递归搜索列表列表并返回原始列表中的索引位置

Question

Title says most of the problem. WE ARE REQUIRED TO WRITE THIS RECURSIVELY . We are given a list of presidents:

    preslst=[['George Washington',1732,1788],['John Adams',1735,1796],['Thomas Jefferson',1743,1800],['James Madison',1751,1808],
     ['James Monroe',1758,1816],['John Quincy Adams',1767,1824],['Andrew Jackson',1767,1828],['Martin Van Buren',1782,1836],
     ['William Henry Harrison',1773,1840],['John Tyler',1790,1840],['James K. Polk',1795,1844],['Zachary Taylor',1784,1848],
  ['Millard Fillmore',1800,1850],['Franklin Pierce',1804,1858],['James Buchanan',1791,1857],['Abraham Lincoln',1809,1861],
     ['Andrew Johnson',1808,1865],['Ulysses S. Grant',1822,1869],['Rutherford B. Hayes',1822,1877],['James A. Garfield',1831,1881],
     ['Chester A. Arthur',1829,1881],['Grover Cleveland',1837,1885],['Benjamin Harrison',1833,1889],['Grover Cleveland',1837,1893],
     ['William McKinley',1843,1897],['Theodore Roosevelt',1858,1901],['William Howard Taft',1857,1909],['Woodrow Wilson',1856,1913],
     ['Warren G. Harding',1865,1921],['Calvin Coolidge',1872,1923],['Herbert Hoover',1874,1929],['Franklin D. Roosevelt',1882,1933],
     ['Harry S. Truman',1884,1945],['Dwight D. Eisenhower',1890,1953],['John F. Kennedy',1917,1961],['Lyndon B. Johnson',1908,1963],
     ['Richard M. Nixon',1913,1969],['Gerald Ford',1913,1974],['Jimmy Carter',1924,1977],['Ronald Reagan',1911,1981],
     ['George H. Bush',1924,1989],['Bill Clinton',1946,1993],['George H.W. Bush',1946,2001],['Barack Obama',1961,2009],
     ['Donald Trump',1946,2017]]

We are asked to define a function (arg1 = presidents name as formatted in the list, arg2 = preslst) that will recursively go through the list and return the index position of the list that holds the right name.

This is what I have currently:

def presPosition(pres,preslst):
    for i in range(0,len(preslst)):
        print('search')
        presname = preslst[i][0]

        if presname ==  pres:
            print("FOUND")
            foundpres = preslst[0]
            return preslst.index(foundpres)

        else:
            return presPosition(pres,preslst[1:]) 

Ex: presPosition('John Adams',preslst) the returned value should be 1.

I keep getting 0 as the returned value, and I understand why because the preslst being passed in the else statement will have the name I'm looking for as the first item in the passed list. So how do I get the function to return the position of the where it is in the original list?

Thanks in advance.

Answer 1

Try the below if you need recursion.

def presPosition(pres, preslst, i=0):
    if i == len(preslst):
        return "Pres Not Found!"

    if preslst[i][0] == pres:
        return i
    else:

        return presPosition(pres, preslst, i+=1)

In your code, you are removing the first element, therefore your index will always be 0.

Answer 2

This is how to do it with enumerate() and a for-loop :

 def myFunc(pname, plist)    
    presname = pname
    preslist = plist
    for index, tup in enumerate(plist):
        if tup[0] == pname:
            return index
        raise ValueError('value not found in list')

Answer 3

This is a weird exercise and definitely not fit for recursion. However, here's something you could do. Return the length of the list when you find the element and at the end you subtract it from the original list's size:

def presPosition(pres,preslst):
    print('search')
    presname = preslst[0][0]

    if presname ==  pres:
        print("FOUND")
        return len(preslst)

    else:
        return presPosition(pres,preslst[1:])

preslst = [...] # your list
print(len(preslst) - presPosition('James Madison', preslst)) # outputs 3

BTW, you don't need the for loop as you're only using the first element and then returning, so I removed it.

Answer 4

You are getting 0 because after each iteration you pass preslst[1:] to the next iteration. And you return preslst.index(foundpres) which is from the above list and its always the 0th index.

Instead I suggest follow.

def presPosition(pres,preslst, next_index=0):
    print('search')
    if next_index == len(preslst):
        print("NOT FOUND")
        return

    presobj = preslst[next_index]
    presname = presobj[0]

    if presname ==  pres:
        print("FOUND")
        return preslst.index(presobj)    # return next_index

    else:
        return presPosition(pres,preslst, next_index+1)