K&R 5-10

[mdh]

I wonder if I could ask for help. I have been struggling with this
most of the day, and cannot understand what is going on. The problems
are (1) it does not work!! (but exits normally on compilation...and
(2) when stepped through ( I use Xcode) it crashes without any error
message, at the indicated function. I have dissected it, run
individual parts on their own...which seem to work fine. Thanks in
advance.
I have supplied 3 command line arguments ( 22.3, 11.9, and '+' so the
argc is 4).
I am including the full code, as there are probably a lot I am missing
and messing up.
#include <stdio.h>
#include <math.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:
push(atof(s));
break;
case '+':
push(pop() + pop());
break;
default:
printf("Error: Unknown input");
break;

}

}
printf("\t %.8g", pop());
return 0;
}

/*********/
#define MAXSTACK 50
double static stack[MAXSTACK];
int static stkp=0;

double pop(void){

if ( stkp > 0)

return stack[--stkp];

else{
printf("Error: Stack empty");
return 0.00;
}

}

void push(double i){

if (stkp < MAXSTACK)
stack[stkp++]=i;
else
printf("Error: Stack overflow");
}

/**********/
#include <string.h>
void ungetch( int );


void ungets( char *s){ /*** I think the problem is here ***/
int i;
i=strlen(s);
while ( i > 0)
ungetch(s[--i]);
}


/**********/

# define MAXBUF 25
char buf[MAXBUF];
static int bufp=0;

void ungetch( int c ){
if (bufp < MAXBUF)
buf[bufp++]=c;
else
printf("Error: Ungetch buffer overflow");
}

int getch(void){

return ( (bufp==0) ? getch(): buf[--bufp]);

}

#include <ctype.h>
int getch(void);

int getop(char *s){
int c;

while( (c=s[0]=getch()) == " " || c == '\t');
s[1] = '\0';

if ( !isdigit(c) && c != '.'){

return c; /** c is not a number **/

if ( isdigit(c))
while ( isdigit(*s++=c=getch()));

if ( c == '.')
while ( isdigit(*s++=c=getch()));
*s='\0';

if ( c != EOF)
ungetch(c);

return NUMBER;
}




return 0;

}

 

[=?ISO-8859-1?Q?Bj=F8rn_Augestad?=]

I wonder if I could ask for help. I have been struggling with this
most of the day, and cannot understand what is going on. The problems
are (1) it does not work!! (but exits normally on compilation...and
(2) when stepped through ( I use Xcode) it crashes without any error
message, at the indicated function. I have dissected it, run
individual parts on their own...which seem to work fine. Thanks in
advance.
I have supplied 3 command line arguments ( 22.3, 11.9, and '+' so the
argc is 4).
I am including the full code, as there are probably a lot I am missing
and messing up. [snip]

int getop(char *s){
int c;

while( (c=s[0]=getch()) == " " || c == '\t');
s[1] = '\0';

if ( !isdigit(c) && c != '.'){

-------------------------------^
Are you sure this is correct? Doesn't look right to me.

return c; /** c is not a number **/

if ( isdigit(c))
while ( isdigit(*s++=c=getch()));


if ( c == '.')
while ( isdigit(*s++=c=getch()));
*s='\0';

if ( c != EOF)
ungetch(c);

return NUMBER;
}




return 0;

}

HTH
Bjørn

 

[mdh]

int getop(char *s){
int c;

while( (c=s[0]=getch()) == " " || c == '\t');
s[1] = '\0';

if ( !isdigit(c) && c != '.'){

-------------------------------^
Are you sure this is correct? Doesn't look right to me.

I have taken most of it from K&$R and from Tondo and Gimpel. I am not
sure at this stage what is going on, so you may well be correct, but I
think it is correct.

 

[=?ISO-8859-1?Q?Bj=F8rn_Augestad?=]

int getop(char *s){
int c;
while( (c=s[0]=getch()) == " " || c == '\t');
s[1] = '\0';
if ( !isdigit(c) && c != '.'){

-------------------------------^
Are you sure this is correct? Doesn't look right to me.

I have taken most of it from K&$R and from Tondo and Gimpel. I am not
sure at this stage what is going on, so you may well be correct, but I
think it is correct.

My point(maybe lost due to font issues) was that you probably have a
block where you just wanted a single statement.

