 
           for i in range(len(tube) // 40):  # how many times can we chunk the tube? 
            piece = tube[i*chunk:(i+1)*chunk]  # this is selecting [0:20] first, then [20:40] etc 
            position = piece.positions.mean(axis=0) 
            lobf.append(position) 

           print (ts,lobf) 
           mol=29627 
           f.write('Generated by trjconv : Ionic liquid simulation t=  '"%s\n"%t) 
           f.write('18\n') 
           f.write('    1C08      C29626   4.247   4.253   7.544\n') 
           cold = 7.544 
           for position in lobf: 
            a=position[0]/10   
            b=position[1]/10 
            c=position[2]/10 
            cnew=c 
            print(cnew) 
            f.write('    1C08      ')     
            f.write('C')   
            f.write("%0.8s"%mol) 
            f.write('   ')      
            f.write("%0.5s"%a)  
            f.write('   ') 
            f.write("%0.5s"%b)   
            f.write('   ') 
            f.write("%0.5s\n"%c) 
            mol=mol+1 
            BF4=universe.atoms.select_atoms("resname BF4 and prop z >= cold and prop z<= cnew") 
            #EMI=universe.atoms.select_atoms("resname EMI and prop z >= cold and prop z<= cnew") 
            #TMA=universe.atoms.select_atoms("resname TMA and prop z >= cold and prop z<= cnew") 
            #print(EMI) 
            #print(BF4) 
            #print(TMA) 
            cold=cnew 
           f.write('    1C08      C29643   4.247   4.253   12.64\n')  
           f.write('6.15000   6.20000  20.18420\n') 
           t=t+50.00000   
           mol=29627 

Answer 1

BF4=universe.atoms.select_atoms("resname BF4 and prop z >= cold and prop z<= cnew") 

MDAnalysis期望实际值(浮动，如5.7)，而不是字符串(“冷”)。它告诉你它需要一个数字而不是字符串。例如：

BF4=universe.atoms.select_atoms("resname BF4 and prop z >= 5.2 and prop z<= 7.5"

您有一个变量"cold"，但是您可能需要学习如何将选择字符串与该变量连接起来的技巧。

下面是一个解释它的小教程：https://www.pythonforbeginners.com/concatenation/string-concatenation-and-formatting-in-python