Strncat() not doing the "cat" part

Projects Programming
1

Having an issue with strncat(). It doesn't seem to be doing what its supposed to. currWord never gets populated (line 66), at least not with character data. I have a similar program that uses it just fine.

This is reading in a file, parsing it, and trying to load it into an array.

This is the part that doesn't work

default:
          strncat(currWord, currChar,1);
          Serial.print(currChar);
          Serial.print('-');
          Serial.println(currWord);
          break;

Lots of prints for troubleshooting in here, sorry. I've been staring at it and trying nonsense for hours...

#include <SPI.h>
#include <SD.h>


// change this to match your SD shield or module;
const int chipSelect = 10;
int array_size = 0;
double steps[10][5];
File myFile;

void setup() {
  Serial.begin(9600);

  Serial.print("Initializing SD card...");

  if (!SD.begin()) {
    Serial.println("initialization failed!");
    return;
  }
  Serial.println("initialization done.");

  readSD();
}

void loop() {

}

void readSD() {
 
  int array_size = 0;
  int arraycount = 0;
  int col, row = 0;
  char currChar;
  char currWord[8] = {"\0"};

  // READ FROM CARD

  myFile = SD.open("1.txt");
  if (myFile) {

    // READ VALUES TO ARRAY

    while (myFile.available()) {
      currChar = myFile.read();
      
      //Serial.print(currChar);
      switch (currChar) {
        case ',':
         // Serial.print(currWord);
          //Serial.print(',');
          steps[row][col] = atof(currWord);
          strcpy(currWord,"\0");
          col++;
          break;
        case '\r':
         // Serial.println(currWord);
          steps[row][col] = atof(currWord);
          strcpy(currWord,"\0");
          row++;
          col=0;
          myFile.read();
          //Serial.println();
          break;
        default:
          strncat(currWord, currChar,1);
          Serial.print(currChar);
          Serial.print('-');
          Serial.println(currWord);
          break;
      }
    }
  }
  array_size = row;
  myFile.close();
  Serial.println("file closed");
  for (int i = 0; i <= row; i++){
    for (int j = 0; j <= 5; j++){
      Serial.print(steps[i][j]);
      Serial.print(',');
    }
    Serial.println();
  }
}
2

this is the data in the file

1,3.555,1,1,76
2,3.675,0,1,76
3,7,2,2,0
4,2.755,1,1,83
5,2.295,1,0,83
6,2.875,0,0,83
7,2.295,0,1,83
8,1.410,0,1,90
9,1.410,1,1,90

3

this is the output (it wouldn't all paste in for some reason. currWord seems to have non character data that won't copy/paste. I can see the squares in the serial monitor.

Initializing SD card...initialization done.

1-

3-

.-

5-

5-

5-

1-

1-

7-a

6-a

2-

3-

.-

6-

7-a

4

"currChar" is not a C-string, as the function str(n)cat requires (it expects a pointer, not a value, as the source string argument).

This works:

void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  char currWord[10] = {0};
  char currChar[] = "a";
  strncat(currWord, currChar, 1);
  Serial.print(currChar);
  Serial.print('-');
  Serial.println(currWord);
}

void loop() {
  // put your main code here, to run repeatedly:

}

Or this:

void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  char currWord[10] = {0};
  char currChar = 'a';
  strncat(currWord, &currChar, 1);
  Serial.print(currChar);
  Serial.print('-');
  Serial.println(currWord);
}

void loop() {
  // put your main code here, to run repeatedly:

}
5

Thanks for the response.
to make sure I understand...
this
char currChar[] = "a";
makes currChar a pointer because the double quotes makes a String and setting it = is really establishing a pointer to it, even though currChar wasn't defined as a pointer? Why does it need to be declared as an array?

char currChar = 'a'; is setting a single character variable which is holding only the character a, and then has to be dereferenced as &currChar to supply its address to the function?

OF course I am reading in 1 char at a time via the .read method of File so I assume the 2nd example is the more appropriate one to use.

6

not the same as passing a c-string which is an array of chars terminated by a Nul '\0'.

a more conventional approach is to read a buffer, the read returns the number of chars read, whether the size of the buffer, something less or 0 at EOF.

chars can then be processed as desired, one at a time or in some blocks possibly delimited by a termination char (e.g. \n)

7

An array name is a pointer, which is required for strncat().

char currChar[] = "a"; makes currChar a pointer because the double quote

No, the square brackets declare an array.

8

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