How would I "concatenate" a number by a float variable?

I already learned prior about concatenating strings together. I am now trying to apply this information to multiplying together a float variable and a basic number. There is other stuff going on in my program so I will paraphrase my code with notes:

//GLOBAL VARIABLES

int yellowLED = A1;
int numYellowBlinks;
int yellowOnTime=500;
int yellowOffTime=500;
float yellowLEDVolts;


void setup(){
  //INTIALIZES PORTS  

  pinMode (redLED, OUTPUT);
  pinMode (yellowLED, OUTPUT);
  Serial.begin(2000000);

  //BEGINS SERIAL PROMPTS

  Serial.println("How Many Times Do You Want the Yellow LED to Blink? ");   //SERIAL: Output Message 

  while (Serial.available()==0 ){                                           //SERIAL: Wait for User Input
      }                                                                         
    numYellowBlinks = Serial.parseInt();                                    //SERIAL: Read and Assign Data

  Serial.println("");                                                       //SERIAL: Break Line

  Serial.println("How Many Volts Would You Like to Use?");                  //SERIAL: Output Message

  while (Serial.available()==0){                                            //SERIAL: Wait for User Input
    }
  yellowLEDVolts = Serial.parseFloat();                                     //SERIAL: Read and Assign User Input

  Serial.println("");


}


void loop(){

  for (int j=1; j<=numYellowBlinks; j=j+1){           //Blinks LED # of Times
    Serial.print("You are on Blink #: ");               //SERIAL: Output Message
    Serial.println(j);                                               //SERIAL: Output Data
    
    analogWrite (yellowLED, 51*yellowLEDVolts);  //Applies analog voltage
    delay (yellowOnTime);               //Creates a delay (in MS) from global int var 'yellowOnTime'
    analogWrite (yellowLED, 0);         //Applies analog voltage off
    delay (yellowOffTime);              //Creates a delay (in MS) from global int var 'yellowOffTime'

}

The video series I'm watching is going over how to convert digital integer ON/OFF (aka 0 or 5volts) into analog 0-255 (0-5volts). In his example, he enters the 0-255 number directly where "51*yellowLEDVolts" is in in the bottom section of my code loop and it seems to work. However, I wanted to add a global float variable so I can prompt the user to enter number of volt number through the serialPort and have it automatically multiply the value by 51 in order to change the brightness of the circuit. But it does not work. It won't even turn on unless it is exactly 0 or 5 volts. Why?

So I figured it out. There was not much wrong with my code. I was using the "analog i/o" marked section of the board for the circuit which I thought would be correct. But it instead needed ports 7 and 6 which then got it working. I only intuitively figured this out because of my outside research into other projects that use PWM signals more frequently and I know it is a way of dynamically changing a current.

So as far as the hardware I/O is concerned on the Arduino Uno; do all digital ports handle PWM as well? or are some ports only limited to ON/OFF (HIGH,LOW)? If so, why would you not just always use a dynamic range?

And for the analog ports A0-A5 -are they only used for reading analog circuit values?

You might want to print out the values after you read them in so you can see what is wrong. Your variable 'yellowLEDVolts' is most likely 0.0 which forces the LED to always be off.
When you call parseInt(), I believe it leaves the linefeed ('\n') char in the buffer which may be the problem. I personally don't like those functions.

You could also have the person enter a value between 0-255 directly, and then just write that to the pin rather than trying to convert it after input.

You are specifying A1 as an output. A0-A5 are analog inputs attached to the ADC (Analog/Digital Convertor). They take an INPUT between 0 an 5V and convert it to an integer between 0 and 1023. If you configure it as an output it behaves as a digital pin, LOW (0v) or HIGH (5V).

The analogWrite() function is intended to take a value between 0 and 255 and convert into a PWM (Pulse Width Modulation) signal between 0 and 5V on specified pins. Not all pins support PWM and none of the A0-A5 pins do. If you look closely at the board you'll see a tilde (~) next to the digital pins that do support analogWrite().

You need to spend some time familiarizing yourself with PWM. Google Arduino PWM secrets and you'll find a great discussion on it's use. Read that and make a few small changes to your code vis-a-vis the pin you use for yellowLED and I think you'll fing your code works.

DKWatson:
[...]. Not all pins support PWM and none of the A0-A5 pins do. If you look closely at the board you'll see a tilde (~) next to the digital pins that do support analogWrite(). [...].

Not all pins support hardware timers (TC0, TC1, and TC2) based PWM signals except digital pins ~3, ~5, ~6, ~9, ~10, and ~11 of UNO. However, all digital pins including A0-A5 can be used to deliver software generated PWM signals as is done by the Servo.h Library.