How can I evaluate an expression with the operator only known as a characte

I am writing a program that implements an rpn type calculator while getting the values for it from a file. I am trying to figure out how I can Evaluate an expression when the operator is retrieved from the file. Right now I have this..

Code:

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double rpn_eval(char * fileName, double x){
double a,b,c;
File *fd;
fd = fopen(fileName);

char * word;
while (word = nextword(fd)){
if (word >= '0' && word <= '9'){
 stack_push(word);
}else{
 a = stack_pop();
 b = stack_pop();
 //I want to evalute b (word) a
 }
}

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If you could also redirect me to some source material so I could read about it that would be greatly appreciated, Thanks!

Perhaps (ignoring all the syntax errors in your pseudo-code)

Code:

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if ( word == '+' ) {
  stack_push(a+b);
}

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