### 1. How do you read an expression?

- Posted by Andrew Baron <abaron at CWAVE.COM> Dec 22, 1996
- 1118 views

Hi all, I am a little new at Euphoria and would like to know how I might convert a user input of say: x^2+3*4 to a expression that would get 'x' from a for loop . This is for a graphing program that I wrote in BASIC and want to convert. The expression could be just 'x' or even '2x^4+3x^2-9x+7' Andrew Baron E-Mail:abaron at cwave.com

### 2. How do you read an expression?

- Posted by Robert Craig <72614.1667 at COMPUSERVE.COM> Dec 24, 1996
- 1098 views

Andrew Baron asks: > Hi all, I am a little new at Euphoria and would like to know how I > might convert a user input of say: x^2+3*4 to a expression that > would get 'x' from a for loop. > This is for a graphing program that I wrote in BASIC and want to convert. > The expression could be just 'x' or even '2x^4+3x^2-9x+7' There's no simple way in Euphoria to read in a string containing an mathematical expression with variables, and evaluate it. There is however a tricky way to do this. You could have a program something like: 1. ask user for input 2. read in his input string 3. write out a file, expr.e, that you create with open(), puts() etc. as: global function user(atom x) return 2*power(x,4) + 3*power(x,2) - 9*x + 7 -- plug in user's strin g end function -- expr.e will have been created by the time Euphoria tries -- to include it: include expr.e procedure plot(atom x, atom y) -- your graphing routine end procedure for x=0.0 to 100.0 by 1.0 do -- call user() function in the expr.e file that you just created plot(x, user(x)) end for The program would probably just do one plot each time. It would be hard to loop around and get another expression from the user since an include statment can't be inside a loop. The general idea is that you can create a .e file just before Euphoria tries to include it. This trick might be used for other purposes as well, such as selecting from several possible versions of a .e file using a system("copy ...", 2) command just before the include statement. I hope this hasn't confused you too much! -- Rob Craig Rapid Deployment Software

### 3. How do you read an expression?

- Posted by David Cuny <HW1.DCUNY at HW1.CAHWNET.GOV> Dec 24, 1996
- 1090 views

*** Reply to note of 12/23/96 21:18 *** WARNING: a lot of code to follow *** this will read an expression. there are a number of things that it will *not* do, such as double unary signs (--3) and operator precedence. on the other hand, it *will* handle the expressions you've got, as long as you use parenthesis. this code is *not* extensively tested. it does a parse-on-the-fly, rather than a robust tokenize/parse/execute. on the other hand, it is fairly small, and almost readable. if this is too large of a post, let me know, and i will avoid such in the future. *please* no flames, etc. there are no guarantees to the fitness of the code; etc. etc. etc. i put it together fairly rapidly. --- CODE STARTS HERE --- global atom x -- your "x" variable global integer p -- current character being parsed in string function math( atom a, integer op, atom b ) -- perform operation OP with A and B if op = '+' then return a + b elsif op = '-' then return a - b elsif op = '*' then return a * b elsif op = '/' then if b = 0 then -- your error handler here. division by zero. return 0 else return a / b end if elsif op = '^' then return power( a, b ) elsif op = '=' then return b elsif op = 0 then return a else -- your error handler here. -- op was not recognized return 1 end if end function function isNumber( integer i ) -- return true if character is part of a number if i = '.' then return -1 -- decimal place else return i >= '0' and i <= '9' -- true if ranges from '0' to '9' end if end function function isOp( integer i ) -- return true if character is an operand return i = '+' or i = '-' or i = '*' or i = '/' or i = '^' end function function parse1( sequence s ) integer op, decimal, sign atom accum, num op = '=' -- first operation is assignment to accumulator accum = 0 -- initialize accumulator sign = 1 -- unary minus while p <= length(s) do -- scan if s[p] = '(' then -- left paren p = p + 1 -- move past the paren num = parse1( s ) -- recurse accum = math( accum, op, num ) -- accumulate elsif s[p] = ')' then return accum -- return parsed expression elsif s[p] = '-' then -- handle unary minus if p = 1 then -- first character in string sign = -1 -- unary minus elsif isOp( s[p-1] ) then -- prior character was op sign = -1 -- unary minus else op = s[p] -- operand end if elsif isOp( s[p] ) then op = s[p] -- assign operand to pending operation elsif s[p] = 'x' then accum = math( accum, op, x * sign ) -- variable sign = 1 -- clear the sign elsif isNumber( s[p] ) then -- number num = 0 -- clear accumulator decimal = 0 -- no decimal place while isNumber( s[p] ) do -- while it's a number if s[p] = '.' then decimal = 1 else num = ( num * 10 ) + ( s[p] - '0' ) -- accumulate decimal = decimal * 10 -- decimal place end if p = p + 1 -- increment pointer end while if decimal > 0 then -- was there a decimal place? num = num / decimal -- divide by the decimal amount end if p = p - 1 -- back up one character (we went too far) num = num * sign -- add sign sign = 1 -- clear sign if s[p+1] = 'x' then -- handle stuff like 3x p = p + 1 -- move past x accum = math( accum, op, num * x ) -- include in operation else accum = math( accum, op, num ) -- perform pending operation end if else -- your error handler here. -- there is an error in the expression end if p = p + 1 -- increment to the next character end while return accum end function function parse( sequence s ) -- main parsing function call p = 1 -- set pointer to first character return parse1( s ) -- parse expression end function -- test it sequence expression atom result x = 22 -- put value in x expression = "" -- initialize while compare( expression, "\n" ) do -- until a blank line is entered puts( 1, "\n Give me an expression >" ) -- prompt expression = gets(0) -- get expression result = parse( expression )-- parse it printf( 1, "\nThe result is: %g", result ) -- print results end while --- CODE ENDS HERE merry christmas, all. -- David Cuny

### 4. Re: How do you read an expression?

- Posted by Andrew Baron <abaron at CWAVE.COM> Dec 26, 1996
- 1087 views
- Last edited Dec 27, 1996

Like to thank David Cuny for his code. Works perfect. Anyone want a program that graphs like a graphing calculator? Cannot do X^X, SIN, COS, TAN, and gets a few divid by zeros, but those will be fixed. ----------------------------------------------------------------------------- Andrew Baron E-Mail:abaron at cwave.com -----------------------------------------------------------------------------

### 5. Re: How do you read an expression?

- Posted by Or Tamir <o_oded at NETVISION.NET.IL> Dec 27, 1996
- 1096 views

I'd like a copy too... I just downloaded the program yesterday and we'll be glad if anyone will tell me where its potential is hidden and give me and send me any programs they make.... Or