| 1. | Positive semi-definite matrices are positive definite if and only if they are nonsingular .
正半定矩阵是正定的，当且仅当它们是非奇异矩阵。 |
| 2. | Two sets are said to be equivalent if and only if they can be placed in one-to-one correspondence .
当且仅当两个集合中的元素一一对应时，才称它们为等势。 |
| 3. | Theorem 1 and are equivalent infinitesimals if and only if
定理1与是等阶无穷小的充分必要条件为 |
| 4. | A node - set is true if and only if it is non - empty
一个节点当且仅当它不是空节点时，表达式的值为真。 |
| 5. | Specifies the block of code to execute if and only if the conditions of all
元素的条件的计算结果均为 |
| 6. | If and only if exactly one of
对于boolean比较，当且仅当 |
| 7. | A string is true if and only if its length is non - zero
一个字符串当且仅当它的长度不为0是，表达式的值为真。 |
| 8. | If and only if both its operands are
时，结果才为 |
| 9. | A string is true if and only if its length is non - zero . in
字符串的值为真的充分必要条件就是字符串的长度不为零。 |
| 10. | Every thing has its solution , if and only if you can be farseeing
每个问题都有其解决之道，只要你把目光放得远一点！ |
