How to extract value from a string: thingspeak​.getFieldV​alue(3));

Hi,

I built, with a NodeMCU ESP8266, a receiver of the data transmitted by a weather station connected to ThingSpeak, based on the project by Daniel Eichhorn (http://blog.squix.ch).

From the string "display-> drawString (40 + x, 15 + y, thingspeak.getFieldValue (3) + "m/s");" of the sketch I get the value of the wind speed expressed in m/s, while I would need the value expressed in Km/h. I can't find a solution to "extract" the numeric value from the string <thingspeak.getFieldValue (3)> to then multiply it by 3.6, and get the value in Km/h.

Could anyone suggest me a solution?

Thank you.

Giuseppe

[Christopher Stapels]

create a variable before that line.

float myFieldValue = thingspeak.getFieldValue (3) ;
myConvertedValue = myfieldValue * conversionFactor;

then

display-> drawString (40 + x, 15 + y, String(myConvertedValue) + "m/s");

[Peppino Lanni]

First of all I want to thank you, Christopher, for your prompt reply.

I corrected my sketch exactly as you suggested, but from Arduino IDE I get this error message:

"cannot convert 'String' to 'float' in initialization".

To provide more information about my sketch, these are the lines to show the data on the display:

void drawThingspeak1(OLEDDisplay *display, OLEDDisplayUiState* state, int16_t x, int16_t y) {

display->setTextAlignment(TEXT_ALIGN_CENTER);

display->setFont(ArialMT_Plain_10);

display->drawString(64 + x, 0 + y, "Kalena");

display->setFont(ArialMT_Plain_16);

float myFieldValue = thingspeak.getFieldValue (3);

myConvertedValue = myFieldValue * 3.6;

display-> drawString (40 + x, 15 + y, String(myConvertedValue) + "m/s");

//display->drawString(40 + x, 15 + y, thingspeak.getFieldValue(3)+ " m/s");

I hope you can help me.

Have a good night.

[Christopher Stapels]

Is the error on this line?

float myFieldValue = thingspeak.getFieldValue (3);

?

It is probably becasue getFieldValue(3) returns a string. So read it into a string and then use the .toFloat function to turn it into a float. Then convert it, then trun it back to a string to display.

https://www.arduino.cc/reference/en/language/variables/data-types/string/functions/tofloat

[Peppino Lanni]

Thank you very much.

I wrote these line of code and now it works:

String stringVal = thingspeak.getFieldValue(3);

float myFieldValue = stringVal.toFloat();

float conversionFactor = 3.6;

float myConvertedValue = myFieldValue * conversionFactor;

display-> drawString (40 + x, 15 + y, String(myConvertedValue) + " Km/h");