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I am trying to run this code:

const int buttonPin = 7; 
const int ledPin13 = 13; 

int buttonState = 0; 
int lastButtonState = buttonState;
bool flag = true;

void setup() 
 // initialize the LED pin as an output:
 pinMode(ledPin13, OUTPUT);
 // initialize the pushbutton pin as an input:
 pinMode(buttonPin, INPUT);


void loop() 

 buttonState = digitalRead(buttonPin);


 if (buttonState = HIGH && lastButtonState != buttonState) 
 flag = !flag;
 if (flag)
 digitalWrite(ledPin13, LOW);
 else 
 digitalWrite(ledPin13, HIGH);
 
 

on this SimulIDE model:

<circuit reactStep="50" animate="0" type="simulide_0.1" noLinStep="10" noLinAcc="5" speed="1000000">

Node-16: 
<item labelrot="0" y="-204" valLabRot="0" valLabelx="0" valLabely="0" hflip="1" vflip="1" x="-220" labelx="-16" labely="-24" Show_id="false" objectName="Node-16" itemtype="Node" id="Node-16" rotation="0"/>

Arduino Uno-4: 
<item hflip="1" Show_id="true" valLabely="0" itemtype="Arduino" vflip="1" labelx="0" objectName="Arduino Uno-4" labely="-20" y="-244" Ser_Port="false" labelrot="0" rotation="0" Program="../../Google Drive/Active projects/SRL/Students/Damian2019/Simulation/20190409/noDelay_20190409/noDelay_20190409.ino.standard.hex" Ser_Monitor="false" id="Arduino Uno-4" valLabelx="0" Mhz="16" valLabRot="0" x="-148"/>

Resistor-3: 
<item Unit=" O" hflip="1" Show_id="false" valLabely="6" itemtype="Resistor" Show_res="true" vflip="1" labelx="-12" objectName="Resistor-3" labely="-24" y="-164" labelrot="0" rotation="-90" Resistance="100" id="Resistor-3" valLabelx="-16" valLabRot="0" x="-220"/>

Push-2: 
<item labelrot="0" y="-308" valLabRot="0" valLabelx="0" valLabely="0" hflip="1" vflip="1" x="-100" labelx="-16" labely="-24" Show_id="false" objectName="Push-2" itemtype="Push" id="Push-2" rotation="0"/>

Connector-5: 
<item hflip="1" Show_id="false" valLabely="0" itemtype="Connector" vflip="1" startpinid="Resistor-3-lPin" labelx="-16" objectName="Connector-5" labely="-24" y="-148" endpinid="Arduino Uno-4-GND0" enodeid="Circ_eNode-6" labelrot="0" pointList="-220,-148,-220,-108,-140,-108" rotation="0" id="Connector-5" valLabelx="0" valLabRot="0" x="-220"/>

Connector-11: 
<item hflip="1" Show_id="false" valLabely="0" itemtype="Connector" vflip="1" startpinid="Push-2-rnod" labelx="-16" objectName="Connector-11" labely="-24" y="-308" endpinid="Arduino Uno-4-V5V" enodeid="Circ_eNode-12" labelrot="0" pointList="-84,-308,44,-308,44,-148,4,-148" rotation="0" id="Connector-11" valLabelx="0" valLabRot="0" x="-84"/>

Connector-9: 
<item hflip="1" Show_id="false" valLabely="0" itemtype="Connector" vflip="1" startpinid="Push-2-lnod" labelx="-16" objectName="Connector-9" labely="-24" y="-308" endpinid="Node-16-0" enodeid="enode-15" labelrot="0" pointList="-116,-308,-220,-308,-220,-204" rotation="0" id="Connector-9" valLabelx="0" valLabRot="0" x="-116"/>

Connector-15: 
<item hflip="1" Show_id="false" valLabely="0" itemtype="Connector" vflip="1" startpinid="Arduino Uno-4-PD7" labelx="-16" objectName="Connector-15" labely="-24" y="-172" endpinid="Node-16-1" enodeid="enode-15" labelrot="0" pointList="-140,-172,-172,-172,-172,-204,-220,-204" rotation="0" id="Connector-15" valLabelx="0" valLabRot="0" x="-140"/>

