Python 小高考篇（5）自定义函数

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​违​者​必​究​，​谢​谢​配​合​。​
​个​人​主​页​：​blog.csdn.net/jzwalliser

在​p​y​t​h​o​n​中​，​如​果​有​一​个​代​码​片​段​需​要​频​繁​使​用​，​则​可​以​考​虑​将​其​写​为​一​个​自​定​义​函​数​。​

​d​e​f​ ​定​义​函​数​

通​过​d​e​f​关​键​词​，​可​以​自​定​义​一​个​函​数​，​之​后​再​在​代​码​内​调​用​它​。​如​：​

defhello():print("Hello World!")print("Hello!")hello()

运​行​结​果​：​

Hello World! Hello!

第​一​行​代​表​定​义​一​个​函​数​，​函​数​名​字​叫​hello；​第​2​行​到​第​3​行​代​表​该​函​数​需​要​执​行​的​东​西​，​第​4​行​代​表​执​行​该​函​数​。​

​传​入​参​数​

就​像​数​学​公​式​内​，​我​们​可​能​会​这​么​写​：​f ( x ) = x + 1 f(x)=x+1f(x)=x+1。​可​以​注​意​到​，​其​实​它​有​一​个​参​数​x xx。​
p​y​t​h​o​n​内​定​义​函​数​也​可​以​有​参​数​，​像​这​样​：​

defsayhello(who):print("Hello",who+"!")sayhello("Jzwalliser")

运​行​结​果​：​

Hello Jzwalliser!

对​比​一​下​，​发​现​第​一​个​例​子​中​，​在​定​义​函​数​名​时​，​后​面​的​括​号​中​没​有​任​何​内​容​（​def hello():）​，​而​在​第​二​个​例​子​中​，​括​号​中​有​个​“​who”​（​def sayhello(who):）​。​其​中​，​这​里​的​“​who”​就​是​一​个​参​数​，​可​以​理​解​为​一​个​变​量​。​再​看​最​后​一​行​（​sayhello("Jzwalliser")）​，​其​实​在​这​里​就​把​who的​值​设​置​为​了​"​J​z​w​a​l​l​i​s​e​r​"​，​所​以​第​二​行​在​输​出​有​关​内​容​时​，​读​取​到​的​who的​值​即​为​"​J​z​w​a​l​l​i​s​e​r​"​。​

​返​回​值​

返​回​值​，​就​是​一​个​函​数​运​行​完​后​还​给​外​界​的​数​值​。​如​：​

defplus(a,b):print(a+b)defplus2(a,b):returna+b result1=plus(114000,514)result2=plus2(114000,514)print("result1",result1)print("result2",result2)

运​行​结​果​：​

114514 result1 None result2 114514

同​样​是​加​法​计​算​，​plus函​数​只​是​直​接​输​出​结​果​，​但​plus2函​数​更​为​“​高​明​”​，​它​可​以​将​计​算​结​果​保​存​到​指​定​的​变​量​中​。​
这​就​是​返​回​值​，​在​函​数​最​后​面​添​加​一​行​return sth就​可​以​将​数​据​返​回​给​外​界​，​就​像​是​函​数​内​部​和​外​部​的​桥​梁​一​样​。​
不​过​注​意​，​return后​面​的​代​码​是​不​执​行​的​，​像​这​样​：​

deftest():print("return 之前")return114514print("return 之后")test()

运​行​结​果​：​

return 之前

​l​a​m​b​d​a​ ​定​义​函​数​

lambda是​一​种​很​简​洁​（​偷​懒​）​的​函​数​定​义​方​法​，​它​的​代​码​量​比​def定​义​的​函​数​少​很​多​，​但​是​正​因​如​此​它​所​能​实​现​的​功​能​也​远​没​有​def多​。​
先​看​个​例​子​，​将​f ( x ) = 2 x f(x)=2xf(x)=2x分​别​用​lambda和​def变​成​函​数​：​

f=lambdax:2*xprint(f(5))

deff(x):returnx*2print(f(5))

