[Tutor] python arthematics

[ALAN GAULD]

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i have tried  like this way but i want to read all integers and operators in one string. how can i do that.
>
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>As described below.
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>expr = raw_input("Type an expression: ")
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>Now you have the expression as a string in expr.
>You just need to analyze it to identify the various components  (this is called parsing).
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>One simplistic approach to get you started is to treat white-space and the operators 
>as separators between operands. You can then scan the string to extract a list of 
>operands and a list of operators. So " 3 + 4 * 6" yields:
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>operators = ["+", "*"]
>operands = ["3", "4", "6"]
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>Since we are expecting the operands to be integers we can go further
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>operands =  [int(n) for n in operands]
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>For the operators you need to do a lookup table and maybe map them 
>to the operators module operations...
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>You can then start matching them and performing the operations just like you did 
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>in your code with the values read separately. eg. You can apply each operator in 
>turn to the first pair of operands in the list:
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>3 + 4  -> 7
>7 * 6 -> 42
> 
>That doesn't take any account of the normal rules of precedence and 
>thats when things start to get much more complex. Also it doesn't allow 
>for parenthesised sub expressions etc. This is actually quite tricky to get right, 
>but a very good learning exercise! (But much harder than you probably 
>thought it would be!)
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>HTH
>
>
>
>________________________________
> From: Alan Gauld <alan.gauld at btinternet.com>
>To: tutor at python.org 
>Sent: Thursday, 22 March 2012 8:14 PM
>Subject: Re: [Tutor] python arthematics
> 
>On 22/03/12 22:14, Sukhpreet Sdhu wrote:
>> i want to write a program that reads simple arithematic expressions and
>> calculates the result.
>
>OK, there are many ways to do this but they pretty much
>fall into three categories:
>
>1) read the string and exec() it - easy but very
>   risky from a security point of view.
>
>2) Write a general text parser to break the input
>   string into operands and operators and combine
>   the results.
>
>3) build a state machine that looks for valid input
>  (this will be easier if you opt for reverse
 polish
>   notation BTW)
>
>You can do No 1 real quick just for fun, but don't try
>that in a real world program.
>
>So that leaves 2 and 3. If you understand the concepts
>(or research them on wikipedia) you can have at it,
>write some code and lets see what you get to.
>Hint: Start with a limited subset - only 2 operands and
>one operator allowed, say.
>
>HTH
>-- Alan G
>Author of the Learn to Program web site
>http://www.alan-g.me.uk/
>
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