on 20-May-2017 (Sat)

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Some of the most-used operations on strings are to check their length ,

There are two ways to access an individual character in a string. The first is the charAt() method:

 return 'cat' . charAt ( 1 ) ; // returns "a" 

The other way to access an individual character on a screen is to treat the string as an array-like object, where individual characters correspond to a numerical index:

 return 'cat' [ 1 ] ; // returns "a" 

return 'cat' [ 1 ] ; // returns "a"

C developers have the strcmp() function for comparing strings. In JavaScript, you just use the less-than and greater-than operators:

 var a = 'a' ; var b = 'b' ; if ( a < b ) { // true console . log ( a + ' is less than ' + b ) ; } else if ( a > b ) { console . log ( a + ' is greater than ' + b ) ; } else { console . log ( a + ' and ' + b + ' are equal.' ) ; } 

String literals (denoted by double or single quotes) and strings returned from String calls in a non-constructor context (i.e., without using the new keyword) are primitive strings

JavaScript automatically converts primitives to String objects, so that it's possible to use String object methods for primitive strings

String literals (denoted by double or single quotes) and strings returned from String calls in a non-constructor context (i.e., without using the new keyword)

JavaScript automatically converts primitives to String objects, so that it's possible to use String object methods for primitive strings

In contexts where a method is to be invoked on a primitive string or a property lookup occurs, JavaScript will automatically wrap the string primitive and call the method or perform the property lookup.

 var s_prim = 'foo' ; 
 var s_obj = new String ( s_prim ) ;
 console . log ( typeof s_prim ) ; // Logs "string"
 console . log ( typeof s_obj ) ; // Logs "object" 

String primitives and String objects also give different results when using eval(). Primitives passed to eval are treated as source code; String objects are treated as all other objects are, by returning the object. For example:

 var s1 = '2 + 2' ; // creates a string primitive var s2 = new String ( '2 + 2' ) ; // creates a String object console . log ( eval ( s1 ) ) ; // returns the number 4 console . log ( eval ( s2 ) ) ; // returns the string "2 + 2" 

String primitives and String objects also give different results when using eval() . Primitives passed to eval are treated as source code; String objects are treated as all other objects are, by returning the object. For example:

For these reasons, code may break when it encounters String objects when it expects a primitive string instead,

For these reasons, code may break when it encounters String objects when it expects a primitive string instead,

A String object can always be converted to its primitive counterpart with the valueOf() method.

In this article, I’ll explain how to solve freeCodeCamp’s “Repeat a string repeat a string” challenge. This involves repeating a string a certain number of times.

There are the three approaches I’ll cover:

1. using a while loop
2. using recursion
3. using ES6 repeat() method

A while statement executes its statement as long as a specified condition evaluates to true.

A while statement looks like this:

while (condition)
 statement

A while statement looks like this:

while (condition)
 statement

with a condition which is evaluated before each pass through the loop. If the condition is true, the statement is executed. If the condition is false, the execution continues with any statement after the while loop.

The statement is executed as long as the condition is true. Here’s the solution:

Recursion is a technique for iterating over an operation by having a function call itself repeatedly until it arrives at a result. There are a few key features of recursion that must be included in order for it to work properly.

• The first is a base case: this is a statement, usually within a conditional clause like if, that stops the recursion.
• The second is a recursive case: this is the statement where the recursive function is called on itself.

You can use a ternary operator as a shortcut for the if/else statement, like this:

times > 0 ? string.repeat(times) : "";

This can be read as:

if (times > 0) { 
 return string.repeat(times);
} else {
 return "";
}

Approach #3: Repeat a String using ES6 repeat() method

For this solution, you’ll use the String.prototype.repeat() method:

• The repeat() method constructs and returns a new string which contains the specified number of copies of the string on which it was called, concatenated together.

You can use a ternary operator as a shortcut for the if/else statement, like this:

times > 0 ? string.repeat(times) : "";

Classical conditioning focuses on responses that are automatic and involuntary, responses that are not deliberate and don't require effort. The theory is also known as respondent (i.e., involuntary), Pavlovian, or stimulus-response conditioning. The two pioneers of this theory were Ivan Pavlov (of the famous dog experiments) and John Watson.

