Postfix evaluation in C

Let's suppose you input is in an array of characters.

char input[] = "4 9 * 0 - ="; 

you can access individual characters by accessing each individual array element

if (input[4] == '*') /* deal with star */; 

or you can use pointer arithmetic and parse from a different point in the input (remember to #include <stdio.h> for the prototype for `sscanf´)

if (sscanf(input + 2, "%d", &number) != 1) /* deal with error */; 

Or, as suggested by Chris Lutz in a comment, use strtol (after the proper #include <stdlib.h>)

number = strtol(input + 2, &next, 10); /* don't forget to check for errors! */ /* `next` now points to the character after the `long` at position 2 in the array */ 

C strings are arrays of chars: char[] or char*. you can use a for loop to iterate it and get each characher by it's index:

for (int i = 0; i < strlen(yourString); i++) { char ch = yourString[i]; // ... } 

Also there is a function, strtok() that might be helpful here for tokenizing the string:

#include <string.h> #define NULL (void*)0 char yourString[] = "4 9 * 0 - ="; char delimiters[] = " "; // could be " +*/-=" depending on your implementation char *token = NULL; token = strtok(yourString, delimiters); while(token != NULL) { printf("current token is: %s\n", token); // do what ever you want with the token token = strtok(NULL, delimiters); // next token } 

You can also know with sscanf how many items have been read (the counter of well read data items is the result of sscanf) and what is the relative position (using the %n format specifier).

so you could also code

int pos = 0; int endpos = 0; int val = 0; if (sscanf(input + pos, "%d %n", &val, &endpos) >= 1) { // val has been read as an integer, handle it stack[top++] = val; pos += endpos; // skip to next token in input } 

There are many more ways of doing that. You might want to read about lexers and parsers, e.g. with flex and bison, or antlr, etc.