What's the meaning of "&&" when operated by a char * on a char in C++?

Question

I came across a code which extracted a token(c string) from a given string,but I am confused by the 5th line:tok && *tok. I just cannot figure out this operation's result. Does anybody happen to know?

const char* getfield(char* line, int num)
{
    const char* tok;
    for (tok = strtok(line, ";");
            tok && *tok;
            tok = strtok(NULL, ";\n"))
    {
        if (!--num)
            return tok;
    }
    return NULL;
}

Thank you!

this code has not sense (but is formally ok). Compute expression and be not interrest in result. Maybe wrong copy&paste? — Jacek Cz, Apr 21 '18 at 18:23
`&&` is logical and. It's taking the logical and of `tok` (effectively `tok != NULL`) and `*tok` (effectively `*tok != 0`). — Stephen Newell, Apr 21 '18 at 18:23
@StephenNewell is correct. In other words, the expression evaluates to `true` if and only if `tok` points to an non-nul character. — Jive Dadson, Apr 21 '18 at 18:25
@Jacek Cz I don't understand it either... The codes aimed to read a CSV file. If you are curious about that...here is the link where I saw these codes: [link](https://stackoverflow.com/questions/12911299/read-csv-file-in-c#answer-12911465). Share me your opinion if you figure out what he is doing! — JadenFero, Apr 22 '18 at 18:28

score 5 · Accepted Answer · edited Apr 21 '18 at 18:30

5

It checks first if the pointer (tok) isn't nullptr (pointing to nothing), then if the pointer isn't nullptr, it checks if it points to memory where the value (*tok) isn't '\0'.

edited Apr 21 '18 at 18:30

Jive Dadson

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answered Apr 21 '18 at 18:24

Sébastien S.

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Thank you! And one more thing, will error message occur if `tok` turns out to be a null pointer or is a `'\0'`? – JadenFero Apr 22 '18 at 18:36
You're welcome. If `num > 0` and `tok == NULL || *tok == '\0'` it will return `NULL`, else it will return `tok`. An error message can appear if you check the return value of the function when you call it and print a message if it returned `NULL`. – Sébastien S. Apr 22 '18 at 18:45

score 3 · Answer 2 · edited Apr 21 '18 at 18:33

This is the conditional expression, and it is checking that tok is not nullptr and that *tok, or the character, is not the "nul" character '\0'.

In a C-style for-loop, you have the initalizer, the condition, and the increment expressions. The condition here is that tok evaluates to true, then *tok evaluates to true.

This can only occur if tok is not nullptr, and the character pointed to (*tok) is not '\0'.

score 0 · Answer 3 · edited Apr 21 '18 at 18:58

0

The meaning of && remains the same. It is still a logical operator. That statement is equivalent to

(tok != nullptr) && (*tok != '\0');

edited Apr 21 '18 at 18:58

Jive Dadson

16,680
9
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answered Apr 21 '18 at 18:48

user1451348

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score 0 · Answer 4 · answered Sep 27 '21 at 17:12

Can any point out what the below portion means?

{
    if (!--num)
        return tok;
}

I'm most interested in the if (!--num). It's not making sense to me. In my head it reads as if NOT num = num - num. It's really cramping my brain.

I know ! is the NOT operator, but the --num just doesn't compute in my brain.

What's the meaning of "&&" when operated by a char * on a char in C++?

4 Answers4