Conditioning is just another word for learning. An unconditioned response is simply an unlearned response. A conditioned response is simply a learned response

classical conditioning, which posits that learning is based on pairing; operant conditioning, which posits that learning is based on rewards and punishments; and social learning theory, which posits that learning is based on observation and imitation.

Pavlov focused on the linksbetween stimuli and responses; these stimulus-response links are known as reflexes.

Unconditioned reflexesby definition are inborn, require no learning, and are generally the same for all members of a species.

When learning has not yet taken place and the link between the stimulus and response is inborn, we have an unconditioned reflex. An unconditioned reflex involves an unconditioned stimulus (US) that evokes an unconditioned response (UR). It is depicted graphically as: US -> UR. In Pavlov's classic experiment with dogs, the unconditioned reflex involved meat powder (US) leading to salivation (UR). Other examples include turning the lights off (US) eliciting pupil dilation (UR), and a loud noise (US) eliciting a startle response (UR).

A conditioned reflex consists of a conditioned stimulus (CS) and a conditioned response (CR).

How is a conditioned reflex learned? First, a stimulus that naturally evokes no automatic response, a neutral stimulus (NS), is paired repeatedly with the unconditioned stimulus (US). Through these repeated pairings, the neutral stimulus comes to elicit a response similar to the unconditioned response (UR). In the classic Pavlovian example, a tone (neutral stimulus) is repeatedly paired with the meat powder (unconditioned stimulus) until the tone elicits salivation. Once the neutral stimulus can elicitthe response by itself (i.e., the tone by itself elicits salivation), the neutral stimulus is renamed the conditioned stimulus (CS). The response to the conditioned stimulus (CS) is called the conditioned response (CR).

Note that the unconditioned response (UR) and the conditioned response (CR) are the same behavior, e.g., salivation. It's simply that the unconditioned response (UR) occurred naturally without learning while the conditioned response (CR) required learning. In addition, the conditioned response (CR) is frequently of less magnitude than the unconditioned response (UR)

examples of conditioned reflexes include feeling perked up (CR) by the first sip of coffee (CS), feeling nausea (CR) at the first bite of food (CS) that had previously made one sick, feeling happy (CR) on hearing the bell of an ice cream truck (CS).

classical conditioning theory states that unconditioned stimuli (US) elicit respondent behaviors (UR). If the unconditioned stimulus (US) is paired with a neutral stimulus (NS) sufficiently, eventually the neutral stimulus becomes a conditioned stimulus (CS) and elicits a conditioned response (CR). The CR typically mimics the UR but is of lesser magnitude

To decide if a stimulus is unconditioned, ask yourself: Is the response to the stimulus universal among members ofthe species? For example, is a blackfishnet stocking an unconditioned or conditioned stimulus? Since every member of the human species does not respond sexually to such a stocking, it is clearly not an unconditioned stimulus but rather a conditioned one.

it is not simply the closeness in time (contiguity) that causes conditioning. Rather, the unconditioned stimulus must appear to depend on or be contingent on the conditioned stimulus (contingency) (Rescorla, 1967

DELAY CONDITIONING (STANDARD PAIRING) In the standard pairing paradigm, the conditioned stimulus (originally the neutral stimulus) precedes the unconditioned stimulus (US) typically by a short interval (as little as .5 seconds) and overlaps into the presentation of the unconditioned stimulus (US).

In trace conditioning, the conditioned stimulus (CS) precedes the unconditioned stimulus (US) by a period and stops right before the unconditioned stimulus (US). For example, a dog first hears the sound of the can opener (originally the neutral stimulus) and then is fed the dog food (US). Over time, the dog will likely begin to salivate (CR) when it hears the can opener sound (CS). Again, the US appears to depend (be contingent) upon the presentation of the CS.

