Modulo and order of operation in Python

In Zed Shaw's Learn Python the Hard Way (page 15-16), he has an example exercise

 100 - 25 * 3 % 4

the result is 97 (try it!)

I cannot see the order of operations that could do this..

100 - 25 = 75

3 % 4 = 0

开发者_如何转开发or (100-25*3) =225 % 4 = ??? but anyhow not 97 I don't think...

A similar example is 3 + 2 + 1 - 5 + 4 % 2 - 1 / 4 + 6 which yields 7

In what order are the operations done?

---

For the first example: * and % take precedence over -, so we first evaluate 25 * 3 % 4. * and % have the same priority and associativity from left to right, so we evaluate from left to right, starting with 25 * 3. This yields 75. Now we evaluate 75 % 4, yielding 3. Finally, 100 - 3 is 97.

---

Multiplication >> mod >> subtraction

In [3]: 25 * 3
Out[3]: 75

In [4]: 75 % 4
Out[4]: 3

In [5]: 100 - 3
Out[5]: 97

Multiplication and modulo operator have the same precedence, so you evaluate from left to right for this example.

---

I figured out the answer to your second question because it was bugging me too--Zac's response is close, but the loss of the result of 1/4 is because of Python 2.X is truncating integer division results. So it's evaluating the modulo operation first, then the division (which since it isn't float, is returned as 0.

3 + 2 + 1 - 5 + 4 % 2 - 1 / 4 + 6
3 + 2 + 1 - 5 + (0) - (0) + 6 
6 - 5 + 6
1 + 6
7

---

Here's how it goes:

'*' and '%' have the same precendence, so evaluate those from left to right.

1. 25*3 = 75
2. 75 % 4 = 3 (4*18 = 72; remainder is 3)
3. 100 - 3 = 97

Q.E.D.

---

Original problem: 100 - 25 * 3 % 4

Actually, evaluating 25 * 3 and taking 75% of 4 is incorrect, and happened to work out conveniently for this problem.

What the % in python actually is is a modulus operator where x % y gives the remainder of x / y. In this case, what happened was that 75 / 4 is 18, with a remainder of 3, which is why 100 - 3 = 97.

Do not try to multiply the percentages, it's a common mistake.

---

In the second exampe, %has same order as * so we get 3+2+1-5+4%2-1/4+6= 3+2+1-5+(4%2)-(1/4)+6=1+(4%2)-(1/4)+6 =1+0-(1/4)+6=1-(1/4)+6=0.75+6=6.75 and that is what it says when I try it on the console, so whatever you did you must have done something to round it.

---

Mathematics isn't my strong point, so yes this question got me as well, for a moment. But hope you find this useful.

75 divided by 4 is 18.75

18 multiplied by 4 is 72 (leaving 3 remaining from the 75)

The calculation given is 100-25*3%4 with an answer of 97. Now this is how I would get it using PEMDAS as he speaks of in the question section:

#!/bin/python

A = 100
B = 25
C = 3
D = 4
E = B*C # 75
F = E%D # 3
G = A-F # 97
print("B * C ="), E
print("E % D ="), F
print("A - F ="), G

I think you have to treat modulo (%) as a division,

---

Python evaluates % after * but before + or _ .

So,

(100 - 25 * 3 % 4)
(100 - 75 % 4)
(100 - 3) 
(97)