Parsing a string(pointer question...) 1

Susan · Jun 14, 2000

I have a simple function which removes brackets from a given phone number. Eg: If the phoned number passed is (000) 123 4567 then the phone number returned is 000 123 4567. The function goes as follows: int RemoveBrackets(char *PhoneNumber) {     int   Result = 0;     char  *OpenBracket  = NULL,   *CloseBracket = NULL,   *BeginPhNum   = NULL,   *EndPhNum     = NULL;     /* Look for the opening bracket. */     OpenBracket = (char *)strstr(PhoneNumber, "(&quot

;     if(OpenBracket != NULL)     { BeginPhNum = ++OpenBracket; /* Look for the closing bracket.*/ CloseBracket = (char *)strstr(PhoneNumber, &quot

&quot

; if(CloseBracket != NULL) {     EndPhNum = ++CloseBracket;       CloseBracket = --CloseBracket;        *CloseBracket  = '\0';            printf("EndPhNum: %s\n", EndPhNum);     strcat(BeginPhNum, EndPhNum);     strcpy(PhoneNumber, BeginPhNum);     printf("PhoneNumber(out): %s\n", PhoneNumber);      }     } return(Result); } It works correctly, returning the phone number without the brackets but I don't actually understand how the contents of 'EndPhNum' are not null/blank after 'CloseBracket' has been nulled...? I khow it's something to do with the line CloseBracket = --CloseBracket; as when this is taken out EndPhNum is then blanked after *CloseBracket = '\0'; However when CloseBracket = --CloseBracket; is left in then somehow the contents of EndPhNum are preserved.... Much appreciated if anyone could shed some light on this ...

rpet · Jun 14, 2000

hi susan! let's try to trace this: string: "(000) 123 4567\0" CloseBracket = (char *)strstr(PhoneNumber, &quot

&quot

; ->CloseBracket points to &quot

" EndPhNum = ++CloseBracket; ->CloseBracket points to the blank ->EndPhNum points to the blank CloseBracket = --CloseBracket; ->CloseBracket points to &quot

" again ->EndPhNum points to the blank *CloseBracket  = '\0';   -> &quot

" is replaced by "\0" -> EndPhNum points to the blank that's it. EndPhNum starts one character after the \0. if you don't decrease CloseBracket EndPhNum points to the same position (which is set to \0 later). -- ralf

Susan · Jun 14, 2000

thanks ralf,  but how come 'BeginPhNum' only contains the initial 000 after *CloseBracket = '\0'; has been executed(ie: everything after ')' has been blanked, it was '000) 123 4567') but 'EndPhNum' still contains it's original details....??? susan

Nosferatu · Jun 14, 2000

Hei Susan! I guess you have a small missunderstanding of what '\0' does... '\0' represents actually a marker that represents the end of a string by replacing ')' with the string terminator. By setting CloseBracket to '\0' you just marked the end of the BeginPhNumber string (confusing?). But the actual string is still in the memory , that assignement doesnt "blank" (as you put it) the string, whose end part is hold by EndPhNum . Imagine the string as an vector array and the pointers (CloseBracket,EndPhNum ...) as indexes in the array. The operations you made on the string are somehow equivalent to PhoneNumber[CloseBracket] = '\0', for example (don't use this as a code, OK?, i hope it was a good enough example) If you want to "blank" your string, you have to do somethin like free(EndPhNum), which will free the memory contained by the string. To be more clearer try the code:    strcpy(EndPhNum,"&quot

; // setting it to the empty string, right?    EndPhNum++;    printf("%s",EndPhNum);   Have i brought you some light?

Susan · Jun 15, 2000

Thanks very much for that - I understand exactly what's going on now! You're right - I did misunderstand what '\0' does but it's clear now. Thanks again... Susan

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Parsing a string(pointer question...) 1

Susan

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rpet

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Susan

Programmer

Nosferatu

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Susan

Programmer

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