How Do I Create a Loop in Java and Increment an Index to Use As Part of a String?

Question

So for my current program, I am currently doing this:

Java Code

        ArrayList<Section> aMainSection = new ArrayList<Section>();
        Section aSection = new Section();
        aSection.setName("Document 1");
        aSection.setSection("Section 1");
        aSection.setText("Text 1");
        Section aSection2 = new Section();
        aSection2.setName("Document 2");
        aSection2.setSection("Section 2");
        aSection2.setText("Text 2");
        Section aSection3 = new Section();
        aSection3.setName("Document 3");
        aSection3.setSection("Section 3");
        aSection3.setText("Text 3");

But what I want to be able to do is create a for loop in which when the condition is met, I can just create a new Section. However, I do not know how to increment variables in Java. I would assume it should be possible, somehow, but I know it's not as simple as concatenating an integer value to the end of the variable name. Any help would be appreciated, thank you.

We usually uses lists for this. List<Section>, and add to the list. What's wrong with doing that? — S.Lott
– S.Lott, Commented Jul 6, 2011 at 20:36
I'm not sure "increment" is correct terminology here. Generally that means to add some value to something. Do you want to add another 'Section' to your 'aMainSection' variable based on a condition? — aardvarkk
– aardvarkk, Commented Jul 6, 2011 at 20:37
Incrementing variables? like: int i = 0; i++; for for(int i = 0; i < someValue; i++) — Ant Kutschera
– Ant Kutschera, Commented Jul 6, 2011 at 20:39
OP is apparently asking if you can increment the name of a variable. No. You use a Collection or an array. — Stephen P
– Stephen P, Commented Jul 6, 2011 at 20:45
I suspect that the questioner has not understood either (or both) block scoped variables or that variables are only references and "point" to different objects at different times. So they think they need a new variable for each section. Lots of answers here should help. — djna
– djna, Commented Jul 6, 2011 at 20:46

Waylon Flinn · Accepted Answer · 2011-07-06 20:59:50Z

2

Sounds like you want to do this:

ArrayList<Section> aMainSection = new ArrayList<Section>();
for(int i = 0; i < 3; i++)
{
    Section aSection = new Section();
    aSection.setName("Document "+(i+1));
    aSection.setSection("Section "+(i+1));
    aSection.setText("Text "+(i+1));
    aMainSection.Add(aSection);
}

If you don't know how many times you want to do it before hand try this:

ArrayList<Section> aMainSection = new ArrayList<Section>();
int sectionNumber = 1;
boolean done = false;
while(!done)
{
    Section aSection = new Section();
    aSection.setName("Document "+ sectionNumber);
    aSection.setSection("Section "+ sectionNumber);
    aSection.setText("Text "+ sectionNumber);
    aMainSection.Add(aSection);

    sectionNumber++;
    done = <put something interesting here>
}

edited Jul 6, 2011 at 20:59

answered Jul 6, 2011 at 20:41

Waylon Flinn

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6 Comments

Gordon Gustafson Over a year ago

why not just start with i=1? :D

Waylon Flinn Over a year ago

@CrazyJugglerDrummer It's a good point. Sounds like the asker is pretty inexperienced. I wanted to make sure their first exposure was pretty standard. That way when they get to arrays they'll be more likely to use the right indexing convention.

This 0ne Pr0grammer Over a year ago

@Waylon That does look like what I am trying to do, but what if I don't know the exact number of sections I am planning to add before-hand?

Gordon Gustafson Over a year ago

@This then you can just replace the 3 with an int variable that contains the exact number of sections you want. You could even make a method to do this and pass in the number of sections as a parameter each time. :D

Waylon Flinn Over a year ago

@This 0ne Pr0grammer In that case you'll want a while instead. I'll update my answer with one.

|

Ulrich Dangel · Accepted Answer · 2011-07-06 20:41:31Z

2

Try this one:

for (int i=1; i<4; ++i) {
    Section aSection = new Section();
    aSection.setName("Document " + i);
    aSection.setSection("Section " +i );
    aSection.setText("Text " +i);
}

answered Jul 6, 2011 at 20:41

Ulrich Dangel

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2 Comments

sshannin Over a year ago

It doesn't sound like this is what the OP was asking for. Also, your sections disappear pretty soon after you create them...

