如何从R中的字符串向量中基于第一个字符提取字符串？
有时矢量字符串具有模式，有时我们需要根据字符从字符串矢量中创建模式。例如，我们可能想要从包含所有名称的向量中提取美国的州名称。这可以通过使用grepl函数来完成。
示例
考虑下面的包含美国州名的向量-
> US_states<-c("Alabama", "Alaska", "American Samoa", "Arizona", "Arkansas",
"California", "Colorado", "Connecticut", "Delaware", "District of Columbia", "Florida",
"Georgia", "Guam", "Hawaii", "Idaho", "Illinois", "Indiana", "Iowa", "Kansas",
"Kentucky", "Louisiana", "Maine", "Maryland", "Massachusetts", "Michigan",
"Minnesota", "Minor Outlying Islands", "Mississippi", "Missouri", "Montana",
"Nebraska", "Nevada", "New Hampshire", "New Jersey", "New Mexico", "New York",
"North Carolina", "North Dakota", "Northern Mariana Islands", "Ohio", "Oklahoma",
"Oregon", "Pennsylvania", "Puerto Rico", "Rhode Island", "South Carolina", "South
Dakota", "Tennessee", "Texas", "U.S. Virgin Islands", "Utah", "Vermont", "Virginia",
"Washington", "West Virginia", "Wisconsin", "Wyoming")
> US_states[grepl("^A",US_states)]
[1] "Alabama" "Alaska" "American Samoa" "Arizona"
[5] "Arkansas"
> US_states[grepl("^B",US_states)]
character(0)
> US_states[grepl("^C",US_states)]
[1] "California" "Colorado" "Connecticut"
> US_states[grepl("^D",US_states)]
[1] "Delaware" "District of Columbia"
> US_states[grepl("^E",US_states)]
character(0)
> US_states[grepl("^F",US_states)]
[1] "Florida"
> US_states[grepl("^G",US_states)]
[1] "Georgia" "Guam"
> US_states[grepl("^H",US_states)]
[1] "Hawaii"
> US_states[grepl("^I",US_states)]
[1] "Idaho" "Illinois" "Indiana" "Iowa"
> US_states[grepl("^J",US_states)]
character(0)
> US_states[grepl("^K",US_states)]
[1] "Kansas" "Kentucky"
> US_states[grepl("^L",US_states)]
[1] "Louisiana"
> US_states[grepl("^M",US_states)]
[1] "Maine" "Maryland" "Massachusetts"
[4] "Michigan" "Minnesota" "Minor Outlying Islands"
[7] "Mississippi" "Missouri" "Montana"
> US_states[grepl("^N",US_states)]
[1] "Nebraska" "Nevada"
[3] "New Hampshire" "New Jersey"
[5] "New Mexico" "New York"
[7] "North Carolina" "North Dakota"
[9] "Northern Mariana Islands"
> US_states[grepl("^O",US_states)]
[1] "Ohio" "Oklahoma" "Oregon"
> US_states[grepl("^P",US_states)]
[1] "Pennsylvania" "Puerto Rico"
> US_states[grepl("^Q",US_states)]
character(0)
> US_states[grepl("^R",US_states)]
[1] "Rhode Island"
> US_states[grepl("^S",US_states)]
[1] "South Carolina" "South Dakota"
> US_states[grepl("^T",US_states)]
[1] "Tennessee" "Texas"
> US_states[grepl("^U",US_states)]
[1] "U.S. Virgin Islands" "Utah"
> US_states[grepl("^V",US_states)]
[1] "Vermont" "Virginia"
> US_states[grepl("^W",US_states)]
[1] "Washington" "West Virginia" "Wisconsin" "Wyoming"
> US_states[grepl("^X",US_states)]
character(0)
> US_states[grepl("^Y",US_states)]
character(0)
> US_states[grepl("^Z",US_states)]
character(0)