Transforming array variables

I want to use variable instead of numbers as infix. Say I have red*blue+green as infix, where these are red, blue, green are variables names representing each array columns for n*3 array. Get the corresponding numerical row values for each row and use it to substitute the variables in the input to get new infix. eg

Code:

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array[2][3]={34.5,13,21,11,16,9.55}

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where red,blue and green are the column names.

Infix input : red+blue*green
infix output:

Code:

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34.5+13*21

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I haven't got any clue on how to go about this. Can someone help me out?

First question is if these variable names are "constant", or if the variable names are "variable". That is, are all the variable names known to the application, or can variables be defined by the user? (Can the user say "blah=11" somewhere, and then say "blah + 4" expecting to get 15?).

I would also prefer if you use braces inside your array initializer - whilst what you've written is perfectly valid, it's making it hard to read what's what (particularly if you need MORE variables later on, and the list gets quite long, it's hard to keep track of which number goes where.

Code:

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array[2][3]={ {34.5,13,21}, {11,16,9.55} };

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If you have three variables, why is there two sets of them? What is the second set of values for?

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Mats

What have you tried?

I assume you might get different operators between red, blue and green as input? You'd probably parse the input for these operators and output as requested?

Quote:

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Originally Posted by matsp

First question is if these variable names are "constant", or if the variable names are "variable". That is, are all the variable names known to the application, or can variables be defined by the user? (Can the user say "blah=11" somewhere, and then say "blah + 4" expecting to get 15?).

I would also prefer if you use braces inside your array initializer - whilst what you've written is perfectly valid, it's making it hard to read what's what (particularly if you need MORE variables later on, and the list gets quite long, it's hard to keep track of which number goes where.

Code:

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array[2][3]={ {34.5,13,21}, {11,16,9.55} };

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If you have three variables, why is there two sets of them? What is the second set of values for?

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Mats

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the column names are constant the user doesn't need to be modified by the user. Assuming red is the name of column one in the array, red should always refer to the elements in column one. The column names red, blue, green are not part of the elements of the array, just names used to designate each field. So in the above array for example red should refer to 34.5 and 11 respectively for each row assuming red is the name given to column one in the array and so on. So basically, if the user inputs red+blue*green, I want the output string to be of the form
34.5+13*21 for row 1 and 11+16+9.55 for row 2.

Quote:

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Originally Posted by anon

What have you tried?

I assume you might get different operators between red, blue and green as input? You'd probably parse the input for these operators and output as requested?

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Sure different operators *,+,-, / can be between the names which I call psuedo operands.

So what have you tried?

This is a string parsing / concatenating assignment isn't it?

May the user also input: "green+red*blue"?

Code:

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While GetsnextSubstring
    GetColumnNumberFromSubstring //for example
    Concatenate
End While

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Do all expressions have just red, green and blue in them, or are there numbers as well? E.g. are you allowed to say "red *4 + blue *3 / green" (and is that supposed to evaluate in mathematical order, or just "as it's stated" (that is, are you calculating blue *3 / green and red * 4 before adding them)?

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Mats

This is what I've done but not what I wanted cos this is static I need to make it dynamic so users can input expression with any combination operators but this has to be done with infix to postfix that bit I've gotten right.

Code:

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#include<stdio.h>
#include <stdlib.h> 
#include<math.h>
#define red 0
#define blue 1
#define green 2
main(){   
int nrows=2;
int ncol=3;
  /*Calculating the output yi for y=red+blue*green*/
      float y[nrows];     
      b=0;
      do{
      for (a=0; a<nrows; ++a){
      y[a]=0;
      c=0;
      y[a]+= array[a][red] + array[a][blue]*array[a];
        ++b;}}while(b<nrows);
     
      for(a=0;a<nrows; ++a)
     
      printf("&#37;.2f\n", y[a]);
      getchar();
      printf("\n\n");
 }

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Does your current code actually output the correct result?
It looks to me as if you are adding it twice...

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Mats

Quote:

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Originally Posted by matsp

Does your current code actually output the correct result?
It looks to me as if you are adding it twice...

