Query on enhanced FOR loop

[Satya Maheshwari]

Hello

I am new to the enhanced for statement.
Considering this code:

int sum(int[] a) {
        int sum = 0;
        for (int i : a)
            sum += i;
        return sum;
    }

Per my understanding this translates to:

int sum(int[] a) {
        int sum = 0;
        for (int i = 0; i < a.length;i++)
            sum += a[i];
        return sum;
    }

Is this correct? Even if it is, I am still not clear how does enhanced for statement work.

[Barry Gaunt]

What about if we rename some variables like this:

int sum(int[] someNumbers) {
        int sum = 0;
        for (int number : someNumbers) // read as: for each number in someNumbers
            sum += number; // add it in to the accumulated sum
        return sum;
    }

?

Can't make it much simpler than that.

[Barry Gaunt]

For your second for loop:

int sum(int[] someNumbers) {
        int sum = 0;
        for (int i = 0; i < someNumbers.length;i++) { // for each of the numbers
            int number = someNumbers[i];  // take the next number
            sum += number; // add it in to the accumulated sum
        }
        return sum;
    }

[Rob Mech]

I've always considered the "Enhanced For Loop" as a "for each" loop. Uh oh, am I showing my .NET background?

Ok, so I code in many things... Getting back to your question though

an easier way to understand the Enhanced For Loop is in the for each context.


for(int item : itemContainer[])

So there you are saying "For every int in the array container, put a copy of it into item and give me the result in this loop"

i.e.

itemContainer[0]
itemContainer[1]
itemContainer[2]
itemContainer[3]

means that your loop would execute 4 times and each time

"item" would equal the respective array item above.
[ May 09, 2007: Message edited by: Rob Mech ]

[Satya Maheshwari]

Thanks Rob,Barry for your replies. Now I think I have a better understanding of the enhanced for loop. One question still though:

for (int number : someNumbers)

Does this mean that someNumbers should always be an array. Can it be a collection of some type? If yes, then what should be 'number' here?

[Chandra Bhatt]

for (int number : someNumbers)

Does this mean that someNumbers should always be an array. Can it be a collection of some type? If yes, then what should be 'number' here?

for(type args1 : args2)

We know it as compatible type. For each object or primitive returned by arg2, the arg1 must be compatible to that.

public class Ex3 {
 
public static void main(String... args) {
float []farr = {12.43f,234.4f,32.3546f};

for(double d:farr) {
System.out.printf("%.3f\n",d);
}

byte [] barr = {1,4,7,2,65};

for(int a: barr) {
System.out.println(a);
}

String[] obarr = {"hello","howdy","welcome"};
for(String s: obarr) {
System.out.println(s);
}
 
}
}
 

All compile and run fine.


Thanks,

[Satya Maheshwari]

Thanks for replying! Actually what I meant was that could we use collections(like HashMap etc.) in the enhanced for loop.

HashMap h = new HashMap();
h.put("a","aaa");
h.put("b","bbb");
for(obj : h)
{
 ...
}

Here, would this work and what type is 'obj'?

[Chandra Bhatt]

Hi,

You may try this:

import java.util.*;
 
public class DemoMap {

public static void main(String... args) {
HashMap map = new HashMap();

map.put("One", 1);
map.put("Two", 2);
map.put("Three", 3);
map.put("Four", 4);
map.put("Five", 5);
map.put("Six", 6);

for(Object o : map.keySet()) {
System.out.println(o);
}

for(Object o : map.values()) {
System.out.println(o);
}
}
}

Or in type safe way:

public class DemoMap {

public static void main(String... args) {
HashMap<String,Integer> map = new HashMap<String,Integer>();

map.put("One", 1);
map.put("Two", 2);
map.put("Three", 3);
map.put("Four", 4);
map.put("Five", 5);
map.put("Six", 6);

for(String o : map.keySet()) {
System.out.println(o + " " + map.get(o));
}

}
}

Thanks,
[ May 10, 2007: Message edited by: Chandra Bhatt ]

[Barry Gaunt]

Another simple collection example:

int sum(List<Integer> someNumbers) {
        int sum = 0;
        for (Integer number : someNumbers) // read as: for each number in someNumbers
            sum += number; // add it in to the accumulated sum
        return sum;
    }    

[Barry Gaunt]

For an Integer-valued HashMap you can simply do:

    int sum(HashMap<?, Integer> mapOfNumbers) {
        int sum = 0;
        for (Integer number : mapOfNumbers.values()) // read as: for each number in mapOfNumbers values
            sum += number; // add it in to the accumulated sum
        return sum;
    }
    

[Satya Maheshwari]

Barry can you please explain what does this do:

HashMap<?, Integer>

Is this some new feature?
[ May 10, 2007: Message edited by: Satya Maheshwari ]

[Barry Gaunt]

Originally posted by Satya Maheshwari:
Barry can you please explain what does this do:

HashMap<?, Integer>

Is this some new feature?

[ May 10, 2007: Message edited by: Satya Maheshwari ]

Not special, but new: Angelika Langer's Generic FAQ