Julia快速入门（二）字符串

本篇代码在Julia1.0.2中测试通过

字符串的转换与规格化

字符串类型转换

# 字符串可以使用parse来转换成int或者float
e_str1 = "2.718"
e = parse(Float64,e_str1)

num_15 = parse(Int, "15")
println(3num_15)
# 程序输出： 45

# int和float间的互相转换使用convert
convert(Float64,123)
# 输出123.0
convert(Int,1.0)
# 输出1

字符串规格化

Julia1.0.2中有bug，@printf和@sprintf暂时用不了

s = @sprintf "this is a %s %15.1f" "test" 34.567;
println(s)
# 输出 UndefVarError: @sprintf not defined

假如没有bug，应该是这样的

e_str1 = "2.718"
e = float(e_str1)
# 使用printf可以将数字转换成字符串并加以规格化
@printf "e = %0.2f\n" e
# 程序输出： 2.72

# 或是使用sprintf构建另一个字符串
e_str2 = @sprintf("%0.3f", e)
println(e_str2)
# 程序输出： 2.718

# 同时输出两个字符串：
println("e_str1 == e_str2: $(e_str1 == e_str2)")
# 程序输出： e_str1 == e_str2: true

# 可用的数字格式字符有f, e, g, c, s, p, d:
# (pi是一个预先定义好的常量;但因为它的类型是"MathConst"，它必须先要转换为float类型才能被规格化)
@printf "fix trailing precision: %0.3f\n" float(pi)
# 程序输出： fix trailing precision: 3.142
@printf "scientific form: %0.6e\n" 1000pi
# 程序输出： scientific form: 3.141593e+03
# g这种用法尚未被指定
@printf "a character: %c\n" 'α'
# 程序输出： a character: α
@printf "a string: %s\n" "look I'm a string!"
# 程序输出： a string: look I'm a string!
@printf "right justify a string: %50s\n" "width 50, text right justified!"
# 程序输出： right justify a string:                    width 50, text right justified!
@printf "a pointer: %p\n" 100000000
# 程序输出： a pointer: 0x0000000005f5e100
@printf "print a integer: %d\n" 1e10
# 程序输出： print an integer: 10000000000

字符串长度

length("jμΛIα")
# 5

字符串重复

使用repeat或者^

repeat("ha", 3)
# "hahaha"
"Test "^3
# "Test Test Test "

将数字变成字符串

pad是占几格，base是几进制

string(5, base = 13, pad = 4)
# "0005"
string(13, base = 5, pad = 4)
# "0023"
string("a", 1, true)
# "a1true"

提取子字符串

SubString(字符串, 起点, 终点)

SubString("abc", 1, 2) #SubString(字符串, 起点, 终点)
# "ab"
SubString("abc", 1:2) #SubString(字符串, 切片)
# "ab"
SubString("abc", 2) #SubString(字符串, 起点)
# "bc"

字符串比较

使用==比较

"abc" == "abc" # true
"abc" == "αβγ" # false

使用cmp比较
cmp(a::AbstractString, b::AbstractString) -> Int

• Return 0 if both strings have the same length and the character at each index is the same in both strings.
• Return -1 if a is a prefix of b, or if a comes before b in alphabetical order.
• Return 1 if b is a prefix of a, or if b comes before a in alphabetical order (technically, lexicographical order by Unicode code points).
• 前面比后面‘大’就是1，相等为0，否则为-1

cmp("abc", "abc") # 0
cmp("ab", "abc") # -1
cmp("abc", "ab") # 1
cmp("ab", "ac") # -1
cmp("ac", "ab") # 1
cmp("α", "a") # 1
cmp("b", "β") # -1

字符串左右加空格

lpad("March", 10) # "     March"
rpad("March", 20) # "March               "

查找

findfirst

findfirst("z", "Hello to the world") # returns nothing, but not printed in the REPL
findfirst("Julia", "JuliaLang") # 1:5

findnext
findnext(pattern::AbstractString, string::AbstractString, start::Integer)
findnext(pattern::Regex, string::String, start::Integer)

findnext("z", "Hello to the world", 1) === nothing # true
findnext("o", "Hello to the world", 6) # 8:8
findnext("Lang", "JuliaLang", 2) # 6:9

findlast

findlast("o", "Hello to the world") # 15:15

findprev
findprev("substring", string, i):从第i个位置，往前数，找到第一substring，返回其index

findprev("z", "Hello to the world", 18) === nothing # true
findprev("o", "Hello to the world", 18) # 15:15
findprev("Julia", "JuliaLang", 6) # 1:5

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