Temporal conditioning occurs when the unconditioned stimulus (US) is presented repeatedly at a consistent time interval. Eventually, time itself begins to act as the conditioned stimulus (CS). For example, if zoo animals are consistently fed at noon each day, the animals actually begin to anticipate feeding time. Thus, at noon, the animal may begin to salivate or pace the cage even if there are no other cues that feeding is imminent. Here, time is acting as the conditioned stimulus (CS) eliciting the animal's anticipatory behavior (CR). T

In simultaneous conditioning, the neutral stimulus andthe unconditioned stimulus (US) completely overlap. For example, the tone (NS) is sounded at exactly the same time the meat powder (US) is presented. Although this is termed "simultaneous conditioning," no learning or conditioning actually takes place. The neutral stimulus never becomes a conditioned stimulus and never elicits the conditioned response. Here, the US and the neutral stimulus are close in time (contiguity) buttheUS does not appear to depend onthe neutral stimulus (no contingency).

In backward conditioning, the unconditioned stimulus (US) precedes the neutral stimulus (NS). For example, the meat powder (US) is presented first, and the tone (NS) follows. Again, no learning or conditioning occurs. In this scenario, the tone will never elicit salivation. Again,closeness in time (contiguity) is not sufficient; contingency is requiredfor conditioningto occur.

whileall the approaches are termed "conditioning," true conditioning onlyoccurswhenthe CS is presented beforethe US. Learning does not take placewhen the CS and US are presented simultaneously, nor when the US precedes the CS.

In stimulus generalization (also known as mediated generalization), the subject automatically generalizes from a conditioned stimulus (CS) to other similar neutral stimuli. The subject then demonstrates the same conditioned response (CR) to these similar stimuli. An example of stimulus generalization comes from John Watson's experiment with Little Albert B., who was initially evaluated at 9 months, and conditioned at 11 months. Little Albert was classically conditioned to be afraid of a white rat (neutral stimulus) when the rat was paired with a loud noise (US). Without any further training or exposure, Little Albert generalized from the rat (CS) to other furry white objects (e.g., white rabbit and Santa Claus mask), and demonstrated a fear response to these similar neutral stimuli. Note that stimulus generalization occurs automatically and without any deliberate attempt on the part of the experimenter.

Higher order conditioning is a deliberate process in which a conditioned stimulus (CS) is paired with a neutral stimulus that is typically unrelated until eventually the new neutral stimulus becomes a conditioned stimulus and also elicits the conditioned response (CR). For example, the tone (CSj) is repeatedly paired with a flash of light (NS) until the animal begins to salivate to the light

The flash of light is now the CS2 and second order conditioning has been accomplished

third order conditioning occurs when a third neutral stimulus (e.g., the color red) is repeatedly paired with the flash of light (CS2) until the color red (CS3) elicits salivation. It is impossible to condition beyond the third level.

Stimulus generalization is an automaticprocess in which the subject demonstrates the conditioned response to stimuli similar to the original conditioned stimulus. Higher order conditioning is a deliberateprocess in which conditioning trials cause the subject to demonstrate the conditioned response to new stimuli (typically non-similar) by pairing them with the conditioned stimulus

Classical extinction results from repeatedly presenting the conditioned stimulus (CS) without the unconditioned stimulus (US). When the tone (CS) is repeatedly presented without the meat powder (US), the dog eventually stops salivating to the tone (i.e., the CR is extinguished). If Little Albert had been repeatedly presented with the white rat (CS) without the loud noise (US), his fear response (CR) would have been gradually /^% eliminated.

repeated presentation of the unconditioned stimulus (US) without the conditioned stimulus (CS) (e.g., repeated presentation of the loud noise without the rat) does not result in extinction. This is because the conditioned stimulus (CS) (e.g., the white rat) continues to have signaling value (i.e., the person anticipates the loud noise).

During extinction trials, following a rest period, the conditioned response (CR) to the conditioned stimulus (CS) often briefly reappears; this reappearanceis known as spontaneous recovery. For example, a dog undergoes extinction trials, i.e., he is repeatedly presented with a tone (CS) and no meat powder (US). Eventually, he stops salivating to the tone. However, the next day, when he is first brought back into the laboratory, he may initially salivate (CR) to the tone (CS), i.e., the conditioned response (CR) will spontaneously recover. The conditioned response will once again vanish if extinction trials continue

stimulus discrimination, an animal learns to discriminate between two similar neutral stimuli because one has been paired with the unconditioned stimulus (US) (e.g., a 500 Hz tone) while the other has not (e.g., a 100 Hz /m^ tone).

