How to find the operand and the operation in a string

Question

So I was trying to figure out how to detect the number of operands and operations in a mathematical expression

Ex: 1+2*9/2

I was trying to separate the operands and the operations into their own form using functions because we have to check how many operations must be done and in the correct order (PEDMAS) as well.

I've taken the equation and taken out all the spaces already, now i have to find the number of operands and operations in the equation and then later use return to find the answer of the given mathematical expression by the user.

Any tips?

You have to parse it into an Abstract Syntax Tree (AST) to know for sure. — Sergio Tulentsev, Nov 08 '12 at 06:27
I would suggest you start with infix to postfix conversion. Then you can evaluate expressions using stack and operator precedence. Obviously this does not gives you type-safety (dimensional analysis) but helps you move forward. Refer: http://scriptasylum.com/tutorials/infix_postfix/algorithms/infix-postfix/index.htm — Sarang, Nov 08 '12 at 06:30
thanks, that seems to help me understand more about the order of operations — Jen, Nov 08 '12 at 06:32

score 1 · Answer 1 · answered Nov 08 '12 at 06:11

1

If you're allowed to, I'd recommend checking out the ast module. It's designed to do stuff like this for you using Python's own parser.

For an actual application, you'd probably use a parser generator like Ply.

For a simple homework assignment like this, you're probably expected to handcode a parser. First tokenize it (str.split), find the parenthesis, and then use precedence to group the other operations.

answered Nov 08 '12 at 06:11

Antimony

37,781
10
100
107

the ast module? could you review that for me? I dont think my prof mentioned ast – Jen Nov 08 '12 at 06:12
i though of using split, but it's one of the things we're actually not allowed to use which is a shame, it would make this a lot easier – Jen Nov 08 '12 at 06:21
Well what are you allowed to use? – Antimony Nov 08 '12 at 06:23
well, they wanted us to use functions to try and show and count the number of operands and operations – Jen Nov 08 '12 at 06:24
Yes but the question is what are you allowed to use? – Antimony Nov 08 '12 at 06:25
I was told anything else really, but no using split, we could use slice and to put everything into function forms – Jen Nov 08 '12 at 06:27

Artsiom Rudzenka · Accepted Answer · 2012-11-08T06:51:19.013

1

If expression can be not so simple as your sample you can use RPN - reverse polish notation

If your expression is pretty simple(only base ops and values less then 10) and you only need to count than you can use something like this i guess:

ops = '+-*/'
operationsCount= sum(expr.count(op) for op in ops)
operandsCount = len(expr) - operationsCount

Or you can use this:

def get_cnt(expr):
    ops = '+-*/'
    res = [expr]
    for op in ops:
        tmp = []
        for x in expr:
            tmp.extend(x.split(op))
        res = tmp[:]        
    return len(res), sum(expr.count(op) for op in ops)

Now you have number of operators and operands - and it is quite easily to split row correctly on ops/opd and to calculate expression.

edited Nov 08 '12 at 06:51

answered Nov 08 '12 at 06:14

Artsiom Rudzenka

27,895
4
34
52

does the sum(expr.count(op) for op in ops) just count the position number of the operation? – Jen Nov 08 '12 at 06:25
s.count(i) - total number of occurences of i in s - that means that we summing up occurences not indeces – Artsiom Rudzenka Nov 08 '12 at 06:26
i was taught with something along the lines of using if statements and the adding +1 to the count – Jen Nov 08 '12 at 07:01
I thought i answered your question on count)) – Artsiom Rudzenka Nov 08 '12 at 07:05

Kaifeng Jin · Answer 3 · 2012-11-08T10:05:13.817

1

import re
exp = 1+2*9/2
ops = re.findall(r'[\+\-*/]', exp)
print len(ops)

I don't know why you wanna find the number of operands I think

re.findall(r'[0-9]+|[\+\-*/()]', exp)

is better.

so I think the regular expression can help you

edited Nov 08 '12 at 10:05

answered Nov 08 '12 at 07:04

Kaifeng Jin

19
2

the number of operands was so that i could distinguish if it had only one operand (ex. 1 * 5) vs. a more complex expression (1+2*9/2) – Jen Nov 08 '12 at 07:09
OK, but if you want to build a calculator, I think there is no need to distinguish 1*5 and 1+2*9/2. You can use two stacks build by yourself, or the library numpy may help you – Kaifeng Jin Nov 08 '12 at 07:18
@Kaifeng Jin: take a closer look at your re! this way it won't work! ;-) – Don Question Nov 08 '12 at 07:26

How to find the operand and the operation in a string

3 Answers3