Array index in for loop

Question

I'm new in c++ and have come across a problem which I don't know what to make of. I was doing some examples in a book when I came across the following piece of code.

for(int i=0;string[i];i++){
    cout<<string[i];
}

Here instead of a boolean for the second argument the person has given the array index.Based on that method I ran the following piece of code but it didn't work.

char string[50] ="This is a test!";

for(int i=sizeof(string)-1;string[i];i--){
    cout<<string[i];
}

cout<<"\n";

I debugged it and the 'string[i]' has the value of 0. So my questions are,

1. Why use an array index instead of a boolean (is it OK?).
2. Why didn't my second piece of code work.

Thanks in advance.

Answer 1

The boolean test string[i] is actually just checking if a null-character ('\0') is encountered. All strings in C end in a null-character, and so this works for forward-iteration through a string. But not if you are going in reverse. You should check instead that i is greater than or equal to zero.

Also, sizeof(string) returns the size of the datatype (50 bytes, or possibly more), not the length of the string it contains. To get the string length, use strlen:

for( int i = strlen(string) - 1; i >= 0; i-- ) {
    cout << string[i];
}

I would advise against calling your variables string, as this is a datatype in the standard library, and is confusing to C++ programmers if you use it as a variable name. Especially as people quite commonly import the entire std namespace and refer to the string class as just string.

This is like calling all your numeric values "integer". Doing so doesn't actually tell the programmer what it's used for, only what type it is. Please use more descriptive variable names =)

Answer 2

In the above for loop, a char has been used instead of bool as the second argument. In this case, non-zero values are considered as "true" and zero value is considered as "false".

In the second code, for loop does not execute because string[sizeof(string)-1] is zero and it is evaluated as "false".

If you use in the following way, your code will work

for(int i=0;string[i];i++){

Answer 3

Since this is C++, not C, you might be better off using the builtin string class rather than using character arrays.

Here is one way to print out the reverse of your string, character by character:

 #include <iterator>
 ...
 typedef std::reverse_iterator<std::string::iterator> rev_iter;

 string str("This is a freaking test!");
 for(rev_iter it (str.end()); it != rev_iter(str.begin()); ++it)
     std::cout <<  *it;
 std::cout << endl;

And another (but this modifies the string):

#include <algorithm>
...
string str("This is a freaking test!");
reverse(str.begin(), str.end());
cout << str << endl;