using sprintf function in arduino

Using Arduino Programming Questions
#1

So I am inserting to database using mysql connector,

I have been using but I was suggested to use char buffer, and sprintf() to populate it will not fragment memory the way that Strings can.

so I took a look in mysql arduino example sketch and it uses this:

const char INSERT_DATA[] = "INSERT INTO test_arduino.temps VALUES (%s, NULL)";

INSERT_DATA i.e the query storing variable is declared char of indefinite length !

and later used sprintf 

char query[64];
char temperature[10];
dtostrf(value_read, 1, 1, temperature); 
sprintf(query, INSERT_DATA, temperature);

looking at the insert query it has %s so does that mean in the call to function sprintf the value of temperature is inserted to INSERT_DATA and then it is fed to the query variable which is the first in call to sprintf ?

thats what I don't get ? I am not cpp programmer I have been programming with python and still thinking like a python programmer , so any help would be greatly appreciated

#2

so does that mean in the call to function sprintf the value of temperature is inserted to INSERT_DATA

Not exactly.

In that example, INSERT_DATA is the format statement. The format statement contains one format specifier, so one additional argument is needed (for a total of three). The format specifier is an s, so the third argument needs to be a string.

temperature is a string, so it is appropriate as the third argument.

What happens, then, is that literal parts of the format statement are copied to the output buffer, until a forma specifier is encountered. That means that "INSERT INTO test_arduino.temps VALUES (" will be copied to query.

Then, the appropriate value is substituted in place of the format specifier - the string temperature is added to the end of query.

Finally, the remaining literal characters, ", NULL)", in the format statement are added to the end of query.

#3

aha, that means It would be much convenient to use sprintf then using String and concatenating then with values from other sensors ...

phewww.

but of temperature to be a string I would have to convert any string or float to a string ? you meant char datatype ?

#4

but of temperature to be a string I would have to convert any string or float to a string ? you meant char datatype ?

There is a format specifier for floats (%f), but the Arduino team disabled support for that format specifier. (It can be re-enabled, relatively simply.) So, yes, you need to convert any float to a string using some other means.

You do not need to convert strings to strings. Yes, a string is a NULL terminated array of chars.

#5

thank you , what do you mean by “NULL terminated array of chars.” ?

#6

seaurchin: thank you , what do you mean by "NULL terminated array of chars." ?

I mean that you really, REALLY, need to buy a C programming book, and spend some time reading.

#7

char str = {‘s’,‘t’,‘r’,’\0’};

‘\0’ is the null terminator. It is used by the string library to know where the end of the string is.

#8

PaulS: I mean that you really, REALLY, need to buy a C programming book, and spend some time reading.

yes thats a thing I am going to do first. I am just restricted by lack of time and living alone doing every thing by myself. Its just that in my list of priorities I have a huge pile.

#9

Hi,

Until you get around to buying that book (Kernighan & Ritchie is highly recommended), this may be useful: www.cplusplus.com/doc/tutorial/ntcs/ --Michael

#10

I leanred from office colleague I can use stringStreams !

#include <iostream>
#include <sstream>
#include <stdio.h>
#include <string.h>

using namespace std;

int main()
{
    float outdoorTempInC = 15;
    int outoorHumidity = 23;
    float indorTempinC = 20;
    int humidity = 22;
    int val = 400; // soil sensor reading
    int avgVal = 38; // avaerage of 10 values of soils sensor
    char statusMsg[50] = {"Hello World"} ;
    std::stringstream sqlQuery;
    
    sqlQuery  << "INSERT INTO arduinoSensorData.sensorLog (out_temperature,  " 
        << "out_humidity,  drwngRoom_temperature, drwngRoom_humidity, "
        << "pot1_soilMoisture, pot1_avg_SoilMoisture, wateringPot1) VALUES ('" 
        << outdoorTempInC << "', '" << outoorHumidity << "', '" << indorTempinC
        << "', '" << humidity << "', '" << val << "', '" << avgVal << "', '"
        << statusMsg << "');";

    cout << sqlQuery << "\n";
    char *mychar = new char[sqlQuery.str().length() + 1];
    strcpy(mychar, sqlQuery.str().c_str());
    cout << mychar << "\n";
    return  0;
}

now my only query is : is it performacne intuitive when used in arduino ?

#11

Your code example looks like it was compiled under Visual Studio and that's not going to work too well with the Arduino. First, the statement:

const char INSERT_DATA[] = "INSERT INTO test_arduino.temps VALUES (%s, NULL)";

the const keyword means you cannot change the content of that string at runtime. I doubt that's what you really want. In the example below, if newData[] is what you want to INSERT into the db, this shell test program might help you see how to build the string:

void setup() {

  Serial.begin(9600);
  Serial.println("Enter input data (20 chars max):");

}

void loop() {

  char insertData[100] = "INSERT INTO test_arduino.temps VALUES ('";
  char kbInput[21];
  int charsRead;

  if (Serial.available() > 0) {     // Something's in the Serial buffer...
    charsRead = Serial.readBytesUntil('\n', kbInput, sizeof(kbInput) - 1); // Leave room for NULL
    kbInput[charsRead] = NULL;      // Now it's a C string
    strcat(insertData, kbInput);
    strcat(insertData, "' NULL)");
    Serial.println("SQL string: ");
    Serial.println(insertData);
    Serial.println("\nEnter input data (20 chars max):");
  }
}

Obviously, you'll need to modify it, but it gives you a way to build the query string without using sprintf(), which uses more memory than needed. Make sure you have the input termination textbox set to Newline.

#12

econjack:
Your code example looks like it was compiled under Visual Studio and that’s not going to work too well with the Arduino.

I believe you didnt had a look at my previous reply .
I think I can use StringStream i.e.:

#include <sstream>

however I discovered it is not present in Arduino :confused:

I am quite puzzled :

/Users/seaurchin/Library/Arduino15/packages/arduino/hardware/avr/1.6.16/cores/arduino
already has a header file saying Stream.h ?

is it the same sstream I used at office or different one ?

I googled web and found this: arduino/libs/avr-stl/include at master · ddierickx/arduino · GitHub

how can I install it along with other exiting libraries ? I am on Mac OS Sierra.

#13

I think I can use StringStream

Why do you think that?

is it the same sstream I used at office or different one ?

Why would you think that stringstream would be defined in Stream.h? Did you even bother looking in Stream.h?

#14