If the stimulus discriminations are then made too difficult, such as discriminating

between a 500 Hz tone and a 450 Hz tone, the animal will experience experimental neurosis. A dog, for example, may become agitated, barking, and howling. Interestingly, if the dog is then returned to the original discrimination it had mastered (i.e., the 100 Hz vs. the 500 Hz tone), it will no longer be able to successfully discriminate; it's as though the dog has had a breakdown.

Pseudoconditioning occurs accidentally. A neutral stimulus that was not deliberately paired with either the unconditioned stimulus (US) or the conditioned stimulus (CS) comes to elicit the conditioned response (CR). For example, the dog salivates when the light to the room is first turned on. Pseudoconditioning may be caused by either inadvertent pairing or heightened arousal.

HABITUATION IS LOSING RESPONSE TO THE US; EXTINCTION IS LOSING RESPONSE TO THE CS. In habituation, the subject becomes accustomed to and less responsive to an unconditioned stimulus (US) after repeated exposure. When a subject habituates to an unconditioned stimulus (US), the unconditioned stimulus (US) no longer elicits the unconditioned response (UR). For example, a person who moves to an apartment underneath an elevated railway becomes so used to the noise of passing trains ^ (US) that after a few weeks, he no longer startles (UR) and wakens when the trains come by. Habituation is not possible with all unconditioned stimuli (e.g., extreme electric shock). HINT: Habituation always involves the unconditioned stimulus, not the conditioned stimulus.

GRE Creating a virtual point-to-point link to Cisco routers at remote points over an IP internetwork. By connecting multiprotocol subnets in a single-protocol backbone environment.

GRE add header with tunneling IP header create at least 24 bytes of additional overhead for tunneled packets, that is why we need to decrease MTU to 1400 to free some space to these additional bytes

GRE Tunnel can be used to make two endpoint communicate with each Private subnet while physical interface assigned public IP , we can also use to let two IPv6 islands communicate through a sea of IPv4 networks

A significant limitation of classical conditioning is that it only explains how we learn involuntary behaviors or responses (e.g., salivation, startling)

operant conditioning explains voluntary behavior, such as getting up in the morning, getting dressed, coming to class, doing homework, going to the gym, etc.

Operant conditioning posits that we learn as a result of reward and punishment. Other names for the theory include Skinnerian conditioning and instrumental conditioning, in that behavior is instrumental (or goal-directed) toward obtaining rewards. Its two major theorists are E. L. Thorndike and B. F. Skinner.

According toThorndike's Lawof Effect, behaviors are initially emitted in random, trial and error fashion. Those random behaviors that are followed by pleasurable consequences (rewards) become stronger and more frequent while those behaviors followed by unpleasant consequences (punishers) become weaker and less frequent. Thorndike later revised the Law of Effect by deleting the part about punishers, since he did not find sufficient evidence that punishment effectively decreases behavior. Despite Thorndike's findings, the current theory of operant conditioning includes both punishment and reinforcement.

Reinforcement and punishment are the consequences or contingencies

Reinforcement, by definition, always increases the target behavior. It brings the subject into a more desirable state. Punishment, by definition, always decreases the target behavior.

Positive means that something is "added* following emission of the target behavior and negative means that something is "subtracted."

Positive reinforcement is reward. After the target behavior is performed, something of value is given to the person. It is reinforcement because it brings the person into a more desirable state, thus increasing the target behavior.

Negative reinforcement is relief. After the target behavior is performed, something annoying or aversive is removed. It is reinforcement because it brings the subject into a more desirable state, thus increasing the target behavior. It is negative because something is being subtracted.

Positive punishment is pain. After the target behavior is performed, something aversive is added. It is punishment because it brings the subject into a less desirable state, thus decreasing the target behavior. It is positive because something is added.

Negative punishment is loss. After the target behavior is performed, something valuable is removed. It is punishment because it brings the subject into a less desirable state, thus decreasing the target behavior.

