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Say I got an array 

unsigned char digit[] = {0, 1, 2, 3, 4, 5, 6, 7};

Yet I want to modify part of the array, make the array become something like:

{0, 1, 2, 3, 0, 0, 0, 0}

Enumerate every element I want to modify and alter them might take some effort. Especially when there's a large amount of elements I want to change. I know in some languages like Python I may do something using a single line of code:

a = np.array([0, 1, 2, 3, 4, 5, 6, 7])
a[4:] = [0, 0, 0, 0]
//a: array([0, 1, 2, 3, 0, 0, 0, 0])

So I wonder, is there a similar way to do that in C?

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  • 1
    There is a way to do it in one line of code but I would not recommend doing it this way: memset(&digit[4], 0, sizeof(digit) - (4 * sizeof(digit[0]))); Commented Jan 8, 2018 at 3:16

4 Answers 4

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There are fewer possibilities in C, but in case of an unsigned char and setting its values to zero you could use memset:

memset(&digit[4], 0, 4);

Demo.

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4 Comments

In this case it solves my question, thank you! But since you said possibilities, I wonder if there's some other ways? Like what if I want to change digit to {0, 1, 2, 3, 0, 9, 17, 0}, that is, alter it with several different value.
@AmarthGûl C99 standard does not allow this, but gcc has a nice extension that could be used with memcpy. Here is a demo.
@dasblinkenlight why wouldn't C99 allow that?
@AnttiHaapala Oops, I thought this was an extension, but it's actually part of C99. Thanks for your comment!
3

One options is that you could write a subroutine that would implement the interface that other languages provide "under the cover". You'll probably want to educate yourself on 'VARARGS' to make it take a variable number of arguments.

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3

Others have already mentioned setting the array elements to a single value using memset, as a part of your follow up question you asked if some elements can be set to certain values like {1, 2, 3, 4}.

You can use memcpy here to achieve that. Since your type here is unsigned char I will keep that, but in general this method can be used for any type. 

memcpy(&digit[4], ((unsigned char[4]){1, 2, 3, 4}), 4 * sizeof(unsigned char));

You can see the demo here.

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2 Comments

The sizeof(unsigned char) is unnecessary since a) sizeof returns the size in chars and, b) memcpy copies the given number of chars (bytes).
@AnttiHaapala I agree with the comment that it is unnecessary but I wanted to make the statement general across types. I have also mentioned in the answer that this method can be used for any type. I had a feeling that OP wouldn't know how to extend it to other types (not trying to be disrespectful but careful).
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I think that maybe not the shortest but something you can easy do is just:

digit[] = {0, 1, 2, 3, 4, 5, 6, 7};   %Having this

a=the number in your vector you want to start making ceros;
n=the lenght of digit;
for(i=a;i=n;i++)
{
    digit[n]=0;
}

Is just a way I think you could use. If you want to change an specific one just 

b=position;
digit[b]=c; %Where c is the number you want to put in there.

I hope it works for you, good luck.

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3 Comments

I think I did, maybe I understood something wrong, can I ask you why are you asking?
If it's because of the variable n that you can know with the function sizeof(), I think it's because of that. But correct me if it's not that.
This answer describes the method the OP is specifically trying to avoid.

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