Concatenate multiple String of same array

[abderraouftn]

I want to concatenate multiple String of same array
For example
String num[20];
String con;
num[1]="ghjjvfvj";
num[2]="478gbnn";
Con=num1+num2;
How can I do that correctly;

[anon73444976]

@abderraouftn

Con=num1+num2

Did you mean

con = num[1] + num [2];

Please remember to use code tags when posting code

[abderraouftn]

Yes

[anon73444976]

And ..?

(What's wrong with num [0] ?)

[abderraouftn]

I just gave example no matter num[0] num [20]
Just an example

[anon73444976]

And... what's the problem?
(Note: there is no num [20] )

[abderraouftn]

What's the difference between
Con=String num[1]+ String num[2];
Con=num[1]+ num[2];

[bradley2020]

I'm going to be that annyoing guy, but I would strongly recommend using char[] instead of String for a variable type when using strings. So you could just do this instead:

#define maxLength 10

char num[20][maxLength]; //20 strings, max of 10 chars including '\0'

snprintf(num[0], maxLength, "ghjjvfvj");
snprintf(num[1], maxLength, "478gbnn");

char concatenatedString[2*maxLength];
snprintf(concatenatedString, 2*maxLength, "%s%s", num[0], num[1]);

or, if you would like to make a function to deal with this for you, you could use:

void myConcatenate(const char *str1, const char *str2, char *result, size_t resultSize)
{
    snprintf(result, resultSize, "%s%s", str1, str2);
}

which you could then call using:

char myNewString[20];
myConcatenate("hello ", "world", myNewString, sizeof(myNewString));

Serial.print(myNewString); 

[drmpf]

@abderraouftn

String num[20];
String con;
num[1]="ghjjvfvj";
num[2]="478gbnn";
Con=num1+num2;

You will get a lot of discussion about the 'correct' way to do that but this code just works

String num[20];
String con;
num[1]="ghjjvfvj";
num[2]="478gbnn";
Serial.println(num[1]);
Serial.println(num[2]);
con=num[1]+num[2];
Serial.println(con);

Output

ghjjvfvj
478gbnn
ghjjvfvj478gbnn

As much as I think Strings are useful, what you are doing does not seem usual. In what context (what larger sketch) do you need to do that concatenation.

[drmpf]

This code may be better for your purpose if the 'String [ ]' is constant.

char num[][9] = {"","ghjjvfvj","478gbnn"}; // 3 char *, num[0] num[1] num[2]
Serial.println(num[1]);
Serial.println(num[2]);
String con = num[1];
con += num[2];
Serial.println(con);

Be sure to turn all the compiler warnings on and look out for
warning: initializer-string for array of chars is too long
if the [9] is not large enough for the longest string AND its terminating '\0'
The compile will still work (being C/C++ it assumes you know what you are doing) but you will get odd results due to missing '\0' in some of the strings.

And if you are going to use Strings a lot check out my Taming Arduino Strings tutorial