question about the array type char

thungmail · 11-05-2009, 01:06 AM

There is code in C

Code:

int main(void)
{
    char *a = "Hello, World!";
    printf("%s\n",&a[1]);
    return 0;
}

I thought the output will be H but in fact it is not. Can someone explain it to me and how can modify my code to get letter H from that string.Thanks

RockyMarrone · 11-05-2009, 01:17 AM

Because you printging the string which will start from base address + 1
in your case a + 1

if you will change you code to

Code:

printf("%c\n",a[0]);

you will get your answer then

thungmail · 11-05-2009, 01:37 AM

thanks for your help but i still feel confused if i use

Code:

printf("%s\n",&a[1]);

it will give me

Code:

ello, World!

I dont get it and if we use & operator at here, why the result is like this.

**laserlight** · 11-05-2009, 01:40 AM

Quote:

Originally Posted by thungmail

Why we can not use & operator at here.

You want to print a character, so you should get the character at your desired index and print it as a character, not a string. The use of address-of in your example provides a pointer to the character at the index 1, which when you print as a string, prints the string starting from index 1.

thungmail · 11-05-2009, 02:31 AM

Thanks for all your answers.
For the problem above, according to laserlight

Quote:

Quote:
You want to print a character, so you should get the character at your desired index and print it as a character, not a string. The use of address-of in your example provides a pointer to the character at the index 1, which when you print as a string, prints the string starting from index 1.

I made a small change

Code:

printf("%s\n",*a[1]);

But still get the error.

This error still occurs when I try an example above.
If I have

Code:

char * str

and str is "ABCDEF"
if i have

Code:

1.for(;*(str+i) != '\0';i++)
2.{
3.    if(isupper(str[i]))              // Check if every char in str is capital
4.            do something
5.}

I dont understand at line 1 and 3. As i remember * operator will give you a content of an object which pointer is pointing to. So*(str+i) will give you content at ith position of str.I tried

Code:

isupper(*(str[i]))

but the compiler complains

Code:

 invalid type argument of `unary *'

I dont get it and also, why we have to implement such syntax in line 1

**laserlight** · 11-05-2009, 02:34 AM

Quote:

Originally Posted by thungmail

As i remember * operator will give you a content of an object which pointer is pointing to. So*(str+i) will give you content at ith position of str.

Correct, so the idea is to keep looping until you have found the first null character (since strings in C are null terminated). Of course, you could have also used str[i] != '\0' for the comparison.

Elysia · 11-05-2009, 09:09 AM

The index operator dereferences the pointer, so in effect a[1] is the same as *(a + 1), so no * is necessary.

thungmail · 11-05-2009, 10:13 AM

Thanks a lot it is clear now
There is another question I want to ask about the fgets() in C. Here is my code

Code:

#include <string.h>
#include <stdio.h>
1.int main (int argc, char *argv[]){
2.	char* input;
3.	char* arg;
4.	fgets ( input ,43, stdin ) ;
5.     arg = strtok(input," ");          
6.	printf("%s\n",arg);
7.}

After compiling and executing the code, I got the error

Code:

red 313 % input.out
input from the screen
Segmentation fault

I thought it should print

Code:

input
from
the
screen

but it did not. Can you explain it for me pls.Thanks

Elysia · 11-05-2009, 10:41 AM

SourceForge.net: Common mistakes and errors - cpwiki

11-05-2009, 01:06 AM	#1
thungmail Registered User Join Date: Mar 2008 Posts: 39	question about the array type char There is code in C Code: int main(void) { char *a = "Hello, World!"; printf("%s\n",&a[1]); return 0; } I thought the output will be H but in fact it is not. Can someone explain it to me and how can modify my code to get letter H from that string.Thanks
thungmail is offline

11-05-2009, 01:17 AM	#2
RockyMarrone Registered User Join Date: Oct 2009 Location: While(1) Posts: 316	Because you printging the string which will start from base address + 1 in your case a + 1 if you will change you code to Code: printf("%c\n",a[0]); you will get your answer then
RockyMarrone is offline

11-05-2009, 01:37 AM	#3
thungmail Registered User Join Date: Mar 2008 Posts: 39	thanks for your help but i still feel confused if i use Code: printf("%s\n",&a[1]); it will give me Code: ello, World! I dont get it and if we use & operator at here, why the result is like this.
thungmail is offline

11-05-2009, 10:13 AM	#8
thungmail Registered User Join Date: Mar 2008 Posts: 39	Thanks a lot it is clear now There is another question I want to ask about the fgets() in C. Here is my code Code: #include <string.h> #include <stdio.h> 1.int main (int argc, char argv[]){ 2. char input; 3. char* arg; 4. fgets ( input ,43, stdin ) ; 5. arg = strtok(input," "); 6. printf("%s\n",arg); 7.} After compiling and executing the code, I got the error Code: red 313 % input.out input from the screen Segmentation fault I thought it should print Code: input from the screen but it did not. Can you explain it for me pls.Thanks
thungmail is offline

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