Expression_Eval

A lightwight easy to use expression evaluator library written in vanilla c.

Installation

download the repo & make user the files are in same folder and import eqn.h into your project

#include "eqn.h"

Quick Example

#include <stdio.h>
#include "eqn.h"
int main(){
    Error error;
    Expression exp;
    init_expression(&exp);
    float result=expression_eval(&exp,"2+2",NULL,&error);
    printf("2+2=%f\n",result);
    /* output:
        2+2=4.000000
    */
    return 0;
}

Struct

Expression:

An Expression instance holds all the information about the expression being execuated. It consis of:

• opcode_stack: stack used fo storing opcode_stac
• data_stack: satck used for storing data during evlation
• funciton_dict: dictionary used for storing predefined function
• constant_dict: dictionary used for storing constants.
• variable_dict: dictionary used for storing variable.

Error:

typedef struct{
	Status status; // __sucess__,__failed__,__pending__
	char msg[100];
} Error;

Error is used to hold the state fo evaluation process.

• status: holds either of 3 enum value ( _sucess_,_failed_,_pending_) represeting the state of eval process.
• msg: msg is set usually when status in _failed_. Stores the description on why the eval process may have failed.

Function_struct:

typedef struct{
	char *name;
	float (*func)(float*,int);
	int parms;
} Function_struct;

Used for defining funciton.

• name: the function name (used in evaluation strnig)
• func: pointer to the actual function in c.
• parms: the number of paramers that funciton can accept.

!!! It is important that u define function in c with return type float and parameters (flot*,int) !!!

flaot* -> contains list of argemets that is passed

int-> length of parameters u have defined in funciton struct (passed to help avoid segfault)

Example

#include <stdio.h>
#include "eqn.h"
float power(float *f,int len){
    float p=1;
    for(int i=0;i<f[1];i++)p*=f[0];
    return p;
}
int main(){
    Error error; Expression exp;

    init_expression(&exp);
    Function_struct func[]={{"pow",&power,2},{NULL,NULL,0}}; /*pow(x,y) so parms=2*/
    register_functions(&exp,func);/* registering fncton in exp*/

    float result=expression_eval(&exp,"pow(2)",NULL,&error);
    if(error.status==__failed__)printf("1st exp  error: %s\n",error.msg);
    else printf("1st exp resut:%f\n",result);
    
    result=expression_eval(&exp,"pow(2,2)",NULL,&error);
    if(error.status==__failed__)printf("2nd exp  error: %s\n",error.msg);
    else printf("2nd exp resut:%f\n",result);

    /* output:
        1st exp  error: Error in function call  "pow":arguments required->2, provide->1
        2nd exp resut:4.000000

    */
    return 0;
}

Variable_struct:

A key value pair used to represent variable in process.

typedef struct{
	char *name;
	float value;
} Variable_struct;

API

void init_expression(Expression *exp):

initilalize the expression to a new/ready state before used. If u calling init_expression() after certain used make sure u call flush_expression() before.

void register_functions(Expression *e,Function_struct *function):

stores function into given expression e.

void register_constants(Expression *e,Variable_struct *c):

store constats into given expression e.

float expression_eval(Expression *exp,char *string,Variable_struct *variable,Error *error):

Evaluate the string under the environment of Expression. Variable_struct *variable must contain all the variable thats are subjected in the char *string, failing to do so would result in Variable_error.

Example:

#include <stdio.h>
#include "eqn.h"

int main(){
    Error error;
    Expression exp;
    init_expression(&exp);

    Variable_struct var1[]={{"x",2},{NULL,0}};
    float result=expression_eval(&exp,"x+y",var1,&error);
    if(error.status==__failed__)printf("1st exp  error: %s\n",error.msg);
    else printf("1st exp resut:%f\n",result);
    
    Variable_struct var2[]={{"x",2},{"y",5},{NULL,0}};
    result=expression_eval(&exp,"x+y",var2,&error);
    if(error.status==__failed__)printf("2nd exp  error: %s\n",error.msg);
    else printf("2nd exp resut:%f\n",result);



    /* output:
        1st exp  error: Variable Error: variable->"y" is not provided
        2nd exp resut:7.000000

    */
    return 0;
}

void register_expression(Expression *exp,char *string) :

registers the given string in Expression exp. Registering expression just setups and envorienment for calculation to take place. It also doesnot make new copy of the char *string so its important to not subjct char *string to any change when untill future function call returs errro->status to be _sucess_ or _failed_

float proceed_next_calculation(Expression *e,Variable_struct *variable,Error *error):

Unlike expression_eval(), this funciton call can be used to without before hand knowledge of varaibale involved in the process. executing proceed_next_calculation() requires to register expression first. If proceed_next_calculation() stumbles upon a unknow variabele it takes an snapshot of the state of calculation , sets error->status to _pending_ and updates Variable_struct *variable with name of that unkown variable which a user must update variabele->value and feed it back to proceed_next_calculation() for the process to continue.

Example:

A simple program to ask user for variable value subjected in the expression.

#include <stdio.h>
#include "eqn.h"

int main(){
    Error error;
    Expression exp;
    init_expression(&exp);

    Variable_struct var={NULL,0};
    char *eqn="2*x-y+1";
    register_expression(&exp,eqn);
    float result;
    while(true){
        result=proceed_next_calculation(&exp,&var,&error);
        if(error.status==__pending__){
            printf("Enter the  value for %s : ",var.name);
            scanf("%f",&var.value);
        }else break;
    }
    if(error.status==__failed__)printf("%s\n",error.msg);
    else printf("%s=%f\n",eqn,result);

    /* output:
        Enter the  value for x : 3
        Enter the  value for y : 2
        2*x-y+1=5.000000
    */
    return 0;
}

!!!Note:The varaibale values are not deleated by default, to subject new values for variable make sure to call flush_variable() !!!

float flush_variables(Expression *exp) :

Delete all the variables stored in the Expression exp and init it to new state.