Runtime Error - can only concatenate list (not "int") to list

Question

I've been doing 0-1 Knapsack problem using recursion+memoization.

My Code:

def knapSack(W, wt, val, n):
    '''
    :param W: capacity of knapsack 
    :param wt: list containing weights
    :param val: list containing corresponding values
    :param n: size of lists
    :return: Integer
    '''
    t = [[-1 for x in range(W + 1)] for j in range(n + 1)]
    if n == 0 or W == 0: 
        return 0
    if t[n][W] != -1: 
        return t[n][W] 
    elif wt[n-1] <= W: 
        t[n][W] = max(val[n-1]+knapSack(W-wt[n-1],wt,val,n-1),knapSack(W,wt,val,n-1))
        return t[n][W] 
    elif wt[n-1] > W: 
        t[n][W] = knapSack(wt, val, W, n-1) 
        return t[n][W]

Why am I getting a runtime error.

Runtime ErrorTraceback (most recent call last):
  File "/home/e8c2fc67721232cbee976a6adfc2c990.py", line 41, in <module>
    print(knapSack(W,wt,val,n))
  File "/home/e8c2fc67721232cbee976a6adfc2c990.py", line 12, in knapSack
    t[n][W] = knapSack(wt, val, W, n-1) 
  File "/home/e8c2fc67721232cbee976a6adfc2c990.py", line 5, in knapSack
    t = [[-1 for x in range(W + 1)] for j in range(n + 1)]
  File "/home/e8c2fc67721232cbee976a6adfc2c990.py", line 5, in <listcomp>
    t = [[-1 for x in range(W + 1)] for j in range(n + 1)]
TypeError: can only concatenate list (not "int") to list

This is the runtime error deatils I'm getting.

What about the line number, traceback, and filename for the error. Is that information available? I suspect the error is occurring where this function is being called for the first time (the outermost invocation of the recursion). I don't see any concatenation in this function. — fountainhead, Nov 08 '20 at 11:05

fountainhead · Accepted Answer · 2020-11-08T11:19:08.020

I suspect that there is a mismatch between the parameter-order of your function definition, and the argument-order in which the args are actually being passed to one of the many invocations.

Note that, in the traceback:

At line 41 it shows the arg-order as print(knapSack(W,wt,val,n))

At line line 12 it shows a different arg-order knapSack(wt, val, W, n-1)

For the condition elif wt[n-1] > W:, your recursive invocation of knapSack() is definitely passing the args in the wrong order.

score 0 · Answer 2 · answered Nov 08 '20 at 10:45

0

I think there is an easy fix to this, if you would just put the loops bellow the first if.

if n == 0 or W == 0: 
        return 0
t = [[-1 for x in range(W + 1)] for j in range(n + 1)]
if t[n][W] != -1: 
        return t[n][W] 
elif wt[n-1] <= W: 
    t[n][W] = max(val[n-1]+knapSack(W-wt[n-1],wt,val,n-1),knapSack(W,wt,val,n-1))
        return t[n][W] 
elif wt[n-1] > W: 
    t[n][W] = knapSack(wt, val, W, n-1) 
    return t[n][W]

answered Nov 08 '20 at 10:45

Oliver Hnat

797
4
19

Alright, could you please give me an example of ```W, wt, val``` and ```n``` that you are passing in? – Oliver Hnat Nov 08 '20 at 10:54
I'm not passing anything(All the work is being done by GFG). I'm solving this question on GeeksForGeeks and it states that: Complete the function knapSack() which takes maximum capacity W, weight array wt[], value array val[] and number of items n as a parameter and returns the maximum possible value you can get – Arnav Luhadiya Nov 08 '20 at 10:58

Runtime Error - can only concatenate list (not "int") to list

2 Answers2