Here's the code for getop(), reformatted by me.

int getop(char *s)
{
int c;

while( (c=s[0]=getch()) == " " || c == '\t');
s[1] = '\0';

if ( !isdigit(c) && c != '.') {
return c; /** c is not a number **/

/* THIS CODE IS NEVER EXECUTED */
if ( isdigit(c))
while ( isdigit(*s++=c=getch()));

if ( c == '.')
while ( isdigit(*s++=c=getch()));

*s='\0';

if ( c != EOF)
ungetch(c);

return NUMBER;
}

return 0;
}

Do you see how getop() never returns NUMBER?

BTW, if you think that your problem is related to ungetch(), why not
check the return value from ungetch()?

 

[pete]

Bjørn Augestad wrote:

Here's the code for getop(), reformatted by me.

int getop(char *s)
{
int c;

while( (c=s[0]=getch()) == " " || c == '\t');
s[1] = '\0';

That's a bad format.
It almost looks like the assignment to s[1] is in the loop.

 

[=?ISO-8859-1?Q?Bj=F8rn_Augestad?=]

Here's the code for getop(), reformatted by me.

int getop(char *s)
{
int c;

while( (c=s[0]=getch()) == " " || c == '\t');
s[1] = '\0';

That's a bad format.
It almost looks like the assignment to s[1] is in the loop.

Well spotted, Pete. I missed that ugliness in the original code.
Now that I had another look at that line, I spotted yet another bug.
Note how the OP compares c with " " instead of ' '. I know nothing about
Xcode, but someone should adjust the warning level of their compiler.

Bjørn

 

[pete]

Here's the code for getop(), reformatted by me.

int getop(char *s)
{
int c;

while( (c=s[0]=getch()) == " " || c == '\t');
s[1] = '\0';

That's a bad format.
It almost looks like the assignment to s[1] is in the loop.

Well spotted, Pete. I missed that ugliness in the original code.
Now that I had another look at that line, I spotted yet another bug.
Note how the OP compares c with " " instead of ' '.

Oh my!

I know nothing about Xcode,
but someone should adjust the warning level of their compiler.

Yes.

 

[pete]

static int bufp=0;

int getch(void){

return ( (bufp==0) ? getch(): buf[--bufp]);

}

If bufp equals zero when getch is called,
then getch will never return.

 

[mdh]

Thanks...I missed that. The problem was that I was not able to get
there as the debugger kept on crashing. It turns out that the project
had become corrupted. Taking the program, and inserting it into a new
project solved the issue. I have to say I have not experienced this
before, but I guess this is old hat to the more experienced
programmers.


One other issue that came up was this. Tondo and Gimpel had used
<math.h> for the lib function "atof". Atof did not work. When I
changed the include to <stdlib.h> ( as done in K&R) atof performed as
planned. My question is this. What did the compiler then do with the
atof function when the includes was incorrect? The conversion did not
make any sense at all, but it still gave some figure.

Thank you for your help, as usual.

 

[pete]

Thanks...I missed that. The problem was that I was not able to get
there as the debugger kept on crashing. It turns out that the project
had become corrupted. Taking the program, and inserting it into a new
project solved the issue. I have to say I have not experienced this
before, but I guess this is old hat to the more experienced
programmers.

One other issue that came up was this. Tondo and Gimpel had used
<math.h> for the lib function "atof".

That's pathetic.

Atof did not work. When I
changed the include to <stdlib.h> ( as done in K&R) atof performed as
planned. My question is this. What did the compiler then do with the
atof function when the includes was incorrect?

For cases of undefined behavior, like that,
the compiler is allowed to do whatever it wants to do.

 

[mdh]

mdh wrote:



For cases of undefined behavior, like that,
the compiler is allowed to do whatever it wants to do.

Apparently so. Thanks for your input. T&G have been pretty good up to
now...perhaps atof was moved? This is the first mistake I have seen
them make.

 

[CBFalconer]

Here's the code for getop(), reformatted by me.

int getop(char *s)
{
int c;

while( (c=s[0]=getch()) == " " || c == '\t');
s[1] = '\0';

That's a bad format.
It almost looks like the assignment to s[1] is in the loop.

It has insufficient blanks and parens and 's, excessive "s, for
readability. Try:

while (((c = s[0] = getch()) == ' ') || (c == '\t'))

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"A man who is right every time is not likely to do very much."
-- Francis Crick, co-discover of DNA
"There is nothing more amazing than stupidity in action."
-- Thomas Matthews