Sprintf - conversion specifications (%n$)

Projects Programming
1

Dear,
In the sprinf conversion specification %n$, n is an integer, e.g. $2$
n is 'hard coded' in the program.
I want to use a variable instead to print a variable number of digit to a LCD.
I tried to use string and constr char. No success.
Ideas ar welcome. Thanks.

2

Hello

%n is used to store in a variable the amount of characters that were written in the string by sprintf. This is not what you want, and it's rarely used.

You must use %d or similar, depending on the exact type of the variable.

See this : https://www.cplusplus.com/reference/cstdio/printf/

1 Like
3

I am not entirely clear what you are trying to do, but you can specify the width parameter in a variable like this

void setup()
{
  Serial.begin(115200);
  char * value = "1234567890";
  for (int n = 1; n < 8; n++)
  {
    Serial.printf("value is %.*s\n", n, value); //ESP32
  }
}

void loop()
{
}

The output

value is 1
value is 12
value is 123
value is 1234
value is 12345
value is 123456
value is 1234567
4

Which board are you using -- UNO or ESP32?

5

OK, nice trick, learned (I hope) something today.

I was gonna suggest using sprintf to make the format string ahead of its use in the Serial.print statement.

Hey wait… Serial.printf? Learned something else. :expressionless:

edit: Oh, ESP32, never mind.

a7

a7

6

I'm using
#include <LiquidCrystal_I2C.h>

and a clipping of the code:

int rotEnPos = 0;
char cstr[16];
sprintf(cstr, "%3d", rotEnPos);      //print 3 digits
lcd.setCursor(4, 1);                  //starting at bottom row position 4
lcd.print(cstr);

This works nicely, except that the three 3 digits are fixed
@UKheliBob: it seems that the class LiquidCrystal_I2C has no member printf

7

It will if it inherits from the Print class and you're on a platform whose Print class supports printf (i.e. ARM or ESP).

8

This is the game I had in mind:

char format[32];
char buffer[64];

int anInteger = 777;
byte digits;

void setup() {
  Serial.begin(9600);
  Serial.println("Hello printf Wolrd!\n");

//  Serial.printf("%05d", anInteger); no printf in Serial? didn complle.

  digits = 4;
  sprintf(format, "like %c%c%d%c", '%', '0', digits, 'd');
  sprintf(buffer, format, anInteger);
  Serial.println(buffer);
  Serial.print("\n\n");

  digits = 6;
  sprintf(format, "like %c%c%d%c", '%', '0', digits, 'd');
  sprintf(buffer, format, anInteger);
  Serial.println(buffer);
  Serial.print("\n\n");

  digits = 7;
  sprintf(format, "like %c%c%d%c", '%', '0', digits, 'd');
  sprintf(buffer, format, anInteger);
  Serial.println(buffer);
  Serial.print("\n\n");
}

void loop() {
}

HTH

a7

9

Then you must use * as shown by HeliBob ( and as described in the link I gave )

sprintf(cstr, "%*d", 3, rotEnPos);

10

As the comment in the code says, it is an ESP32

11

sorry missed that comment. I'm using a Nano

12

Building the format string with sprintf() seems to be the obvious choice. I have seen that before. Sketch from @alto777 of Reply #8 in Wokwi: https://wokwi.com/arduino/projects/322451529929851476

13

/alto777 blushes.

THX!

a7

14

Dear,
It took me some time to digest your answers.
Now it works.
Tx for your support!

15

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