Find the number of items in an array of Strings

Projects Programming
1

Hi everyone

I have a project that generates random sentences. There are three String arrays that contain words and they are printed to an LCD in a standard template.

Right now I'm having to manually count how many elements I have in each array so I can set the upper limit for the random number generator. I thought I could use sizeof() but that just returns the number of bytes, not the number of String items in the array.

Is there a function I can use to achieve this? Or if there is not one build in how can I write one?

Many thanks

2

Maybe show us some examples of the arrays.

Using the String class can cause hard to find bugs caused by memory corruption. Better to use null terminated character arrays (strings).

3

Show us your use of sizeof().

4

The number of elements in an array is the number of bytes in the array, divided by the size of a single element.

sizeof (array) / sizeof (array [0])
5 
#include "Arduino.h"

int getArrayLen();
int getArrayLen(String array[]);


String cmds[] = { "c1","c22", "c333", "c4444", "c55555"};


void setup() {
   Serial.begin(9600);
   Serial.println((String)"getArrayLen(): "+getArrayLen());   // Correctly returns 5
   //
   Serial.println(getArrayLen(cmds));

}

// The loop function is called in an endless loop
void loop() {
   //Add your repeated code here
}

// Generic function
int getArrayLen() {
   return (int)sizeof(cmds)/sizeof(cmds[0]);
}


// Specific function
int getArrayLen(String array[]) {
   int arbitrary = 10;
   for (int i = 0; i < arbitrary; ++i) {
      Serial.println((String)"array: "+ *array);
      array = array +1;
   }
   return 0;
}
6

These are slightly modified versions of the arrays I have. The elements aren't all the same size so I can't use sizeof (array) / sizeof (array[0])

String commands[] { "Go away",
                    "Leave now",
                    "Bog off"};
                    
String descriptor[] { "silly",
                      "daft",
                      "naughty",
                      "dirty"};
                      
String insult[] { "thing",
                  "person"};

I have tried the following but it just returns the number of bytes.

Serial.print(sizeof(insult));
7

Don't use code to find the length. That uses memory. sizeof() operates at compile time, and generates no code. Instead it produces a constant that the compiler can use in place of a programmer supplied constant. Thus there is no memory usage penalty.

8

pianomatt:
These are slightly modified versions of the arrays I have. The elements aren't all the same size so I can't use sizeof (array) / sizeof (array[0])

String commands[] { "Go away",

"Leave now",
                    "Bog off"};
                   
String descriptor[] { "silly",
                      "daft",
                      "naughty",
                      "dirty"};
                     
String insult[] { "thing",
                  "person"};




I have tried the following but it just returns the number of bytes.

Serial.print(sizeof(insult));

Sure you can!

String insult[] { "thing",
                  "person"};
                   size_t INSULTSIZE = sizeof ( insult) / sizeof ( insult[]);
9

TheMemberFormerlyKnownAsAWOL:
The number of elements in an array is the number of bytes in the array, divided by the size of a single element.

sizeof (array) / sizeof (array [0])

Okay, this works! Thanks.

I don't understand how it works though, because each element is potentially a different length. I have some with only 3 letters, others much longer.

10

sizeof String is a constant - six, IIRC.