Connector-17: 
<item hflip="1" Show_id="false" valLabely="0" itemtype="Connector" vflip="1" startpinid="Node-16-2" labelx="-16" objectName="Connector-17" labely="-24" y="-204" endpinid="Resistor-3-rPin" enodeid="enode-15" labelrot="0" pointList="-220,-204,-220,-180" rotation="0" id="Connector-17" valLabelx="0" valLabRot="0" x="-220"/>

PlotterWidget-13: 
<item modal="false" childrenRect="" normalGeometry="" baseSize="" geometry="" sizeIncrement="" windowOpacity="1" windowModified="false" enabled="true" maximumSize="" childrenRegion="" maximumHeight="200" inputMethodHints="0" mouseTracking="false" minimumSize="" frameGeometry="" sizeHint="" windowIconText="" locale="" minimumSizeHint="" height="200" isActiveWindow="true" x="0" accessibleName="" layoutDirection="0" autoFillBackground="false" width="200" windowFilePath="" windowModality="0" maximized="false" sizePolicy="" MinVolt="-500" fullScreen="false" windowTitle="" windowIcon="" maximumWidth="1000" objectName="PlotterWidget-13" toolTip="" toolTipDuration="-1" focus="false" MaxVolt="500" palette="" font="MS Shell Dlg 2,8.25,-1,5,50,0,0,0,0,0" whatsThis="" pos="" minimumWidth="200" minimumHeight="200" size="" focusPolicy="0" itemtype="Plotter" updatesEnabled="true" accessibleDescription="" y="0" rect="" frameSize="" minimized="false" acceptDrops="false" styleSheet="" cursor="" visible="false" statusTip="" contextMenuPolicy="1"/>

SerialPortWidget-14: 
<item modal="false" childrenRect="" normalGeometry="" SettingsProp="COM1,0,3,0,0,0" baseSize="" geometry="" sizeIncrement="" windowOpacity="1" windowModified="false" enabled="true" maximumSize="" childrenRegion="" maximumHeight="170" inputMethodHints="0" mouseTracking="false" minimumSize="" frameGeometry="" sizeHint="" windowIconText="" locale="" minimumSizeHint="" height="141" isActiveWindow="true" x="0" accessibleName="" layoutDirection="0" autoFillBackground="false" width="313" windowFilePath="" windowModality="0" maximized="false" sizePolicy="" fullScreen="false" windowTitle="Settings" windowIcon="" maximumWidth="450" objectName="SerialPortWidget-14" toolTip="" toolTipDuration="-1" focus="false" palette="" font="MS Shell Dlg 2,8.25,-1,5,50,0,0,0,0,0" whatsThis="" pos="" minimumWidth="0" minimumHeight="0" size="" focusPolicy="0" itemtype="SerialPort" updatesEnabled="true" accessibleDescription="" y="0" rect="" frameSize="" minimized="false" acceptDrops="false" styleSheet="" cursor="" visible="false" statusTip="" contextMenuPolicy="1"/>

</circuit>

and I expect the LED to be turned on-off everytime I push the button, however the result is unstable:

I would appreciate if you could help me know if there is something wrong with my code or circuit and I should expect the same behavior on an actual hardware, or this is a simulation issue?

P.S.1. All file can also be downloaded from here.

P.S.2. I'm using this example from Arduino.org website.

P.S.3. I also tried using an actual Arduino, also implementing the suggestions below. However the issue is not resolved. You may see the video here.

I identify a few problems here:

• The 100 Ohm pulldown resistor for the button is way too low. 1 kOhm to 100 kOhm is typically used here.


• 
  if( buttonState = HIGH ) does not compare, it assigns buttonState to HIGH (overwriting the value read with digitalRead(buttonPin)). Use the compare-for-equality operator == here, otherwise the if just checks if buttonState has been assigned a "truthlike" value (incidentally, HIGH is one of them).


• As others have already commented, lastButtonState must be set somewhere after the if-block!

