--Originally published at Hector Martinez Alcantara

A validation is when you request an specific type of value from the user, so, you have to force the user to type what you want.

Let’s do an example:

var=input("Type a number\n")
while str.isnumeric(var)== 0:
 var=input("That's not a number, please type a number\n")
print("%s is the number you typed"%(var))

First, you require a string from the user, if the string is not a number, the program will request the number again.

That’s the main idea of a validation, depends on you how many conditions you want in your validation.

--Originally published at Hector Martinez Alcantara

Let’s talk about recursion. Personally, recursion is the  most dificult thing in programming, but I’ll try to explain it by the most common recursive function, the factorial.

Let’s see the example code:

def factorial(var):
    if var>1:
       var=var*factorial(var-1)
       return var
    else:
       return 1
 
x=int(input("Type a number to calculate the factorial:\n"))
print("The factorial of "+str(x)+" is:")
print(factorial(x))

The result of this is:

Type a number to calculate the factorial:
5
The factorial of 5 is:
120

But why?

Let’s see step by step, what happens.

First of all, we call the function with one parameter.

factorial(5) #factorial of 5

The number 5 enters to the function then if the number is 1 it returns 1, and then, the interesting part, if there’s some number higher than 1 the next function will be realized

var=var* factorial(var-1)

What happen here? Let’s see step by step.

First the value is 5, it’s greater than 1, so the function will be

var= 5 * factorial(var-1)

Now we can see that the value of factorial var-1 will be the same function, so we can see that the value of var in the function will be 4 that part will be like this:

var= 5 * (4*factorial((var-1)-1)

As you can see the value of the function factorial is the same function nested in the other sentence, so I’m goin to show you how the function looks doing the same procedure.

var=5 * ( 4 * ( 3 * ( 2 * (factorial ((((var-1)-1)-1)-1) ) ) ) )
#Remember the var==5
#((((var-1)-1)-1)-1) is like ((((5-1)-1)-1)-1) == 1

So…

var=5 * ( 4 * ( 3 * ( 2 * (factorial(1) ) ) )

What happens with factorial of 1? Remember that we typed that if the variable of the function is 1 or less, then the function will return

--Originally published at Hector Martinez Alcantara

This post is about the other loop, the while loop, it’s often used to do an action until a variable changes it’s value.

Here’s the sintaxis of the while loop

while condition:
      statements

And here’s an example of this:

#Introducing 5 lines of text
list=[]
x=1
while x<6:
      list.append(input("type some words"))
      x=x+1
print(list)

An example of while used as a returning menu:

op=""
while op!= "c":
      print("a)Print Hello")
      print("b)Print something")
      print("c)Exit")
      op=input("Select an option\n")
      op=op.lower()
      if(op=="a"):
           print("Hello")
      elif(op=="b"):
           print("Something")
      elif(op=="c"):
           print("Goodbye")
      else:
           print("Thats not a letter of the menu")

The result is

a)Print Hello
b)Print something
c)Exit
Select an option
>>a
Hello
a)Print Hello
b)Print something
c)Exit
Select an option
>>b
Something
a)Print Hello
b)Print something
c)Exit
Select an option
>>z
Thats not a letter of the menu
a)Print Hello
b)Print something
c)Exit
Select an option
>>c
Goodbye

Thanks for reading, ask me in comments.

Special thanks to Python while loop from Tutorialspoint

--Originally published at Hector Martinez Alcantara

Hi, I’m writing this post about the control flow tool called for loop. It’s used to repeat an action many times, but not only that, you can make a loop with specific conditions, and use it as a ‘foreach’ in C.

Another tool that we’re going to use is the range funtion that returns a list of a range of numbers.

I’m going to explain how the different uses of this for loop works.

First the sintaxis of the range tool:

range('initial number','last number range','conditional number')

Examples of use of range function:

#It can be used only as a quantity of numbers
range(5)
#The result is a list of numbers between 0 and 5 = [0,1,2,3,4]

#Used as a range of numbers
range(4,14)
#The result is a list of the range of numbers between 4 and 14 = [4,5,6,7,8,9,10,11,12,13]

#Used as a range of numbers with a jump condition
range(0,100,10)
#The result is a list of the range between 0 and 100 every 10 numbers = [0,10,20,30,40,50,60,70,80,90]

Now the sintaxis of the for tool:

for 'variable' in 'list' :
     #conditions

Examples of for loop function:

#Doing a quantity of actions
for number in range(8):
    print("Hello")
#The result is a print of the word 'Hello' 8 times

#Using a range of numbers
for number in range(10,20):
    print(number)
#The result is a print of numbers from 10 to 19

#Using a range of numbers with a special jump condition
for number in range(0,10,2):
    print(number)
#The result is the pair numbers from 0 to 8

#Using a variable
word="Hello"
for letter in word:
    print(letter)
#The result is a print of every letter in the word 'Hello' = ['H','e','l','l','o']

The explanation is that the for function takes every component in a list of components and use the component as the variable, for each component, the actions