String format replace by value in character in snprintf

Question

I have the following function piece of code in my ESP8266 based NodeMCU:

  snprintf ( temp, 800,
             "<html>\
  <head>\
    <meta http-equiv='refresh' content='25'/>\
    <title>NodeMCU DHT11 Sensor \and Relay Board</title>\
    <style>\
      body { background-color: #cccccc; font-family: Arial, Helvetica, Sans-Serif; Color: #000088; }\
      li { margin: 10px 0;}\
    </style>\
  </head>\
  <body>\
  <h1>Hello from NodeMCU!</h1>\
  <p>Temperature: %02d &#8451;<br>\
  Humidity: %2d %<br></p><ol>\
  <li><a href='/control?relay=5&state=%d'>Turn Relay 1 On\/Off</a>\
  <li><a href='/control?relay=4&state=%d'>Turn Relay 2 On\/Off</a></ol>\
    <p> Uptime: %02d:%02d:%02d </p>\
  </body>\
</html>",
             t, h, !digitalRead(5), !digitalRead(4), hr, min % 60, sec % 60
           );

I want to be able to replace text on with off and vice versa based on the state of pin which comes from digitalRead(5). So I don't have to write Turn Relay 1 On/Off and instead I should get the state using digitalRead(pinNum) and set the text on or off based on state.

Something simple could be to just use the ternary operator to pass the correct string into snprintf as parameter: `digitalRead(5) ? "on" : "off"` — CBroe, Mar 27 '17 at 11:39

score 0 · Answer 1 · edited May 23 '17 at 12:17

The ternary (conditional) operator is your friend here. You could treat it as an inline if-statement. The syntax looks like this

condition ? val1 : val2

The expression will yield a result depending on condition. If the condition is true it will yield val1, otherwise it will yield val2.

You can use this inside of sprintfs argument list to return a string depending on the pin state.

snprintf(temp, 800, 
    "... <li><a href='/control?relay=5&state=%d'>Turn Relay 1 %s</a><li> ... ", 
    !digitalRead(5), (digitalRead(5) ? "Off" : "On");

%s is a placeholder for a string and depending on the state of the pin 5 it will be replaced by "On" or "Off"

it's hard to imagine a library that offers `snprintf` but doesn't support `%s` in its format string... maybe you have a bug elsewhere in your program — M.M, Mar 31 '17 at 07:09

String format replace by value in character in snprintf

1 Answers1