Is there a simpler way to define a condition in a for loop in C++?

Question

Consider an array arr[10] of size 10. While getting/displaying data, we use a for loop of the following common syntax

for(int i=0;i<10;i++)

In case of a string, it is like

for(int i=0;strlen(arr)>i;i++)

But I've read somewhere that a simpler expression like arr[i] could just be used in the place of condition. I've tried running a code with that condition. But I get an error. So is there a similar, simple condition expression that can be used with arrays/string?

Answer 1

As stated in the for statement documentation:

condition may be a declaration

With the following code to demonstrate:

int main(){
    int arr[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 0 };
    for (int i = 0; int x = arr[i]; ++i){
        std::cout << x << std::endl;
    }
}

The loop will exit once arr[i] becomes 0 and gets assigned to x causing the whole condition to evaluate to false. Why that is in more details:

a declaration of a single variable with a brace-or-equals initializer. the initializer is evaluated before each iteration, and if the value of the declared variable converts to false, the loop is exited.

Answer 2

Since you said simple and c++, use std::string.

std::string str = ???;
for(char& c : str) {
    do_things_with(c);
}

Taken from this post.

Answer 3

Yes, for a zero-terminated c-string you can use the following

for(int i=0; arr[i]; i++)

because the loop will stop when it encounters the nul-terminator.

This does not work for arrays, only for zero-terminated strings. (It would also work for arrays terminated by something that converts to false, but this is quite rare IMO. Thanks @François Andrieux)

Answer 4

You probably want something likes this:

int main() {
  char arr[] = "Hello";

  for (int i = 0; char c = arr[i]; i++)
  {
    putc(c, stdout);
  }
}

The output will be

Hello

c = arr[i] will be 0 when yolu come to the end of the string, ans thus the loop will be terminated.

BTW: for(int i = 0; strlen(arr) > i; i++) is correct but inefficient. I let you find out why as an exercise.

Answer 5

I am not sure if this is simplier but it is a bit different and works well for strings, not for other generic types;

You can use array[] to check for the string terminator

char array[10] = "hello";
for (int i = 0; array[i] != 0; i++) 
     myFunction(array[i]);

for string you can use pointer arithmetic instead

for (char *aPtr = array; *aPtr != 0; aPtr++)
    myFynction(*aPtr);

Answer 6

I'm aware that writing this:

for(int i=0; arr[i]; i++)

will save a few microseconds over writing this:

int array_length = strlen (arr);
for(int i=0; i < array_length; i++)

I'm also aware that constructions like the first are pretty idiomatic in C, and experienced C programmers are used to seeing them.

However, in my view, the circumstances have to be very particular for saving a few cycles to be more important than writing code that other folks will understand easily. As a person who spends a significant part of his working day looking for defects in other people's code, I really appreciate expressive coding practices.

Answer 7

you have many options but calling the strlen is the most ineffective one (especially if you do it the way shown). arr has to be the pointer to be incremented

while(*arr)
{
   /* do something */
   arr++;
}

while(arr[i])
{
   /* do something */
   i++;
}

do
{
   /* do something */
}while(*++arr);

do
{
   /* do something */
}while(arr[++i]);



for(;*arr;arr++) 
{
   /* do something */
}

do  // is you need to do somethingh with the trailing zero
{
   /* do something */
}while(*arr++);

do // is you need to do somethingh with the trailing zero
{
   /* do something */
}while(arr[i++]);

etc etc.