Please Help

Mike Jones · 03-26-2002

Does anyone know how to do this?

You are to write a program that evaluates simple arithmetic expressions which are composed by positive numbers (real or integers) and the four basic arithmetic operators: +, -, * and /. The program will have to adhere to the following rules:

The expressions will be evaluated left to right (all operators will have the same precedence).
Blank spaces are ignored.
The expression is terminated by the end of the line, at which point you will display the answer.
The prompt is indicated with >.
You have to guard against illegal characters entered.
The program can be terminated only by pressing <CTRL-C>.
Ex:

> 2 + 3 * 4/6
3.333333
> 2/4+3
3.500000
> 2!/4+3
*** Illegal character !
>

**Prelude** · 03-26-2002

>Does anyone know how to do this?
Yes, and please read the homework thread at the very top of the list.

-Prelude

Mike Jones · 03-26-2002

I understand the homework rules, and I'm not asking for an answer, but rather just some hints or some help. I've been trying to do this problem for the past two days and I haev no idea how to even begin it. I understand that I'm going to have to use getchar, but I dont exactly understand why I'm going to need it. Also I know I'm probably going to have to use a switch to go from + to - to * to \ , and that I'm going to have to use a while loop. But geting rid of the white space problem, is also bugging me. Basically I dont need an answer, but rather sometype of algorithm for how i should go about soving this problem....thanx

**Dave Jay** · 03-26-2002

How would you want the program to handle the following:

> 3456348908687874545.76456489056456 + 23.444

> / 17

> 8 ++ 3

is there a limit to how long an expression can be?

**Prelude** · 03-26-2002

Since this is homework, you don't need to error proof it, that's a future exercise. All you have to do is maintain some semblance of order. If the expression is

3 + 7 / 2 * 4

you can make some assumptions about the order of the expression. A number will always precede an operator, there is always an operator after a number, there is always another number after an operator. The first and last tokens in the input will always be numbers. With these assumptions in mind, you develop an algorithm:

Code:

Read a number and save it
Read an operator and save it
Read a number and save it
Determine what the operator is
Perform the appropriate operation on the two numbers
Save the result
Loop while not EOL
  Read an operator and save it
  Read a number and save it
  Determine what the operator is
  Perform the appropriate operation on the result and the number
  Save the result
  Rinse
Repeat
Print the result

That's all there is to it.

-Prelude

Mike Jones · 03-26-2002

I'm assuming there is no limit to the expression, I know I'll be using a float.

Mike Jones · 03-26-2002

Prelude, Thanx a lot, I'm going to try it out and see if it works, I really appreciate the algorithm but I have just one question, how does this get rid of it not reading blank space as a character? What i mean is the program is supposed to see "3 + 5" and "3+5" as the same thing, and just pass over the " ", how is this accomplished?

Thanx again.

**Prelude** · 03-26-2002

>how is this accomplished?
Read a character, if that character is whitespace then read another one until it isn't whitespace.

-Prelude

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