问为什么我在这个程序中得到“名称错误:名称x没有违抗”？

EN

# UNQ_C2
# GRADED FUNCTION: compute_gradient
def compute_gradient(x, y, w, b): 
 """
    Computes the gradient for linear regression 
    Args:
      x (ndarray): Shape (m,) Input to the model (Population of cities) 
      y (ndarray): Shape (m,) Label (Actual profits for the cities)
      w, b (scalar): Parameters of the model  
    Returns
      dj_dw (scalar): The gradient of the cost w.r.t. the parameters w
      dj_db (scalar): The gradient of the cost w.r.t. the parameter b     
 """
    
# Number of training examples
n = x.shape[0]
    
# You need to return the following variables correctly
dj_dw = 0
dj_db = 0
    
### START CODE HERE ###     
    
for i in range (n):
            
        f_wb = w*x[i] + b
        
        dj_db_i = f_wb - y[i]
        
        dj_db += dj_db_i
    
        dj_dw_i = (f_wb - y[i]) * [i]
        
        dj_dw += dj_dw_i 
        
        dj_dw = dj_dw / m
        dj_db = dj_db / m
    
### END CODE HERE ### 
        
return dj_dw, dj_db