编写程序输入一个3×4的矩阵 求出它的转置矩阵并输出

好的，下面是 Python 代码实现：

# 定义原矩阵
matrix = [
    [1, 2, 3, 4],
    [5, 6, 7, 8],
    [9, 10, 11, 12]
]

# 定义转置矩阵
transpose = [
    [0, 0, 0],
    [0, 0, 0],
    [0, 0, 0],
    [0, 0, 0]
]

# 计算转置矩阵
for i in range(len(matrix)):
    for j in range(len(matrix[0])):
        transpose[j][i] = matrix[i][j]

# 输出转置矩阵
for row in transpose:
    print(row)

输出结果为：

[1, 5, 9]
[2, 6, 10]
[3, 7, 11]
[4, 8, 12]