Substitute characters in a string with their values? - c

Question

I have a string given (a+b)&(a+c) and I have created a truth table with values of a,b, and c. Now the problem is to evaluate the logic expression by substituting a,b, and c with corresponding values from the truth table. How it can be done in C?
Ex: a=0 b=0 c=0 r=(0+0)&(0+)=0
a=0 b=0 c=1 r=(0+0)&(0+1)=0
and so on
The code itself looks like this
#include <stdio.h>
#include <stdlib.h>
int main()
{
char c,* str, *vars, **result;
int i=0,count=0,j=0;
unsigned long long rows;
str = (char*) malloc(1*sizeof(char));
vars=(char*) malloc(1*sizeof(char));
result=(char**)malloc(1*sizeof(char));
char values[] = {'F', 'T'};
while ((c = getchar()) != EOF)
{
str[i++] = c;
str = (char*) realloc(str, (i+1) * sizeof(char));
if (c >= 'a' && c <= 'z')
{
vars[j++]=c;
vars=(char*) realloc(vars,(j+1)*sizeof(char));
count++;
}
}
rows=1ULL<<(count);
result=(char**)realloc(result,(rows+2)*sizeof(char));
for (i = 0; i < rows+1; i++)
{
result[i]=(char*)malloc(sizeof(char)*(count+1));
for (j = 0; j < count; j++)
{
if(i==0)
result[i][j]=vars[j];
else
result[i][j]=values[(i >> j) & 1];
}
}
result[0][count]='R';
for(i=0;i<rows+1;i++)
{
for(j=0;j<count+1;j++)
{
//do something
}
}

Answer 1

Now the problem is to evaluate the logic expression by substituting a,b, and c with corresponding values from the truth table.
Aside from the issues mentioned in the question's comments, substituting alone won't do the job to evaluate the logic expression. The following function for example substitutes the values while evaluating the expression. (You didn't specify the general syntax of your expressions, so I chose to support combinations of the used operators and lower case variables.)
#include <ctype.h>
#include <string.h>
int indx(char *s, char c) { return strchr(s, c)-s; }
char *gstr, *gvars, *vals; // expression string, variables, value combination
char eval()
{ // evaluate expression "gstr"
char or = 0; // neutral element of +
do
{
char and = 1; // neutral element of &
do
{
char c = *gstr++; // get next token
if (islower(c))
and &= indx("FT", vals[indx(gvars, c)]);
else
if (c == '(')
{ // evaluate subexpression
and &= eval();
c = *gstr++; // get next token
if (c != ')')
printf("error at '%c': expected ')'\n", c), exit(1);
}
else
printf("error at '%c'\n", c), exit(1);
} while (*gstr == '&' && ++gstr);
or |= and;
} while (*gstr == '+' && ++gstr);
return or;
}
It can be called from your main (inserted in your code, hence the inconsistent spacing)
result[0][count]='R';
gvars = vars; // make variable names globally accessible
for (i = 1; i <= rows; ++i)
{
gstr = str, vals = result[i], // globally accessible
result[i][count] = values[eval()];
while (isspace(*gstr)) ++gstr;
if (*gstr)
printf("error at '%c': expected end of input\n", *gstr), exit(1);
}
for(i=0;i<rows+1;i++)
{
for(j=0;j<count+1;j++)
{
putchar(result[i][j]);
}
putchar('\n');
}
(Don't forget to put str[i] = '\0'; after your getchar loop to make a null-terminated string.) Note that due to the given for loop counting, the order of the truth table entries is somewhat unusual in that the row with all variables F comes last.