passing String as a parm into a Function

Projects Programming
1

I can't figure out how to take a String input into a function? In the code below,

line 34

positionInside = message.indexOf(inputString); //passing in "parm1="

is interpreted as only getting the first character "p" (thus returning value of 3).

The answer I am looking for is a value of 8.

What am I doing wrong?

//                01234567890
String message = "/y?parm1=98";    //

int positionOutside = 0;
int positionInside = 0;

void setup () {
  Serial.begin(9600);
  positionOutside = message.indexOf('parm1=');
  // CASE #1)  returns 8
  Serial.println(positionOutside);
  
  String parmDoubleQuote = "parm1=";
  Function_Pass_String (parmDoubleQuote);

// String  parmSingleQuote = 'parm1='; //Error compiliing conversion from 'int to 'String is ambiguous
//  Function_Pass_String (parmSingleQuote);

}

void loop() {
} 

void  Function_Pass_String (String  inputString ) {

  positionInside = message.indexOf("parm1=");  
  // CASE #2)   returns 3   with double quote 
  Serial.println(positionInside);

  positionInside = message.indexOf('parm1=');  
  // CASE #3)    returns 8   with single quote 
  Serial.println(positionInside);

  positionInside = message.indexOf(inputString);  
  //  CASE #4)    returns 3   with String parm passed, stops on the first character, not the entire string 
  Serial.println(positionInside);
}
2

Hi, I always find confusing the use of the String type, and given it takes quite a bit of memory I usually use only char arrays.

The concept you are looking for are "pointers" that's the way to pass strings or any array as function parameter. A pointer is basically the address in the memory of the first element of the array. Then you can access the rest of the elements by adding to the pointer.

Here a very simple example of how pointers works:

char command[]="time"; // defined this way command will have 5 elements "t", "i", "m", "e" and "/0" which is the end of string indicator

function printCharN(*char text, int N){
//this function prints the Nth character of the string text

//To make sure N is not longer than the string, we go trough it looking for a '/0'
int i=0;
int textTooShort=1;
while(*text!='/0){
if(i==N){
Serial.println(*text+N);
textTooShort=0;//we record that we found the charecter
break;
}
text++;//this advance the pointer to the next character of the string
i++;
}
if(textTooShort==1){
Serial.println("Text too short");
}
}

Make sense?

3

In setup() and elsewhere in code like

positionOutside = message.indexOf('parm1=');

Watch the use of single ' versus "

You use single quote for a char, a double quote for a string.

'A' and "A" are not the same thing

4

First Thanks for the quick reply.

What is confusing is indexOf seams to behave the opposite based on the rules of single and double quotes.
https://www.arduino.cc/en/Reference/String

Double quotes should treat like a string and return a value of 8, but returns 3 (like a solo character search)

  positionInside = message.indexOf("parm1=");  
  // CASE #2)   returns 3   with double quote treats like looking for just a character 'p'
  Serial.println(positionInside);

and

Single quotes should treat the argument like a solo character but in fact returns 8 like a string (which is what I want).

  positionInside = message.indexOf('parm1=');  
  // CASE #3)    returns 8   with single quote index of treats like a string "parm=1'
  Serial.println(positionInside);

Here is my unsuccessful attempt to pass in a character array

//                01234567890
String message = "/y?parm1=98";    //

int positionOutside = 0;
int positionInside = 0;

void setup () {
  Serial.begin(9600);
  positionOutside = message.indexOf('parm1=');
  // CASE #1)  returns 8
  Serial.println(positionOutside);
  
  char parmDoubleQuote[7] = "parm1="; 
  Function_Pass_String (parmDoubleQuote);
}

void loop() {
} 

void  Function_Pass_String (*char  inputString ) {
  positionInside = message.indexOf(inputString);  
  // I want the value of 8
  //  CASE #4)    returns 3   with String parm passed, stops on the first character, not the entire string 
  Serial.println(positionInside);
}

which is having a problem with *char as the input

How do I set up the function to return a value of 8 for .indexOf?

Any help would be greatly appreciated. . .

5 
String message = "/y?parm1=98";

The string "parm1" DOES start at position 3 (the / is in 0, the y is in 1, the ? is in 2, and the p is in 3).

If you expect the function to return 8, it is your expectations that are WRONG!

6

Agreed, but after research there appears to be a bug in the String function. As I said ,the indexOf() treats single quotes as doubles and doubles as singles.

