C ++：像这样的for循环如何工作（使用指针）？

Question

I just came across this for loop in a reference book but I'm not really sure what's going on in the comparison since it's just a lone pointer.

char input[300], *p, *q[300], **r = q;
cin.getline(input, 300);
for (p = input; *p; p++)

How would it work?

Answer 1

input is a null-terminated string. cin.getline will place a 0 after the last character of the read string or at input[299] if the line exceeds 299 characters.

The char value gets implicitely converted to a boolean. It becomes true if it's not zero and consequently false if it's zero. Thus the loop condition is equivalent to *p != 0 .

Therefor the loop will iterate over the array until it comes across a zero, the end of the string.

Answer 2

The second expression in a for statement is evaluated for "truthiness". A character is true if is not equal to 0. The for loop could be changed to the following equivalent one:

for (p = input; *p != 0; p++)

Since strings are null-terminated, this is how one iterates through the characters in a string, and stop at the end.