Programming Forums > Application Development > C++

rahulsr123 · Feb 13th, 2006, 6:42 AM

hi guys,
thanks for posting on my last thread. i was wondering if pointers could be used to display part of a string instead of arrays. so i tried the following program. but it is not working.

(Toggle Plain Text)

#include<iostream.h>
int main()
{
char tu[]={"hello world"};
char *p;
char se[20];
int i;
for(i=0;i=5;i++)
se[i]=*(p+i)
cout<<se;
return 0;
}

Mjordan2nd · Feb 13th, 2006, 6:50 AM

*Edited to correct code tags*

DaWei · Feb 13th, 2006, 6:51 AM

(The trailing code tag requires a slash, [/code]. Thanks for using them, though.) A character string (ala C) IS an array. Exactly and precisely. It has one additional requirement -- a 'null' character to mark the end of useful characters.

I think maybe you'd like to examine what this means:

(Toggle Plain Text)

for (i = 0; i = 5; i++);

You are setting i to zero. You are specifying that the loop will execute only when i is 5. Hard to see that working. I would suggest you put braces around the body of your loops and such, even though, as a single line, they're not required. A novice will get into the bad habit of leaving them out where they're needed.

nnxion · Feb 13th, 2006, 9:08 AM

Additionally, you should include <iostream>, <iostream.h> is deprecated.
@David: Does the compiler set i to 5 in the for loop, or does it only compare even though one types the = operator instead of == operator?

DaWei · Feb 13th, 2006, 9:27 AM

<me>Dang me, take a new rope and hang me, didn't even pick up on the single '='</me> In that case, the loop will run (i = 5 is true), i will be incremented, set back to 5, ad nauseum. se [i] will get a workout and p+i will definitely have the jitters like a wino on joy juice.

crazykid48x · Feb 13th, 2006, 9:30 AM

I dont use pointers much but it seems to me it wouldnt work because he didnt set p to point to anything

DaWei · Feb 13th, 2006, 9:33 AM

That's true, too. It's hell to be old and blind. I can't even read my own tutorial without a magnifying glass.

aznluvsmc · Feb 14th, 2006, 9:34 PM

Another method that will work is to use pointer notation to pass the starting address of the string to cout. For example, tu itself stores the address of the beginning of the string so cout will start printing from that memory address until it reaches the NULL byte. However, you can do pointer arithmetic by adding a specific number to tu and then cout should start printing from that memory address.

For example to print only the word "world" you can use

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cout << tu + 6 << endl;

Here we start printing 6 characters inward starting from the beginning of the string.

Dameon · Feb 14th, 2006, 9:54 PM

@aznluvsmc: Don't forget a little bounds checking when arbitrarily advancing a pointer. In this case, if the string you are working with isn't long enough, you could end up with a pointer past the end of your useful data. Any string manipulation/display function would keep processing garbage until, by chance, it hits a zero.

DaWei · Feb 15th, 2006, 6:39 AM

...or references protected memory...

. We need a smiley that shows the system running off into the woods and getting eaten by a b'ar.

Feb 13th, 2006, 6:42 AM	#1
rahulsr123 Newbie Join Date: Feb 2006 Posts: 5 Rep Power: 0	how to use pointers to display a part of a string hi guys, thanks for posting on my last thread. i was wondering if pointers could be used to display part of a string instead of arrays. so i tried the following program. but it is not working. (Toggle Plain Text) #include<iostream.h> int main() { char tu[]={"hello world"}; char p; char se[20]; int i; for(i=0;i=5;i++) se[i]=(p+i) cout<<se; return 0; } #include<iostream.h> int main() { char tu[]={"hello world"}; char p; char se[20]; int i; for(i=0;i=5;i++) se[i]=(p+i) cout<<se; return 0; }

Feb 13th, 2006, 6:50 AM	#2
Mjordan2nd The Supreme Ruler Join Date: May 2004 Location: Houston Posts: 1,476 Rep Power: 6	Edited to correct code tags __________________ "Every gun that is made, every warship launched, every rocket signifies, in the final sense, a theft from those who hunger and are not fed, from those who are cold and are not clothed. The world in arms is not spending money alone. It is spending the sweat of its laborers, the genius of its scientists, the hopes of its children." - Dwight D. Eisenhower

Feb 13th, 2006, 6:51 AM	#3
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	(The trailing code tag requires a slash, [/code]. Thanks for using them, though.) A character string (ala C) IS an array. Exactly and precisely. It has one additional requirement -- a 'null' character to mark the end of useful characters. I think maybe you'd like to examine what this means: (Toggle Plain Text) for (i = 0; i = 5; i++); for (i = 0; i = 5; i++); You are setting i to zero. You are specifying that the loop will execute only when i is 5. Hard to see that working. I would suggest you put braces around the body of your loops and such, even though, as a single line, they're not required. A novice will get into the bad habit of leaving them out where they're needed. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

Feb 13th, 2006, 9:08 AM	#4
nnxion Programming Guru Join Date: Jun 2005 Location: elemental plane Posts: 1,429 Rep Power: 5	Additionally, you should include <iostream>, <iostream.h> is deprecated. @David: Does the compiler set i to 5 in the for loop, or does it only compare even though one types the = operator instead of == operator? __________________ "Employ your time in improving yourself by other men's writings, so that you shall gain easily what others have labored hard for." -- Socrates Last edited by nnxion; Feb 13th, 2006 at 9:25 AM.

Feb 13th, 2006, 9:27 AM	#5
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	<me>Dang me, take a new rope and hang me, didn't even pick up on the single '='</me> In that case, the loop will run (i = 5 is true), i will be incremented, set back to 5, ad nauseum. se [i] will get a workout and p+i will definitely have the jitters like a wino on joy juice. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

Feb 13th, 2006, 9:30 AM	#6
crazykid48x Programmer Join Date: Apr 2005 Posts: 96 Rep Power: 4	I dont use pointers much but it seems to me it wouldnt work because he didnt set p to point to anything __________________ I had my portable CD player, and took it in the bathroom with me while I went to pee. And the second I whipped my penis out, the theme song to 'Rocky' started playing. I've never felt more manly than in that moment.

Feb 13th, 2006, 9:33 AM	#7
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	That's true, too. It's hell to be old and blind. I can't even read my own tutorial without a magnifying glass. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

Feb 14th, 2006, 9:34 PM	#8
aznluvsmc Hobbyist Programmer Join Date: Aug 2005 Posts: 137 Rep Power: 4	Another method that will work is to use pointer notation to pass the starting address of the string to cout. For example, tu itself stores the address of the beginning of the string so cout will start printing from that memory address until it reaches the NULL byte. However, you can do pointer arithmetic by adding a specific number to tu and then cout should start printing from that memory address. For example to print only the word "world" you can use (Toggle Plain Text) cout << tu + 6 << endl; cout << tu + 6 << endl; Here we start printing 6 characters inward starting from the beginning of the string.

Feb 14th, 2006, 9:54 PM	#9
Dameon Troll Join Date: Apr 2005 Location: Texas Posts: 732 Rep Power: 4	@aznluvsmc: Don't forget a little bounds checking when arbitrarily advancing a pointer. In this case, if the string you are working with isn't long enough, you could end up with a pointer past the end of your useful data. Any string manipulation/display function would keep processing garbage until, by chance, it hits a zero. __________________ MD5(sig) = bcef75433db02e9ad9bf81d6f7c5c270

Feb 15th, 2006, 6:39 AM	#10
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	...or references protected memory... . We need a smiley that shows the system running off into the woods and getting eaten by a b'ar. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

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Programming Forums > Application Development > C++

how to use pointers to display a part of a string