循环停止在列表中的第三个元素上

Question

I am currently doing a project that requires me to create some of Pythons built in modules without the aid of any built in modules. My program creates a 500 element long list with a random number range of 1,999. My problem arises with my find_max(numbers) function.

def find_max(numbers):
    i = 0
    stop = 0
    a = i+1
    count = 0
    if numbers[1] > numbers[2]:
            greatest = numbers[1]
    elif numbers[2] > numbers[1]:
            greatest = numbers[2]
    while count != 500:
            if greatest > numbers[i]:
                    i += 1
                    count += 1
            elif numbers[i] > greatest:
                    greatest = numbers[i]
                    i += 1
                    count += 1
            else:
                    count+=1
    print("The greatest: "+str(greatest))
    mainmenu(numbers)

When I use this before or after sorting my list it will always stop on the third element. My find_min(numbers) function is a mirror copy of max with < intstead of > and always manages to find the lowest value before or after sorting my list. I have tried using a for count in range(len(list)): as my condition for the loop with the same results.

OUTPUT:

Please enter your menu option: max

The greatest: 10

LIST(after sorting):

Please enter your menu option: sort

[1, 7, 10, 11, 12, 13, 13, 14, 20, 29, 34, 38, 38, 39, 41, 44, 45, 51, 55, 56, 57, 57, 57, 62, 63, 69, 72, 73, 77, 78, 82, 83, 83, 95, 96, 98, 100, 102, 103, 104, 105, 106, 106, 111, 114, 114, 115, 116, 117, 123, 123, 125, 126, 127, 132, 134, 134, 135, 136, 138, 139, 140, 140, 142, 142, 142, 146, 148, 149, 151, 154, 156, 158, 158, 161, 163, 166, 166, 168, 170, 173, 173, 175, 175, 176, 182, 183, 184, 190, 197, 197, 204, 204, 205, 207, 207, 207, 213, 216, 217, 217, 219, 219, 221, 221, 221, 221, 222, 225, 227, 230, 233, 236, 243, 244, 249, 250, 250, 250, 253, 254, 256, 260, 261, 262, 265, 267, 268, 275, 276, 277, 278, 286, 290, 293, 294, 297, 299, 301, 302, 304, 305, 307, 308, 308, 309, 315, 318, 319, 322, 325, 332, 333, 334, 337, 338, 338, 341, 342, 342, 343, 344, 345, 347, 348, 352, 354, 355, 355, 355, 357, 357, 359, 362, 363, 367, 368, 370, 373, 374, 374, 378, 385, 387, 387, 391, 391, 392, 393, 399, 401, 403, 409, 417, 418, 419, 419, 421, 421, 424, 424, 428, 428, 430, 431, 432, 432, 43 4, 436, 436, 436, 438, 441, 441, 446, 447, 449, 450, 450, 452, 454, 456, 457, 457, 457, 457, 459, 463, 464, 467, 470, 470, 471, 475, 475, 476, 478, 479, 484, 489, 490, 501, 503, 504, 504, 507, 511, 513, 513, 513, 516, 516, 522, 532, 533, 534, 534, 537, 538, 538, 540, 548, 552, 557, 557, 562, 563, 563, 564, 572, 573, 574, 576, 577, 579, 583, 583, 584, 587, 588, 589, 592, 592, 592, 594, 595, 596, 598, 601, 602, 604, 605, 605, 606, 606, 607, 617, 617, 619, 622, 622, 625, 626, 627, 629, 631, 634, 636, 637, 642, 642, 643, 643, 653, 654, 656, 658, 658, 663, 669, 672, 672, 673, 674, 674, 679, 680, 681, 682, 683, 685, 692, 695, 696, 696, 698, 699, 709, 709, 710, 710, 711, 717, 727, 727, 735, 740, 741, 743, 746, 754, 754, 755, 757, 764, 765, 765, 768, 769, 773, 773, 783, 786, 788, 791, 792, 792, 793, 798, 800, 803, 803, 809, 809, 809, 813, 813, 813, 814, 815, 815, 816, 817, 817, 819, 821, 822, 823, 824, 824, 826, 827, 829, 829, 830, 830, 831, 831, 834, 835, 839, 840, 841, 841, 842, 842, 842, 84 3, 843, 843, 844, 850, 853, 854, 863, 863, 864, 868, 869, 870, 875, 877, 879, 880, 880, 882, 884, 884, 886, 887, 887, 889, 893, 894, 896, 896, 897, 897, 899, 901, 903, 903, 903, 905, 906, 909, 910, 913, 916, 916, 917, 917, 919, 920, 920, 921, 922, 922, 923, 926, 926, 926, 927, 928, 931, 932, 932, 935, 935, 936, 937, 938, 943, 949, 951, 951, 953, 954, 956, 958, 958, 959, 960, 966, 974, 974, 975, 976, 978, 979, 983, 984, 987, 991, 993, 993, 994, 995, 998, 999]

UPDATE:

Here is the code that applies to the question:

def initial():
    numbers = []
    i = 0
    while i <= 499:
            temp = random.randint(1,999)
            numbers.append(temp)
            i += 1
    return numbers

def find_min(numbers):
    i = 0
    if numbers[1] < numbers[2]:
            least = numbers[1]
    elif numbers[2] < numbers[1]:
            least = numbers[2]
    for element in range(len(numbers)):
            if least < numbers[i]:
                    i += 1
            elif numbers[i] < least:
                    least = numbers[i]
                    i += 1
    print("The lowest: "+str(least))
    mainmenu(numbers)

Answer 1

You didn't increment your list index, i . I found the problem with a simple print :

while count != len(numbers):
        print (count, i, numbers[i], greatest)
        if greatest > numbers[i]:

This shows the difficulty, called with:

test = [3, 1, 4, 1, 5, 9]
find_max(test)

Output:

0 0 3 4
1 1 1 4
2 2 4 4
3 2 4 4
4 2 4 4
5 2 4 4
The greatest: 4

First of all, your final else clause doesn't increment i . You don't need two counters running in parallel: use either count or i , not both.

Second, since you know in advance how many times you go through the loop, use a for loop, not a while :

for i in range(len(numbers)):

Even better, since you don't really need to know where the max element is in the list, just iterate through the list (I see that Mark Tolonen also hit this in a comment):

for element in numbers:

Finally, making this change will get rid of updating counters within the if statements, so that you don't have all that nasty, redundant code.

Answer 2

I struggle with weather to answer this and potentially deprive you of the value of figuring it out. However, I think that there is also value in seeing a cleaner way of doing what you are trying to accomplish. I learn a lot myself from seeing other's code. You decide if you want to use this or work through your own.

>>> def find_max(numbers):
    greatest = numbers[0]
    for x in numbers:
        if x > greatest:
            greatest = x
    return greatest

>>> find_max([3,400,200,100,500,250])
500

>>> find_max([250,500,100,200,400,3])
500

Notice, that python will automatically step through the numbers - you don't need to manage the increments. In this case, this is similar to what you would see in a for-each loop in other languages if you want to look it up.

Also notice how much easier the comparison logic can be. This is a pretty common pattern for these sorts of things so its probably worth your getting comfortable with this approach.