The acquisition phase refers to the period during which new learning r & G/- occurs

extinction refers to the period in which reinforcement is withheld.

The strength of a behavior, termed operant strength [OPERANT...THEY'RE OPERATING ON THIER ENVIRONMENT, TRYING TO CHANGE SOMETHING], is measured by the rate of responding (i.e., how often do the pigeons peck) during both acquisition and extinction trials.

Continuous reinforcement simply means reinforcing every occurrence of the behavior. For example, every time a first grader reads a word, she receives a candy. Continuous reinforcement is the best schedule for acquiring new behaviors

satiation describes this phenomenon of a reinforcer losing its value through overuse.

although continuous reinforcement is best for the acquisition phase, it is highly susceptible to satiation and extinction.

after acquisition, the schedule of reinforcement is best changed from continuous to intermittent, a change termed thinning.

In intermittent reinforcement, the subject is not reinforced for every occurrence of the behavior.

In ratio schedules, the subject is reinforced based on how often the subject has emitted a target behavior (e.g., 2:1,for every two pecks, one pellet).

In interval reinforcemtn schedules, reinforcement is limited by time, not how many times the behavior happened. (e.g., if the pigeon has pecked 50 times during the 60 seconds, it still only gets one treat).

In fixed schedules, the ratio or time interval doesn't change whereas in variable schedules, the ratios or time intervals do change.

FIXED INTERVAL (FI): You can only get one reinforcer in a certain period fo time (e.g., Getting sex after taking out trash, but at most once per week).

On a fixed interval schedule the response rate is high near the end of the interval and low (or non-existent) during most of the interval.

FIXED INTERVAL (FI) Reinforcement occurs the first time the target behavior is o i V JU 3 ¥ emitted after the fixed time interval has elapsed. For example, a child is taken to Baskin- Robbins for an ice cream cone if she completes her homework unassisted (the target behavior) provided at least a week has passed (fixed interval) since she was last

VARIABLE INTERVAL (VI) Reinforcement occurs the first time the target behavior is emitted after a variable/unpredictable interval of time has elapsed. For example, Skinner's pigeons might receive feed if they pecked once at the end of a 30 second interval, then at the end of a 20 second interval, then after a 40 second interval, etc. Here, the subject can't anticipate when reinforcement might occur and therefore continues to perform the behavior at a moderate rate and without pause.

FIXED RATIO (FR) Reinforcement occurs after a fixed/set/exact number of responses are emitted. For example, in piecemeal work, the person is paid after every 500 envelopes stuffed. Response rate is typically moderate to high, and the subject may pause after reinforcement is provided, especially if the ratio is large (i.e., many responses are required before reinforcement).

VARIABLE RATIO (VR) Reinforcement occurs after an unpredictable number of c# responses are emitted. Slot machines are the classic example: one cannot predict how many times of playing are required to win the jackpot (reinforcement). This schedule Oy results in subject continuing to perform the behavior at high rates and with little pause.

Fixed schedules are still intermittent schedules because reinforcement does not occur for every response.

RATES OF RESPONDING DURING ACQUISITION During acquisition, variable ratio (VR) results in the greatest operant strength followed by fixed ratio (FR), variable interval (VI), and fixed interval (FI). Fixed interval (FI) has the lowest response rates during new learning

Variable ratio (VR) is the most resistant to extinction (just think of those slot machines!), followed by fixed ratio (FR), variable interval (VI), and fixed interval (FI). Behaviors learned through fixed interval (FI) extinguish most easily and quickly.

the fixed schedules result in pauses after reinforcement. When graphed, this pattern is termed scalloped. Variable schedules result in more uninterrupted behavior. When graphed, this pattern looks smooth. The fixed interval (FI) schedule appears most scalloped.

At first, withholding reinforcement will usually result in an increase in the behavior, termed a response burst. A child, for example, who's used to getting her way by whining will actually whine more if the father suddenly stops responding to her whining. Over time, however, the unreinforced behavior will diminish, i.e., it will be extinguished.