Since many Arduinos do have pullup resistors built in, I'd say: drop the resistor completely, instead initialize the button input pin (7) to use its built-in pullup resistor ... this will invert the logic and you'll have to connect the button between pin 7 and GND (i.e. swapping button and resistor), though:

 pinMode(buttonPin, INPUT_PULLUP);

This will have the same result as if you replaced the 100 Ohm resistor with a 20 kOhm resistor after swapping button and resistor -- but that resistor is now inside the Arduino. (Take care if you use "Arduino-compatible" boards; sometimes they lack the pullup resistors or have them fixed!).

There still is one problem: bouncing. Pressing a mechanical button leads to the contacts inside the button to bounce a bit against each other, leading to a very fast HIGH-LOW-HIGH-LOW-etc signal before it settles on HIGH. You can alleviate this a bit by adding a delay(5) at the end of your loop(), but using a button library that debounces the signal will lead to better results most of the time.

This will lead to this code (also including the ternary operator hint by Michel Keijzers):

const int buttonPin = 7; 
const int ledPin13 = 13; 

int buttonState = 0; 
int lastButtonState = buttonState;
bool flag = true;

void setup() 
 // initialize the LED pin as an output:
 pinMode(ledPin13, OUTPUT);
 // initialize the pushbutton pin as an input:
 pinMode(buttonPin, INPUT_PULLUP);


void loop() 
 buttonState = digitalRead(buttonPin);

 if (buttonState == LOW && lastButtonState != buttonState) 
 flag = !flag;
 digitalWrite(ledPin13, flag ? LOW : HIGH);
 

 lastButtonState = buttonState;
 delay(5);

This should do the trick (I didn't try it, though).

Certain Changes in your code:

const int buttonPin = 7; 
const int ledPin13 = 13; 

int buttonState = 0; 
int lastButtonState = buttonState;
bool flag = true;

void setup() 
 // initialize the LED pin as an output:
 pinMode(ledPin13, OUTPUT);
 // initialize the pushbutton pin as an input:
 pinMode(buttonPin, INPUT);


void loop() 

 buttonState = digitalRead(buttonPin);


 if (buttonState == HIGH && lastButtonState != buttonState) 
 lastButtonState = buttonState;
 flag =!flag;
 if (flag)
 digitalWrite(ledPin13, LOW);
 else 
 digitalWrite(ledPin13, HIGH);
 
 

Secondly your hardware model is not good.
ANODE of LED must be connected to PIN13 and Cathode to GND. And switch must be placed between 3.3 V and Pin7 only.
In your model by closing switch you are shorting 3.3V and GND. If you had provided an external pullup then you should use at minimum a 4.7K resistor value. 100ohm never works.

I don't have software for Design so I used Paint and made a sketch for you.

^{simulate this circuit – Schematic created using CircuitLab}

There are already great answers, but I also found out about the interrupt functionality:

const byte ledPin = 13;
const byte interruptPin = 2;
volatile byte state = LOW;

void setup() 
 pinMode(ledPin, OUTPUT);
 pinMode(interruptPin, INPUT_PULLUP);
 attachInterrupt(digitalPinToInterrupt(interruptPin), blink, FALLING);


void loop() 
 digitalWrite(ledPin, state);


void blink() 
 state = !state;

some point:

• no matter using interrupt or the methods above tho process will not achieve stable / deterministic result. The issue lies in the hardware of the push-button and there will be fluctuations which can be interpreted by the MCU. One way to alleviate is to use delay.


• The


• 
  attachInterrupt has four modes LOW, Change, FALLING and RISING. The FALLING and RISING modes are the one to be used. the other two also lead to very unpredictable result.


• Only pins 1 and 2 can be used for interrupts.


• There is also the option to mix the two methods which you can see here, but I'm not sure if it will improve the stability.

I used this video and information on this page.

This is a comment (but could not use text alignment in a comment):

Instead of

if (flag)
 digitalWrite(ledPin13, LOW);
 else 
 digitalWrite(ledPin13, HIGH);

You can use:

digitalWrite(ledPin13, flag ? LOW : HIGH);

(It is functionally 100% equal though).