编写代码来计算一个字符串中所有字母的出现次数。
一个字符串：hello world!
以下是Python 3中的实现：

```python
string = "hello world!"
frequency = {}

for char in string:
    if char in frequency:
        frequency[char] += 1
    else:
        frequency[char] = 1

print("出现次数：
")
for char, count in frequency.items():
    if char.isalpha():
        print(f"{char}: {count}")
```

输出：

```
出现次数：

h: 1
e: 1
l: 3
o: 2
w: 1
r: 1
d: 1
```