Superstitious behavior results from accidental reinforcement or from non-contingent reinforcement,

DISCRIMINATION LEARNING (STIMULUS CONTROL) In some experiments, and usually in the real world, target behaviors are reinforced in certain circumstances but not in others. In discrimination learning or stimulus control, the subject learns to "discriminate" between those situations (i.e., stimuli) in which reinforcement will be forthcoming and those in which it will not. The stimulus that signals reinforcement will take place is known as the discriminative stimulus (SD); the stimulus that signals reinforcement will not take place is known as the S delta (SA). For example, a child whines in the presence of his grandmother because he knows that his grandmother will pay attention but he does not whine around his mother who ignores the whining behavior. Here, the grandmother is serving as an SD (signaling reinforcement) while mother is serving as an SA (signaling no reinforcement).

Stimulus generalization occurs when a subject begins to emit the target behavior in the presence of stimuli similar to but not exactly the same as the discriminative stimulus. For example, the pigeon that gets food when it pecks to a green light might also peck to blue light, expecting reinforcement.

Response generalization refers to performing a -^ behavior that is similar but not identical to the one that has been previously been reinforced.

To decide whether a phenomenon is stimulus generalization or response generalization, ask yourself: What's changing, the stimulus or the response? In the above examples, the child's behavior of whining is the same, but the stimulus to which he does it has changed (grandmother to other elderly); hence it is stimulus generalization. When the dog starts doing different dog tricks, his response has changed, and hence it is response generalization.

Prompting involves cueing the subject about what behavior to perforin. For example, to teach a child good manners, a parent may prompt the child with the instruction "Say thank you." Over time, the parent gradually reduces the prompt to "What do you say?" and eventually to justa look. This gradual reduction in prompting is known as fading.

Shaping involves is when you reinforce someone for getting closer and closer to what you want them to do.

Chaining involves stringing together different behaviors to accomplish a goal. Each behavior in the sequence is mildly reinforced (minor reinforcement) and serves as a cue (discriminative stimulus) to perform the next behavior in the chain. The major reinforcement occurs at the end of the chain.

Every step must be reinforced or else the chain will stop. For instance, if the person gets up to the ticket window and finds that he has left his wallet at home, he is not reinforced and the whole chain is interrupted.

The Premack principle, also known as Grandma's rule. Use a high frequency behavior (i.e., something the person likes to do) to reinforce a low frequency behavior (i.e., something the person doesn't much care to do). For example, "If you eat your spinach (low frequency behavior), you can go out to play(high frequency behavior)."

Imagine a situation in which two behaviors have been equally reinforced (e.g., both cleaning one's room and raking the yard earns allowance). Then, only one of the behaviors is reinforced (only cleaning one's room is reinforced). Typically the behavior that is being reinforced (cleaning) increases in frequency, while the behavior that is no longer being reinforced (raking) decreases. This phenomenon is termed behavioral contrast.

According to Social Learning Theory (also known as the theory of observational learning), human learning and behavior cannot fully be explained by behavioral principles of reinforcement (operant conditioning) or association (classical conditioning). Social Learning Theory proposes that learning, especially complex learning (e.g., brain surgery), occurs through the observation and modeling (imitation) of behavior that has been witnessed. In the classic Bobo doll study Bandura, Ross, and Ross (1963) found that children exposed to violent models tended to imitate the exact violent behavior they observed. In general, models who were high status, nurturant, and of the same sex as the observer were more likely to be imitated than models who were low status, non-nurturant, or of the opposite sex.

Bandura's social learning theory says we anticipate reinforcers and act accordingly; classical and operant conditioning believe that we are programmed by past reinforcement/punishment.

The radical behaviorist viewpoint contends that all behavior is shaped by contingencies in the environment. In contrast, social learning theorists believe in reciprocal determinism, according to which an interactive triad of the person, his/her behavior, and the environment regulate the individual's behavior. For example, a car accident victim anticipates positive reinforcement (i.e., money) for complaints of pain and therefore files a disability claim. Receiving this reinforcement leads the victim to become even less active which then weakens his muscles further and thereby causes him more (and very real) pain.

According to Bandura, Observational Learning has 4 steps: attention (attending to model), retention (remembering what is seen/heard), production (reproducing the memory during imitation), and motivation (reinforcement for accurate performance)