Python 用 matplotlib 绘制方程
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graphing an equation with matplotlib
提问by ?then?
I'm trying to make a function that will graph whatever formula I tell it to.
我正在尝试制作一个函数来绘制我告诉它的任何公式。
import numpy as np
import matplotlib.pyplot as plt
def graph(formula, x_range):
x = np.array(x_range)
y = formula
plt.plot(x, y)
plt.show()
When I try to call it the following error happens, I believe it's trying to do the multiplication before it gets to y = formula.
当我尝试调用它时会发生以下错误，我相信它正在尝试在乘法之前进行乘法运算y = formula。
graph(x**3+2*x-4, range(-10, 11))
Traceback (most recent call last):
File "<pyshell#23>", line 1, in <module>
graph(x**3+2*x-4, range(-10, 11))
NameError: name 'x' is not defined
采纳答案by rputikar
This is because in line
这是因为在线
graph(x**3+2*x-4, range(-10, 11))
x is not defined.
x 未定义。
The easiest way is to pass the function you want to plot as a string and use evalto evaluate it as an expression.
最简单的方法是将要绘制的函数作为字符串传递并用于eval将其作为表达式求值。
So your code with minimal modifications will be
所以你的代码修改最少
import numpy as np
import matplotlib.pyplot as plt
def graph(formula, x_range):
x = np.array(x_range)
y = eval(formula)
plt.plot(x, y)
plt.show()
and you can call it as
你可以称之为
graph('x**3+2*x-4', range(-10, 11))
回答by DSM
Your guess is right: the code is trying to evaluate x**3+2*x-4immediately. Unfortunately you can't really prevent it from doing so. The good news is that in Python, functions are first-class objects, by which I mean that you can treat them like any other variable. So to fix your function, we could do:
您的猜测是正确的：代码正在尝试x**3+2*x-4立即评估。不幸的是，您无法真正阻止它这样做。好消息是，在 Python 中，函数是一等对象，我的意思是你可以像对待任何其他变量一样对待它们。因此，要修复您的功能，我们可以这样做：
import numpy as np
import matplotlib.pyplot as plt
def graph(formula, x_range):
x = np.array(x_range)
y = formula(x) # <- note now we're calling the function 'formula' with x
plt.plot(x, y)
plt.show()
def my_formula(x):
return x**3+2*x-4
graph(my_formula, range(-10, 11))
If you wanted to do it all in one line, you could use what's called a lambdafunction, which is just a short function without a name where you don't use defor return:
如果您想在一行中完成所有操作，您可以使用所谓的lambda函数，它只是一个没有名称的简短函数，您不使用defor return：
graph(lambda x: x**3+2*x-4, range(-10, 11))
And instead of range, you can look at np.arange(which allows for non-integer increments), and np.linspace, which allows you to specify the start, stop, and the number of points to use.
而不是range，您可以查看np.arange（允许非整数增量）和np.linspace，它允许您指定开始、停止和要使用的点数。
回答by jelde015
To plot an equation that is not solved for a specific variable (like circle or hyperbola):
要绘制未针对特定变量（如圆或双曲线）求解的方程：
import numpy as np
import matplotlib.pyplot as plt
plt.figure() # Create a new figure window
xlist = np.linspace(-2.0, 2.0, 100) # Create 1-D arrays for x,y dimensions
ylist = np.linspace(-2.0, 2.0, 100)
X,Y = np.meshgrid(xlist, ylist) # Create 2-D grid xlist,ylist values
F = X**2 + Y**2 - 1 # 'Circle Equation
plt.contour(X, Y, F, [0], colors = 'k', linestyles = 'solid')
plt.show()
More about it: http://courses.csail.mit.edu/6.867/wiki/images/3/3f/Plot-python.pdf
更多相关信息：http: //courses.csail.mit.edu/6.867/wiki/images/3/3f/Plot-python.pdf
