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listing_5_10.c
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listing_5_10.c
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#include <stdio.h>
#include <math.h>
#include <stdlib.h>
#define MAXOP 100
#define NUMBER '0'
int getop(char []);
void ungets(char []);
void push(double);
double pop(void);
int main(int argc, char *argv[])
{
char s[MAXOP];
double op2;
while (--argc > 0) {
ungets(" ");
ungets(*++argv);
switch (getop(s)) {
case NUMBER:
//why atoi is right but atof is zero?
//because i lose the stdlib.h
//but how can it run without the .h?
/**
* http://stackoverflow.com/questions/4800102/not-including-stdlib-h-does-not-produce-any-compiler-error
* For historical reasons -- specifically, compatibility with very old C programs (pre-C89) -- using a function without having declared it first only provokes a warning from GCC, not an error. But the return type of such a function is assumed to be int, not double, which is why the program executes incorrectly.
* If you use -Wall on the command line, you get a diagnostic:
* $ gcc -Wall test.c
* test.c: In function ‘main’:
* test.c:5: warning: implicit declaration of function ‘atoi’
* test.c:6: warning: implicit declaration of function ‘atof’
* You should use -Wall basically always. Other very useful warning options for new code are -Wextra, -Wstrict-prototypes, -Wmissing-prototypes, -pedantic, and -Wwrite-strings, but compared to -Wall they have much higher false positive rates.
* Tangentially: never use atoi nor atof, they hide input errors. Use strtol and strtod instead.
*/
push(atof(s));
break;
case '+':
push(pop() + pop());
break;
case '-':
op2 = pop();
push(pop() - op2);
break;
case '*':
push(pop() * pop());
break;
case '/':
op2 = pop();
if (op2 == 0.0)
printf("error: zero divisor\n");
else
push(pop() / op2);
break;
default:
printf("error: unknow command %s\n", s);
argc = 1;
break;
}
}
printf("\t%.8g\n", pop());
return 0;
}