Put an array in the end of another array C++

Question

Normally it's a question about a buffer with a null-terminated string, but we can extrapolate it to a general case.

I have a big array of a fixed length, let's say 10:

char outputArray[10] = {'-','-','-','-','-','-','-','-','-','-'};

And I have some other (Edited: smaller) array (in my case it's a char buffer with null terminator) with a variable length. Let's say it's a buffer of 6 elements, but the actual length is indicated by another variable.

char inputArray[10] = {'h','i','/0',...some other values, i'm not interested in};
int arrLength = 2; // For my task it means a strlen(inputArray);

How to put a small array at the end of a big array, to get this:

outputArray = {'-','-','-','-','-','-','-','-','h','i'} // the null terminator isn't important, it's not about the strings, it's about arrays.

Constraints:

I can't use std, so only "native" solutions (it's for Arduino)
C++11
Code should be memory and time efficient (some elegant algorithm without too much loops and too much temporary variables or calculations please)

Thank you in advance

Edited:

Thank to @ThomasWeller for an answer. I have a small precision though. What if I need to clean all the elements before the inserted array?

For example I had some garbage

{'a','k','$','-','n','"','4','i','*','%'};

And I need to get

{'-','-','-','-','-','-','-','-','h','i'};

Do I need 2 loops? First to reset an array and the second one to set the actual result?

"I have a big array of a fixed length, let's say 10" Hmm, interesting definition of "big"... (I'm not the downvoter.) — Intelligent Shade of Blue
– Intelligent Shade of Blue, Commented Sep 10, 2022 at 16:35
outputArray[8] = inputArray[0]; outputArray[9] = inputArray[1]; — Eljay
– Eljay, Commented Sep 10, 2022 at 16:38
memcpy(outputArray + sizeof outputArray - arrLength, inputArray, arrLength); — Intelligent Shade of Blue
– Intelligent Shade of Blue, Commented Sep 10, 2022 at 16:45

Thomas Weller · Accepted Answer · 2022-09-10 17:01:52Z

2

It can be done with a single for loop and a single variable:

  for (char i=0; i<arrLength; i++)
  {
    outputArray[10-arrLength+i] = inputArray[i];
  }

If you make arrLength a char instead of an int, this will even save you 2 bytes of memory ;-)

Use memset() to set all memory to an initial value and then memcpy() the contents at the end:

char output[10];
char input[10]  = "hi\0------";
char arrLength = 2;
void setup() {
  memset(output, '-', 10);  // "----------"
  memcpy(output+10-arrLength, input, arrLength);
  Serial.begin(9600);
  Serial.write(output, 10);
}
void loop() { }

edited Sep 10, 2022 at 17:01

answered Sep 10, 2022 at 16:38

Thomas Weller

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7 Comments

GZstudio GZstudio Over a year ago

Thank you for your response. I've edited my post to ask also how can I possibly reset all the values before the actual result? Thank you

Thomas Weller Over a year ago

@GZstudioGZstudio: you can use memset(). Adapted the answer

GZstudio GZstudio Over a year ago

You're awesome dude! :) As I see, memset is more efficient than a loop, am I right?

Thomas Weller Over a year ago

@GZstudioGZstudio: I don't see a statement on the performance of memset. At least it should not be slower than a loop. Note that this might not apply, since Arduino does not have SSE or MMX instructions.

Thomas Weller Over a year ago

How important is performance and why?

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