CODEWITHSUNDEEP
c++pointers
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New Progammer

Reputation: 112

Back to the position of pointer

I have such function:

bool filtruj(char* text, bool(*f)(char)) {
    while(*(text)!='\0')
    {
        bool sprawdzWartosc = f(*(text));
        if(sprawdzWartosc)
        {
            printf("run");
            b=*(text);
            while(b!='\0')
            {
                printf("run2");
                *(text) = *(text+1);
                text++;
            }
        }
    }
    return false;
}

which when the sprawdzWartosc is true, delete this char and move other characters one to the left.

The question is: how to back to the position of pointer which is used in var sprawdzWartosc? Becuase when I'm moving characters my pointer is at the end of *(text).

I can't use int, short, long variables, only pointers.

Upvotes: 1

Views: 71

Answers (2)

sehe
sehe

Reputation: 393567

 b=*text;
 while(b!='\0')
 // ...

This just keeps comparing the same character b, and it will never be \0. Perhaps you wanted to use another pointer?

Here's what I think you were trying to achieve: Live on Coliru

bool filter(char* text, bool(*pred)(char)) {
    char* out = text;
    while (*text)
    {
        if (pred(*text))
        {
            ++text;           // just skip
        } else
        {
            *out++ = *text++; // copy out
        }
    }
    *out = '\0';
    return (out != text); // true if characters were filtered
}

#include <cctype>
#include <cstring>
#include <cstdio>

bool foo(char c)
{
    return isalpha((int)(unsigned char)c);
}

void test(const char* input)
{
    char* s = strdup(input);
    printf("%s -> ", s);
    bool b = filter(s, foo);

    printf("%s (%s)\n", s, b?"true":"false");
}

int main()
{
    test("12346  234890-98 .");
    test("12a46  awesome-98!");
}

Printing

12346  234890-98 . -> 12346  234890-98 . (false)
12a46  awesome-98! -> 1246  -98! (true)

Upvotes: 1

kfsone
kfsone

Reputation: 24269

I think what you are trying to do is iterate over the body of a string and delete characters for which a call to f returns true.

bool filtruj(char* text, bool(*f)(char)) {
    while(*(text)!='\0') {
        bool sprawdzWartosc = f(*(text));
        if(sprawdzWartosc) {
            for (char* cur = text; *cur; ++cur) {
                *cur = *(cur+1);
            }
        }
    }
    return false;
}

Note that your original code did this

b = *(text);
while (b != '\0')

The assignment, b = *(text) copies the current value at the address text currently points to into the variable b - it does not make b magically track the current value of text.

int i = 5;
int* ptr = &i;
int j = *ptr;
++i;
printf("j = %d\n", j);

prints 5, not 6.

Upvotes: 0

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