Please show me an example of this

positionInside = message.indexOf('parm1='); // returns 8, the ending of the string

by passing a variable instead of an explicit single quoted value. I can't figure it out.

Perhaps it can't be done.

something like this

positionInside = message.indexOf(aStringVarible); // returns 8, not 3!

The solution I am going with is just to add 5 to the result. It works.

7

Single quote denotes a character literal which is composed of a constant character. It is represented by the character surrounded by single quotation marks.

--> Single-quotes denote a character literal. Double-quotes denote a string literal.

'parm1=' is not a legit C++ expression for a string. What do you expect?

8

The solution I am going with is just to add 5 to the result. It works.

Utter nonsense.

9

come on...

consider this code

String message = "/y?parm1=98";

void setup() {
  Serial.begin(115200);
  int positionSingleQuote = message.indexOf('parm1=');
  int positionDoubleQuote = message.indexOf("parm1=");
  Serial.println(positionSingleQuote);
  Serial.println(positionDoubleQuote);

  Serial.println((char) 'parm1=');
}

void loop() {}

when you try to compile you get:

[color=orange]/var/folders/xk/mwl4yt3j5rsgdmgmgjlqsk7w0000gn/T/arduino_modified_sketch_875666/sketch_nov17a.ino:5:45: warning: [b][color=red]character constant too long for its type[/color][/b]
   int positionSingleQuote = message.indexOf('parm1=');
                                             ^
/var/folders/xk/mwl4yt3j5rsgdmgmgjlqsk7w0000gn/T/arduino_modified_sketch_875666/sketch_nov17a.ino:10:25: warning: [color=red][b]character constant too long for its type[/b][/color]
   Serial.println((char) 'parm1=');
                         ^
/var/folders/xk/mwl4yt3j5rsgdmgmgjlqsk7w0000gn/T/arduino_modified_sketch_875666/sketch_nov17a.ino: In function 'void setup()':
/var/folders/xk/mwl4yt3j5rsgdmgmgjlqsk7w0000gn/T/arduino_modified_sketch_875666/sketch_nov17a.ino:5:53: [color=red][b]warning: overflow in implicit constant conversion[/b][/color] [-Woverflow]
   int positionSingleQuote = message.indexOf('parm1=');[/color]

That should be enough to tell you to start thinking at what you do. why are you ignoring the compiler warnings?

But as it's just a warning so it lets you compile et run the code. indeed you get

8
3
=

and then it all starts to makes sense

with the single quote you get the position 8 because the compiler (as evidence with the last print which shows '=') has transformed your illegal string 'parm1=' into just '=' --> so you are asking what is the index of the character '=' into "/y?parm1=98" which is indeed at index 8

if you use the correct notation, then you get 3 - because indeed 3 is the index in "/y?parm1=98" where "parm1=" starts.

don't try to mess around with adding or subtracting stuff to fix your programing mistakes or claim "after research there appears to be a bug in the String function"... that's total nonsense.

As often said - the bug had its hands on the keyboard and was staring at the screen... :slight_smile: (gentle joke)

Just program the right way.

if you want to look for "parm1=" and then point at the starting position of "98" then the right way to do this to meet the general case is

String message = "/y?parm1=98";
const char param1_string[] = "parm1=";

void setup() {
  Serial.begin(115200);
  int position = message.indexOf(param1_string) + strlen(param1_string);
  Serial.print("Number will start at position "); Serial.println(position);
  Serial.print("Number is "); Serial.println(message.c_str() + position); // message.c_str() = pointer to the array that contains a null-terminated sequence of characters
}

void loop() {}

and obviously if you could drop the String class altogether that would be even better for your memory footprint: the code above uses 3506 bytes and 262 of RAM.

this does the same thing:

char message[] = "/y?parm1=98";
const char param1_string[] = "parm1=";

void setup() {
  Serial.begin(115200);
  int position = strstr(message, param1_string)-message+strlen(param1_string);
  Serial.print("Number will start at position "); Serial.println(position);
  Serial.print("Number is "); Serial.println(message + position);
}

void loop() {}

and uses 2150 bytes of flash (you save 1356 bytes) and 246 bytes of RAM. (as tested on a MEGA)

of course in real life you would just do:

char message[] = "/y?parm1=98";
const char param1_string[] = "parm1=";

void setup() {
  Serial.begin(115200);
  int value = atoi(strstr(message, param1_string)+strlen(param1_string));
  Serial.print("Number is "); Serial.println(value);
}

void loop() {}

and have the value of your param1 in an integer.

Makes sense ?