tokenize_expr.c

/**
 * A simple arithmetic expression tokenizer. Use this example to get you 
 * started on the first task.
 */
#include <stdio.h>

/**
 * Is the given character a digit?
 */
int is_digit(char ch) {
  // this relies on the fact that digits are ordered in the ASCII table
  return ch >= '0' && ch <= '9'; 
}

/**
 * Read the next integer as a string from the input into the output.
 */
int read_integer_string(const char *input, char *output) {
  int i = 0;
  // while we have input and the character is a digit,
  while (input[i] != '\0' && is_digit(input[i])) {
    output[i] = input[i]; // copy character to output buffer
    ++i;
  }
  output[i] = '\0'; // add the terminating byte

  return i; // return the length of the string
}

int main(int argc, char **argv) {
  char expr[] = "12+4 - 20543 /         12"; // example expression string
  char buf[256]; // temp buffer

  int i = 0; // current position in string
  while (expr[i] != '\0') { // while the end of string is not reached
    // first check if the current char is a digit
    if (is_digit(expr[i])) {
      // read the integer from the output AND
      // advance the current position by the length of the string
      i += read_integer_string(&expr[i], buf); 
      // could convert it here...
      printf("Integer token: %s\n", buf);
      continue; // skip the rest of this iteration
    }

    // if not number, consider the current character and print its type    
    switch (expr[i]) { 
      case '+':
        printf("PLUS token\n");
        break;
      case '-':
        printf("MINUS token\n");
        break;
      case '*':
        printf("TIMES token\n");
        break;
      case '/':
        printf("DIVIDE token\n");
        break;
      case ' ':
        // skip spaces
        break;
      default: 
        printf("ERROR: Unknown token '%c'.\n", expr[i]);
    }

    ++i; // advance to the next character
  }

  return 0;
}