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Mats

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I missed out the ( ) around (array[a][red] + array[a][blue]*array[a])

But I don't know where to go from here to achieve what I want to do

Check the sum of your variables. Is it really the same value as you get when you calculate by hand or using a calculator?

Now, this isn't getting you towards a solution really - as your colours aren't really available as variables to a user entering an expression, as you pointed out earlier. So perhaps we should discuss possible solutions here...

What do you think you need to do in order to translate the string "red+green+blue" into the numerical equivalent using your array of values? What is the first thing you need to do with this string? And how do you do that?

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Mats

Quote:

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Originally Posted by matsp

Check the sum of your variables. Is it really the same value as you get when you calculate by hand or using a calculator?

Now, this isn't getting you towards a solution really - as your colours aren't really available as variables to a user entering an expression, as you pointed out earlier. So perhaps we should discuss possible solutions here...

What do you think you need to do in order to translate the string "red+green+blue" into the numerical equivalent using your array of values? What is the first thing you need to do with this string? And how do you do that?

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Mats

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to read in the string but I don't know what funtion to use in reading it in

What function do you use to read strings in when you need to do it in other applications you've written?

I was actually referring to the next step, once the string has been read in - so perhaps while you are digging round your old files, you could ponder that one...

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Mats

I can use fgets but that will read in the entire expression at once, provideing a pointer to the string array

And what's the problem with reading all the string at once? Yes, you can do it one char at a time too - do you prefer to do that? If so use getchar() or fgetc().

--
Mats

Quote:

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Originally Posted by matsp

And what's the problem with reading all the string at once? Yes, you can do it one char at a time too - do you prefer to do that? If so use getchar() or fgetc().

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Mats

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Mats this is where I've gotten to. What step should I take next?

Code:

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#include<stdio.h>
#include<stdlib.h>
#define MAXCOLS 80

int main()
{
 
    int i;
    char string[MAXCOLS];
  char expr[];
    double array[2][3]={{4.5,13,12},{32,12.6,2.5}};
               

    printf("Enter an expression:\n");
    while ((expr[i++]=getchar()) != '\n');
    expr[--i] ='\0';

    /* this will come in later
    printf("Expression\n");
    printf("&#37;s", string);*/
   
    getchar();
    return 0;
 }

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Quote:

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Originally Posted by ben2000

Mats this is where I've gotten to. What step should I take next?

Code:

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#include<stdio.h>
#include<stdlib.h>
#define MAXCOLS 80

int main()
{
 
    int i;
    char string[MAXCOLS];
  char expr[];
    double array[2][3]={{4.5,13,12},{32,12.6,2.5}};
               

    printf("Enter an expression:\n");
    while ((expr[i++]=getchar()) != '\n');
    expr[--i] ='\0';

    /* this will come in later
    printf("Expression\n");
    printf("%s", string);*/
   
    getchar();
    return 0;
 }

---

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And why would you want to use getchar() instead of "fgets()"? In the above code, it achieves almost exactly the same thing - the only main difference is the lack of checking for expr overflowinng.

I do believe you should use "string" to input things into, rather than "expr" - as the latter is not defined to a length, and you are printing "string", rather than "expr".

--
Mats

Quote:

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Originally Posted by matsp

And why would you want to use getchar() instead of "fgets()"? In the above code, it achieves almost exactly the same thing - the only main difference is the lack of checking for expr overflowinng.

I do believe you should use "string" to input things into, rather than "expr" - as the latter is not defined to a length, and you are printing "string", rather than "expr".

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Mats

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Well I used string there cos when and after extracting the values and concatenating them I'll probably need to store it in the string array.

So, I've completed my variant:

Code:

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Enter expression using red, green and blue as variables:green+red+blue
result[0]: 68.500000
result[1]: 36.549999

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Slight rounding error on the second case, but that's normal for "float" - need "double" to make it more precise (or print with fewer digits).

--
Mats

Quote:

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Originally Posted by ben2000

Well I used string there cos when and after extracting the values and concatenating them I'll probably need to store it in the string array.

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Yes, probably a good idea to use two strings, one for the original and one for the "modifed" - if nothing else, it makes it easier to cope with the resulting variable being longer or shorter than the variable without having to do a bunch of complex string-shuffling.

