Understanding Pointers and Strings

[JonathaThurston]

Hey Guys,

I have a question regarding pointer arithmetic as I find it very hard to grasp it. Well to be totally honest I do not understand the concept at all so I wrote this program which sends a char array to a function and prints out a string of characters from a certain offset to a certain length.

I am having trouble with the loop in the function can you please help me understand it.

Thanks.

#include<stdio.h><br /> <br /> void draw(char *w, int len);<br /> <br /> int main(){<br /> char words[100] = ("This is my hundredth attept st this extremely annoying task :)");<br /> char *index = NULL;<br /> int len = 21;<br /> <br /> draw(words + 11, len);<br /> <br /> return 0;<br /> }<br /> <br /> void draw(char *w, int len){<br /> char *index = NULL;<br /> index = w;<br /> <br /> for(index; *index != *(index + len); index++)<br /> putchar(*index);<br /> }

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#include<stdio.h><br /> <br /> void draw(char *w, int len);<br /> <br /> int main(){<br /> char words[100] = ("This is my hundredth attept st this extremely annoying task :)");<br /> char *index = NULL;<br /> int len = 21;<br /> <br /> draw(words + 11, len);<br /> <br /> return 0;<br /> }<br /> <br /> void draw(char *w, int len){<br /> char  *index = NULL;<br /> index = w;<br /> <br /> for(index; *index != *(index + len); index++)<br /> putchar(*index);<br /> }

[GWILouisaxwzkla]

To understand the problem completely, remember that arrays are configured as contiguous blocks of memory ( the first index in memory precedes the next and so on ) and realize the fact that pointers are simply variables that hold the address of other variables ( the size depends on your hardware and operating system ). Also keep in mind that in C the semantic value of an array’s name is the address of the first item in the array ( array [ 0 ] ). So that in your program you have a situation like :

<br /> <br /> draw(words + 11, len); //pass in the address of the 11th 8-bit item in memory, words [11]<br />

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<br /> <br /> draw(words + 11, len); //pass in the address of the 11th 8-bit item in memory, words [11]<br />

since the array looks like this in memory ( let 0x0065fd94 be the first address of the array ):

memory address item
0x0065fd94 ‘T’
0x0065fd95 ‘h’
0x0065fd96 ‘i’
0x0065fd97 ‘s’
…………..

and so on your loop:

<br /> <br /> for(index; *index != *(index + len); index++)<br /> putchar(*index);<br /> }<br />

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<br /> <br /> for(index; *index != *(index + len); index++)<br /> putchar(*index);<br /> }<br />

starts at index == words [ 11 ] which is ‘h’ and continues until ( *index == *(index + len); ) . But since * ( index + len ) is words [ 32 ] == ‘h’ your loop never executes. I think what you wanted to do is this:

<br /> #include <stdio.h><br /> #include <iostream.h><br /> <br /> void draw(char *w, int len);<br /> <br /> int main()<br /> {<br /> <br /> char words [ 100 ] = ("This is my hundredth attept st this extremely annoying task :)");<br /> char * index = NULL;<br /> int len = 21;<br /> <br /> <br /> <br /> draw( words + 11, len );<br /> <br /> <br /> <br /> return 0;<br /> }<br /> <br /> void draw ( char * w, int len )<br /> <br /> {<br /> char * index = NULL;<br /> int i = 0;<br /> <br /> index = w;<br /> <br /> putchar ( 'n' );<br /> for( ; i < len ; index ++ , i ++ )<br /> {<br /> putchar ( * index );<br /> }<br /> putchar ( 'n' );<br /> }<br /> <br /> <br /> <br />

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<br /> #include <stdio.h><br /> #include <iostream.h><br /> <br /> void draw(char *w, int len);<br /> <br /> int main()<br /> {<br /> <br /> char words [ 100 ] = ("This is my hundredth attept st this extremely annoying task :)");<br /> char * index = NULL;<br /> int len = 21;<br /> <br /> <br /> <br /> draw( words + 11, len );<br /> <br /> <br /> <br /> return 0;<br /> }<br /> <br /> void draw ( char * w, int len )<br /> <br /> {<br /> char  * index = NULL;<br /> int i = 0;<br /> <br /> index = w;<br /> <br /> putchar ( 'n' );<br /> for( ; i < len ; index ++ , i ++ )<br /> {<br /> putchar ( * index );<br /> }<br /> putchar ( 'n' );<br /> }<br /> <br /> <br /> <br />

[JonathaThurston]

Hey dman thanks for explaining it makes more sense now. Is it possible to avoid making *index a pointer to the array and do the manipulations on *w instead?

[GWILouisaxwzkla]

Heres how I might do this ( this uses pointer arthimatic instead of indexes ) :

<br /> <br /> #include <stdio.h><br /> #include <iostream.h><br /> <br /> void draw(char *w, int len);<br /> <br /> int main()<br /> {<br /> <br /> char words [ 100 ] = ("This is my hundredth attept st this extremely annoying task :)");<br /> char * index = NULL;<br /> int len = 21;<br /> <br /> draw( words + 11, len );<br /> <br /> <br /> <br /> return 0;<br /> }<br /> <br /> void draw ( char * words, int lengthOutput ) //words points to words [ 11 ]<br /> <br /> {<br /> <br /> char * upperLimit; //pointer for storing upper limit of output<br /> <br /> <br /> upperLimit = words + lengthOutput; //upperLimit points to the 32nd char of words [ 100 ]<br /> putchar ( 'n' ); //output newline<br /> while ( words != upperLimit ) //while not at the 32nd char of array<br /> {<br /> putchar ( * words); //output the current char<br /> words ++; //goto next char in the array<br /> }<br /> putchar ( 'n' ); //output a newline<br /> <br /> }<br /> <br /> <br />

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<br /> <br /> #include <stdio.h><br /> #include <iostream.h><br /> <br /> void draw(char *w, int len);<br /> <br /> int main()<br /> {<br /> <br /> char words [ 100 ] = ("This is my hundredth attept st this extremely annoying task :)");<br /> char * index = NULL;<br /> int len = 21;<br /> <br /> draw( words + 11, len );<br /> <br /> <br /> <br /> return 0;<br /> }<br /> <br /> void draw ( char * words, int lengthOutput ) //words points to words [ 11 ]<br /> <br /> {<br /> <br /> char * upperLimit;   //pointer for storing upper limit of output<br /> <br /> <br /> upperLimit = words + lengthOutput; //upperLimit points to the 32nd char of words [ 100 ]<br /> putchar ( 'n' ); //output newline<br /> while ( words != upperLimit ) //while not at the 32nd char of array<br /> {<br /> putchar ( * words); //output the current char<br /> words ++; //goto next char in the array<br /> }<br /> putchar ( 'n' ); //output a newline<br /> <br /> }<br /> <br /> <br />

[JonathaThurston]

Yea that makes more sense actually, and its more readable than a for loop.
Thanks

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