strange question

[Jack]

Below is a simple code:

int main(){
char str[] = "How are you.";

int len;
len = strlen(str);
char *p1;

p1 = str + len-1;
printf("str:%s\n", str);
int i;
for( i = len-1; i >= 0; ){
printf("p1:%s\n", p1);
if(*(p1+i) != ' ') { //LINE1
p1--;
i--;
}
else{
i--;
printf("REACH HERE!\n");
}
}

}

The output is:

str:How are you.
p1:.
p1:u.
p1u.
p1:you.
p1: you.
p1:e you.
p1:re you.
p1:are you.
p1: are you.
p1:w are you.
p1w are you.
p1:How are you.

Why LINE1 does not work?

Thanks.

 

[manoj1978]

Below is a simple code:

int main(){
char str[] = "How are you.";

int len;
len = strlen(str);
char *p1;

p1 = str + len-1; Now p1 points to str[11]
printf("str:%s\n", str);
int i;
for( i = len-1; i >= 0; ){
printf("p1:%s\n", p1);
if(*(p1+i) != ' ') { //LINE1

First time i is 11 and you are trying to look at str[22] which is not
valid in this program.

 

[boa]

* Jack wrote, On 26.08.2006 07:02:

Below is a simple code:

int main(){
char str[] = "How are you.";

int len;
len = strlen(str);

len = 12

char *p1;

p1 = str + len-1;

Now p1 points to str+11

printf("str:%s\n", str);
int i;
for( i = len-1; i >= 0; ){

i starts out as 11

printf("p1:%s\n", p1);
if(*(p1+i) != ' ') { //LINE1

p1 + i == (str+11+11), way off.
Try: if(str == ' ') or just if(*p1 == ' ')

p1--;
i--;
}
else{
i--;
printf("REACH HERE!\n");
}
}

}


The output is:

str:How are you.
p1:.
p1:u.
p1u.
p1:you.
p1: you.
p1:e you.
p1:re you.
p1:are you.
p1: are you.
p1:w are you.
p1w are you.
p1:How are you.

Why LINE1 does not work?

See above.

Boa

 

[spibou]

Below is a simple code:

int main(){
char str[] = "How are you.";

int len;
len = strlen(str);
char *p1;

p1 = str + len-1;
printf("str:%s\n", str);
int i;
for( i = len-1; i >= 0; ){
printf("p1:%s\n", p1);
if(*(p1+i) != ' ') { //LINE1
p1--;
i--;
}
else{
i--;
printf("REACH HERE!\n");
}
}

}

The output is:

str:How are you.
p1:.
p1:u.
p1u.
p1:you.
p1: you.
p1:e you.
p1:re you.
p1:are you.
p1: are you.
p1:w are you.
p1w are you.
p1:How are you.

Why LINE1 does not work?

I can only speculate what you intended for
LINE1 to accomplish but what it does do
is undefined behaviour. That's because in
most repetitions of the loop and certainly the
first repetition , p1+i points past the end of str[]
so you are not allowed to dereference it.

Just run in your head the programme or use
a piece of paper or add printf statements which
print the values of variables and it will become
obvious.

Spiros Bousbouras