--
Mats

Quote:

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Originally Posted by matsp

So, I've completed my variant:

Code:

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Enter expression using red, green and blue as variables:green+red+blue
result[0]: 68.500000
result[1]: 36.549999

---

Slight rounding error on the second case, but that's normal for "float" - need "double" to make it more precise (or print with fewer digits).

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Mats

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You mean you've written a code that did what I'm trying to do just now, then you must be too good. No doubt. I wish I'm in your position you're vast in this.

Mats please work me through.

Well, I've been programming for about 24 years now, and the last 20 or so I've been paid to do so by some small and large companies.

Writing little bits of code to scan strings and do math expressions is quite easy compared to working on a Linux kernel, Xen Hypervisor or a Windows graphics driver.

Of course SOME OF the code in these projects aren't particularly challenging, but other pieces certainly are - and all of the code I wrote today is MUCH smaller than almost any of the many dozens of files that are part of those big projects - the source code for all of the above counts in megabytes. The file I just created for this project is about 3.5KB, and less than 200 lines of code.

Oh, and I cheated - I don't transform the expression to postfix first, I just parse the string and calculate the infix operations, using recursion and push/pop functions to evaluate in correct order:

Code:

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Enter expression using red, green and blue as variables:5+5*5
result[0]: 30.000000

Enter expression using red, green and blue as variables:5*5+5
result[0]: 30.000000

Enter expression using red, green and blue as variables:5+gray
Error - invalid variable gray

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The last one shows that it also supports error handling. I haven't implemented parenthesis, so you can't do things like "5*(3+4)". I'm sure that could be done quite easily, but it didn't seem like it was necessary for what you are trying to do.

--
Mats

Quote:

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Originally Posted by ben2000

Mats please work me through.

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Yes, sure - but I'm not giving you my code - because that's not the point here, is it?

So, what have you done so far?

You got the string read in, yes? [And I don't really care if you use fgets() or a sequence of getchar() - it is REALLY not important]

So, next step is to replace the "red", "green" and "blue" with the respective values from the "array", right? How do you think you should go about that?

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Mats

That's shocking, can you imagine 24yrs and I'm just one month into it. Well I hope to be like you someday, I envy you a lot. Like you know I've gotten into my infix-to-postfix-evaluate working, just trying to get this bit, so I can parse the string of values to it. If you can help me out here, I'll crown you.

Well, I'm trying to get you to think about ONE part of the problem at a time...

Say your string is "red+green+blue", what do you need to do to make that into the numerical form (sorry, can't remember the numbers).

Work on that - in fact, you could even ignore the input, just do

Code:

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strcpy(string, "red+green+blue");

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Obviously, try to write it so that it supports all math operations and perhaps also numbers in the expression - but start with the above bit. [using strcpy means that you don't have to type the expression every time you "try out your current code" - which is good until you have it working to the level where you need to try "different variations"].

I must say, I probably couldn't write an "infix to postfix" interpreter when I'd been programming a month - but then I never tried that - I was programming basic and assembler with no teacher available...

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Mats

well I'm thinking of how to go about it but have not come up with something yet.

Mats please can you give your code even at a price, I'm working on a project and I'm really short of time. Please, I've spent so much time on this whole code. I'm statistically inclined, due to the fact that this whole thing came impromptu, I was reading the language up and at the same time trying to put it together. The major code, code I needed to write, I've managed to put it together, but this bit I thought would have been easy but it appears quite difficult. I can't quite think clearly, probably because I'm racing against time. Please if you can assist me I'll so much appreciate it, this is the only bit holding me down. Even at a price I'm ready to pay. Please, and please, just assist me.

That's not really my style.

Is this a "work project" or a "school project"?

Really, the part of parsing a variable so that it becomes the value of the string is quite easy.

Look at the hint I gave earlier, then think about what you need to do. It's not very different from parsing a string for digits - the operators are of course similar in either case.

Sorry I didn't reply to your post earlier, but it didn't appear in my "new posts" - I only just went back to see if you posted anything last night after I gone to bed.

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Mats