我可以用一个循环来解决这个问题，但是为什么没有使用这个递归 function 显示数组的所有组合？

Question

The code prints out all the combinations except for index [0:4:3] and I don't understand why. I want to solve it using only recursion. So i am wondering why it skipping that one index. If someone can explain that would help

combination=[]          #create array
re=[3,34,2,1]                   #list to find all combinations
#name of function
def all_combinations(arr,x,y,z):
    # if less than length of array for x let it pass
    if x < len(arr)+1:
        # if less than length of array for y let it pass
        if y < len(arr)+1:
            # if less than length of array for z let it pass
            if z < (len(arr)+2):
                if arr[x:y:z] not in combination: 
                    combination.append(arr[x:y:z])       #add to array
                    z +=1       #increase z
                    all_combinations(arr,x,y,z)      #recurse
                y += 1      #increase y
                z=1         #reset index
                all_combinations(arr,x,y,z) #recurse
            x += 1           #next x
            y =0             #reset y
            z=1              #reset z
            all_combinations(arr,x,y,z)      #recurse

all_combinations(re,0,1,1)        #call function
print(combination)                #print result

Answer 1

You reset of y places it at zero (instead of x+1). This will generate an empty combination arr[x:0:1] which will be present in the combinations the second time, thus breaking the recursion for all starting values of x after reaching the maximum end value for a given step size.