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expr.cpp
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expr.cpp
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/*
* expr.cpp --
*
* stack based expr parse/calculation
*
* 2010/01/10: initial release by Todd Xue
*/
#include <limits.h>
#include <stdarg.h>
#include <string.h>
#include <stdio.h>
#include <ctype.h>
#include "expr.h"
/*
*-------------------------------------------------------------------------
*
* expr --
*
* RETURN:
* the expr value
*
*-------------------------------------------------------------------------
*/
static int left_prec[256];
static int right_prec[256];
struct PrecInit {
PrecInit() {
struct PREC {
char ch;
int prec;
};
PREC l[] = {
{ '(', 10 },
{ '+', 20 },
{ '-', 20 },
{ '*', 30 },
{ '/', 30 },
{ '^', 40 },
{ ')', 1000 },
};
for (int i = 0; i < sizeof(l)/sizeof(PREC); ++i)
left_prec[l[i].ch] = l[i].prec;
PREC r[] = {
{ ')', 10 },
{ '+', 20 },
{ '-', 20 },
{ '*', 30 },
{ '/', 30 },
{ '^', 40 },
{ '(', 1000 },
{ '\0', 1 },
};
for (int i = 0; i < sizeof(r)/sizeof(PREC); ++i)
right_prec[r[i].ch] = r[i].prec;
}
} gPrecInit;
void infix_to_postfix(char* str, char* dest) {
char ops[256];
int ops_c = 0;
do {
int c = (unsigned char)*str;
if (isdigit(c)) {
*dest++=c;
}
else {
int rprec = right_prec[c];
if (rprec == 0)
break;
int op;
while (ops_c > 0 && left_prec[(op = ops[ops_c-1])] >= rprec) {
if (op != ')' && op != '(')
*dest++=op;
--ops_c;
}
ops[ops_c++] = c;
}
} while (*str++);
*dest = 0;
}