Converting potentiometer value to string for GLCD display - arduino

Question

I'm trying to display a potentiometer's value on an Adafruit ST7565 GLCD. My serial monitor is giving me values between 1.62-1.67, while the GLCD ranges from -20,000 to +20,000. I'm not sure whether the arithmetic/data type is wrong or whether I am allocating memory improperly for the "sprintf" conversion.
#include "ST7565.h"
#include "stdlib.h"
char buffer[5];
int ledPin = 13; // LED connected to digital pin 13
char str[8];
// the LCD backlight is connected up to a pin so you can turn it on & off
#define BACKLIGHT_LED 10
// pin 9 - Serial data out (SID)
// pin 8 - Serial clock out (SCLK)
// pin 7 - Data/Command select (RS or A0)
// pin 6 - LCD reset (RST)
// pin 5 - LCD chip select (CS)
ST7565 glcd(9, 8, 7, 6, 5);
#define LOGO16_GLCD_HEIGHT 16
#define LOGO16_GLCD_WIDTH 16
void setup() {
Serial.begin(9600);
// turn on backlight
pinMode(BACKLIGHT_LED, OUTPUT);
digitalWrite(BACKLIGHT_LED, HIGH);
// initialize and set the contrast to 0x18
glcd.begin(0x18);
glcd.display(); // show splashscreen
delay(3000);
glcd.clear();
Serial.println(" ");
digitalWrite(BACKLIGHT_LED, HIGH);
glcd.drawstring(0,0," ");
glcd.display();
glcd.clear();
}
void loop() {
// read the input on analog pin 0:
int sensorValue = analogRead(A0);
// Convert the analog reading (which goes from 0 - 1023) to a voltage (0 - 5V):
float voltage = sensorValue * (5.0 / 1023.0);
// print out the value you read:
Serial.println(voltage);
digitalWrite(BACKLIGHT_LED, HIGH);
Serial.println(voltage);
sprintf(str,"%d",voltage); // converts to decimal base.
glcd.drawstring(0,0,str);
glcd.display();
delay(500);
glcd.clear();
}
Any insight is appreciated. I don't have much formal programming experience, so linking a tutorial about data types won't be of any use. I need to see a specific example like this worked out to truly understand.

Answer 1

You used %d to print out a float; this is undefined behaviour (in your case, it probably dumped out the integer representation of some part of the float's bit sequence).
Instead of using sprintf (since sprintf(..., "%f", val) is reportedly broken on Arduino), use dtostrf:
dtostrf(voltage, 0, 2, buf);
Also, if you're interested, you can see how Arduino prints floats here.