Initialising Arrays

This is a discussion on Initialising Arrays within the C++ Programming forums, part of the General Programming Boards category; I'd like to know if it's possible to initialize arrays while defining it? The same with pointers. Is it possible ...

  1. #1
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    Initialising Arrays

    I'd like to know if it's possible to initialize arrays while defining it? The same with pointers. Is it possible to automatically allocate it, like calling ptr = new int ?
    More than just saving some syntax, I have a class in whose constructor, a global array is modified. So unless it's all already initialised and set to 0, I'll have problems.
    If this can't be done, then could you tell me how to get rid of the junk that is there when the array is initialized, and not affect the data that has been put into the array, when I call the constructor (via assigning a class variable)?

  2. #2
    CSharpener vart's Avatar
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    Quote Originally Posted by studiesrule
    I'd like to know if it's possible to initialize arrays while defining it?
    yes
    Code:
    int a[5] = {0,1,2,3,4};
    Quote Originally Posted by studiesrule
    The same with pointers. Is it possible to automatically allocate it, like calling ptr = new int ?
    yes

    Quote Originally Posted by studiesrule
    I have a class in whose constructor, a global array is modified. So unless it's all already initialised and set to 0, I'll have problems.
    Constructor is a good place to do things like this.
    Use some class for your global array and do stuff you need to initialize array in its constructor.

    Quote Originally Posted by studiesrule
    If this can't be done, then could you tell me how to get rid of the junk that is there when the array is initialized, and not affect the data that has been put into the array, when I call the constructor (via assigning a class variable)?
    the good old memset is already not working?
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

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    A vector is usually a better idea than an array anyway, and if you use a vector you can initialize it to all 0's in its constructor.
    Code:
    std::vector<int> arr(20);
    If you have to stick to arrays, you can initialize it to 0's when you define it depending on the type. Is it an array of ints?
    Code:
    int arr[20] = { 0 };

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    >> you must "turn that off"
    There's no reason you must turn that off. It's not even necessary in most cases.

  5. #5
    Just Lurking Dave_Sinkula's Avatar
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    Quote Originally Posted by vart
    the good old memset is already not working?
    This does not guarantee useful null pointer values or floating-point zero values.
    7. It is easier to write an incorrect program than understand a correct one.
    40. There are two ways to write error-free programs; only the third one works.*

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    Wow, I'm overwhelmed by the superfast reply (10 minutes). I needed something like int arr[20] = 0. I'm pretty sure I can do the same with ptrs, and initializing them to NULL. Thanks a lot.

    Though this is unrelated, I'd like to know how to use escape sequences to variables in C++. In C, I would just use %d, or %f. For example the below code (which just prints %d 20 times):

    Code:
    #include <iostream>
    using namespace std;
    
    int main()
    {
    	int i,a[20]={0};
    	for (i=0;i<=19;i++)	{
    		cout<<"%d ", a[i];
    	}
    	return 0;
    }

  7. #7
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    What do you mean by escape values? There is no need for format strings with cout. Just output the variable:
    Code:
    cout << a[i];
    Your C++ book should probably have shown how to do that in an example.

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    Ok here it's very simple, but suppose I'd like to print lets say 5-6 variables in a line, then I'd have to do cout <<var1<<" "<<var2<<" "... I find this tedious, and irritating. It's also hard to nicely format the output.

  9. #9
    Code Goddess Prelude's Avatar
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    >I find this tedious, and irritating.
    Bummer.

    >It's also hard to nicely format the output.
    It's hard to concisely format the output. The iostreams library supports modifiers for formatting (which are briefly looked at in the FAQ).
    My best code is written with the delete key.

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    Ok thanks. I wonder why they removed this small feature from C++

  11. #11
    Code Goddess Prelude's Avatar
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    >I wonder why they removed this small feature from C++
    They didn't. You can still use printf if you want. However, format modifiers in a string are error prone and very difficult to make type safe. Add to that the awful hack of variable length argument lists and the difficulty of extending the modifiers for user-defined types, and it's easy to see why iostreams don't use them.
    My best code is written with the delete key.

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    printf is sloooow. And they never removed it (that's one of my beefs though, they never remove anything ), they just shifted all of it into <iomanip>.
    Code:
    #include <stdio.h>
    
    void J(char*a){int f,i=0,c='1';for(;a[i]!='0';++i)if(i==81){
    puts(a);return;}for(;c<='9';++c){for(f=0;f<9;++f)if(a[i-i%27+i%9
    /3*3+f/3*9+f%3]==c||a[i%9+f*9]==c||a[i-i%9+f]==c)goto e;a[i]=c;J(a);a[i]
    ='0';e:;}}int main(int c,char**v){int t=0;if(c>1){for(;v[1][
    t];++t);if(t==81){J(v[1]);return 0;}}puts("sudoku [0-9]{81}");return 1;}

  13. #13
    Just Lurking Dave_Sinkula's Avatar
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    Quote Originally Posted by jafet
    printf is sloooow. And they never removed it (that's one of my beefs though, they never remove anything ), they just shifted all of it into <iomanip>.
    QoI, nothing more. Nothing to do with a language -- other than correct implementation of I/O.
    7. It is easier to write an incorrect program than understand a correct one.
    40. There are two ways to write error-free programs; only the third one works.*

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    I don't mean the printf feature (which I do know exists, but for backward compatibility). I was wondering why cout doesn't have a similar thing.

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    printf is way faster than cout... so don't complain about it's speed.

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