def mean_normalization(x):
    """
    This script is to do mean normalization for DataFrame
    
    Parameter:
        x: format: DataFrame
        rows:feature 
        columns:samples
    
    Returns:
        output:DataFrame; after mean normalization.
        rows:feature 
        columns:samples
    """
    for i in range(x.shape[1]):
        x_i = x.iloc[:,i]
        Whole_mean = sum(x_i)/len(x_i)
        new_i = [ij/Whole_mean for ij in x_i]
        x.iloc[:,i] = new_i
    return (x)