define a 2d list in python

Question

rows = 7  
cols = 6  
mat = []  
for i in range(cols):
    col = []  
    for j in range(rows):
        col.append(0)  
    mat.append(col) 

for i in range(cols):  
    for j in range(rows):
        print(mat[j])
    print('\n')

why it has an //IndexError: list index out of range// error ?

Answer 1

Either you can use

rows = 7  
cols = 6  
mat = []  
for i in range(cols):
    col = []  
    for j in range(rows):
        col.append(0)  
    mat.append(col) 

for i in range(cols):  
    for j in range(rows):
        print(mat[i][j], end = " ")
    print('\n')

Here you will print each values one by one. Or another method is just to print the entire row at once as:

rows = 7  
cols = 6  
mat = []  
for i in range(cols):
    col = []  
    for j in range(rows):
        col.append(0)  
    mat.append(col) 

for i in range(cols):  
    print(mat[i])
print('\n')

IndexError is because, you are trying to access an element which is not there.