# 2.5. Conditional Expression: if-then-else¶ Open the notebook in Colab

The if-then-else statement is supported through te.if_then_else. In this section, we will introduce this expression using computing the lower triangle of an matrix as the example.

import tvm
from tvm import te
import numpy as np
import d2ltvm


In NumPy, we can easily use np.tril to obtain the lower triangle.

a = np.arange(12, dtype='float32').reshape((3, 4))
np.tril(a)

array([[ 0.,  0.,  0.,  0.],
[ 4.,  5.,  0.,  0.],
[ 8.,  9., 10.,  0.]], dtype=float32)


Now let’s implement it in TVM with if_then_else. It accepts three arguments, the first one is the condition, if true returning the second argument, otherwise returning the third one.

n, m = te.var('n'), te.var('m')
A = te.placeholder((n, m))
B = te.compute(A.shape, lambda i, j: te.if_then_else(i >= j, A[i,j], 0.0))


Verify the results.

b = tvm.nd.array(np.empty_like(a))
s = te.create_schedule(B.op)
print(tvm.lower(s, [A, B], simple_mode=True))
mod = tvm.build(s, [A, B])
mod(tvm.nd.array(a), b)
b

produce compute {
for (i, 0, n) {
for (j, 0, m) {
compute[((i*stride) + (j*stride))] = tvm_if_then_else((j <= i), placeholder[((i*stride) + (j*stride))], 0f)
}
}
}

<tvm.nd.NDArray shape=(3, 4), cpu(0)>
array([[ 0.,  0.,  0.,  0.],
[ 4.,  5.,  0.,  0.],
[ 8.,  9., 10.,  0.]], dtype=float32)


## 2.5.1. Summary¶

• We can use tvm.if_then_else for the if-then-else statement.