Stas Jaro Over a year ago

you would simply create an array:Section[] sections = new Section[4]; and instead of aSection use sections[i-1]; this question shouldn't be here anyway

user41871 · Accepted Answer · 2011-07-06 20:41:47Z

2

Yes, in Java you can increment variables. There is even a special operator for it: ++.

answered Jul 6, 2011 at 20:41

user41871

3 Comments

Gordon Gustafson Over a year ago

They OP wanted to know about incrementing variable names, not incrementing integers. This is helpful, but it comes nowhere near actually answering the question...

user41871 Over a year ago

Disagree. He is trying to solve a problem (automatically generating the names), and thinks that he needs to increment strings. The standard approach (illustrated by most of the answers here) would be to use ++ to increment an integer and append the integer to a string prefix to generate a name. Problem solved, question answered.

Gordon Gustafson Over a year ago

He wants to be able to manipulate the actual names of the variables as well, and ++ alone certainly won't help you there. :D

Gordon Gustafson · Accepted Answer · 2011-07-06 20:48:23Z

2

You cannot manipulate the names of variables in Java, so it's impossible to do things like making a bunch of Strings where each variable's name has a different number appended to it. You could hack this together by using a preprocessor that operates on actual source code text, but in this case that's highly unnecessary as there's a much simpler solution.

When you need to create a large group of variables in sequential order like your sections, you can just use a collection like an ArrayList<> to store them and access them by number:

    ArrayList<Section> aMainSection = new ArrayList<Section>();
    int NumberOfTimesYouWantToIncrement = 2;

    for (int i=1; i<NumberOfTimesYouWantToIncrement; i++) {
    Section aSection = new Section();
    aSection.setName("Document + i );
    aSection.setSection("Section" + i );
    aSection.setText("Text" + i );
    aMainSection.add( aSection ); //assuming your MainSection is supposed to contain the other sections
    }

This will create an ArrayList<> of Sections for you that you can then iterate through to get the different sections you created:

for (Section i: aMainSection) {
    //do something with that section
}

This is a lot less cumbersome than manipulating the variable names as it lets you create and store them much more easily. Think about what you would have to do if you needed to create 200 sections in 200 different variables, and then mentioned all of them by name again whenever you wanted to loop through them. :D

edited Jul 6, 2011 at 20:48

answered Jul 6, 2011 at 20:40

Gordon Gustafson

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1 Comment

This 0ne Pr0grammer Over a year ago

Thank you very much. Two things I'm wondering though, when the new sections are added to the arraylist, how is it able to differentiate between them (why is it not writing over "aSection" each time?) Also, if I do not know the # of sections I want to add beforehand, how would I address that issue?

Rostislav Matl · Accepted Answer · 2011-07-06 20:41:58Z

1

What do you want to do ? Just create a few of you Sections ?

ArrayList<Section> aMainSection = new ArrayList<Section>();
int sectionsCount = 3;
for (int i=1; i<=sectionsCount; i++)
{
  Section aSection = new Section();
  aSection.setName("Document " + i);
  aSection.setSection("Section " + i);
  aSection.setText("Text " + i);
  aMainSection.add(aSection);
}

answered Jul 6, 2011 at 20:41

Rostislav Matl

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Comments

djna · Accepted Answer · 2011-07-06 20:43:37Z

1

Short of generating and compiling code (which is possible, but I'm 99% sure you don't want to go there) there's no way to do exactly what you asked for. But I don't believe you actually need to what you're asking for.

Consider this ... what did you plan to do with the variables aSection2, aSection3 ... aSection974 ... later. You can't use them without generating corresponding code to use them.

Instead use collections as many other respondents have suggestions.

answered Jul 6, 2011 at 20:43

djna

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Comments

score 1 · Accepted Answer · 2011-07-06 20:59:06Z

1

final ArrayList<Section> list = new ArrayList<Section>(100);
for (int = 0; i < 100; i++)
{
    final Section s = new Section();
    s.setName(String.format("Document %d", i));
    s.setSection(String.format("Section %d", i));
    s.setText(String.format("Text %d", i));
    list.add(s);
 }

Creates and adds 100 sections.

edited Jul 6, 2011 at 20:59

answered Jul 6, 2011 at 20:41

user177800

Comments

sshannin · Accepted Answer · 2011-07-06 20:38:51Z

0

You can't. All variables must be explicitly typed and known at compile time. Use an array (or some other Collection like the ArrayList you declare on the first line).

answered Jul 6, 2011 at 20:38

sshannin

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