以​上​两​段​代​码​其​实​是​基​本​等​价​的​，​可​以​看​出​第​一​段​代​码​量​要​少​很​多​，​甚​至​连​个​return都​没​有​，​但​实​际​上​它​依​然​有​个​返​回​值​。​
解​释​一​下​第​一​段​代​码​什​么​意​思​：​
1​. ​f为​函​数​名​
2​. ​lambda表​示​需​要​定​义​一​个​函​数​
3​. ​x是​一​个​参​数​
4​. ​2 * x是​一​个​表​达​式​，​同​时​也​是​这​个​函​数​的​返​回​值​

​递​归​

递​归​，​就​是​一​个​函​数​自​己​调​用​自​己​。​递​归​主​要​由​两​个​部​分​组​成​：​递​归​操​作​及​递​归​边​界​。​递​归​操​作​是​函​数​的​主​体​部​分​，​负​责​执​行​运​算​、​实​现​功​能​；​递​归​边​界​即​一​个​条​件​，​ ​满​足​条​件​后​停​止​递​归​，​并​返​回​上​一​层​，​以​避​免​程​序​无​限​递​归​下​去​而​陷​入​死​循​环​。​
比​如​，​计​算​斐​波​那​契​数​列​就​可​以​使​用​递​归​算​法​来​实​现​。​假​设​第​x xx项​用​f ( x ) f(x)f(x)来​表​示​（​x ∈ Z + x\in\mathbb{Z_+}x∈Z+​）​，​那​么​：​

f ( x ) = { 1 ( x = 1 ) 1 ( x = 2 ) f ( x − 1 ) + f ( x − 2 ) ( x ≥ 3 ) f(x)=\left\{ \begin{aligned} &1\space(x=1) \\ &1\ (x=2)\\ &f(x-1)+f(x-2)\space(x\geq 3) \\ \end{aligned} \right.f(x)=⎩⎨⎧​​1(x=1)1(x=2)f(x−1)+f(x−2)(x≥3)​

翻​译​为​p​y​t​h​o​n​如​下​。​

deff(x):ifx==1orx==2:return1else:returnf(x-1)+f(x-2)

由​于​之​前​就​写​过​一​篇​关​于​递​归​的​文​章​，​这​里​就​不​在​赘​述​了​，​欲​知​详​情​，​请​移​步​到​这​里​：​初识递归算法。​

​自​测​

1​. ​阅​读​以​下​代​码​：​

deff(x):ifx>1:returnx*f(x-1)return1print(f(10))

该​程​序​输​出​为​（​ ​ ​ ​ ​ ​ ​）​
A​.​ ​362880
B​.​ ​3628800
C​.​ ​39916800
D​.​ ​10
【​答​案​】​B​

2​. ​阅​读​以​下​代​码​：​

deff(x):ifx==0:return1returnf(x-1)*2

这​段​代​码​表​达​的​等​价​数​学​公​式​是​（​ ​ ​ ​ ​ ​ ​）​
A​.​ ​f ( x ) = 2 x ( x ∈ N ) f(x) = 2^x(x\in\mathbb N)f(x)=2x(x∈N)
B​.​ ​f ( x ) = 2 x ( x ∈ N ) f(x) = 2x(x\in\mathbb N)f(x)=2x(x∈N)
C​.​ ​f ( x ) = 2 x − 2 ( x ∈ N ) f(x) = 2x-2(x\in\mathbb N)f(x)=2x−2(x∈N)
D​.​ ​f ( x ) = 2 x − 1 ( x ∈ N ) f(x) =2^{x-1}(x\in\mathbb N)f(x)=2x−1(x∈N)
【​答​案​】​A​

​总​结​

d​e​f​ ​定​义​函​数​
使​用​def定​义​函​数​的​模​板​：​

deffunction(arg1,arg2,arg3):#定义函数returnvalue=do_sth(arg1,arg2,arg3)#操作和运算returnreturnvalue#返回值

l​a​m​b​d​a​ ​定​义​函​数​
模​板​：​

func=lambdaarg1,arg2,arg3:do_sth(arg1,arg2,arg3)

递​归​
递​归​，​即​函​数​自​己​调​用​自​己​。​层​层​深​入​进​行​计​算​，​之​后​再​回​溯​，​将​计​算​结​果​返​回​到​上​一​层​参​与​运​算​。​

​结​尾​

好​啦​，​今​天​就​分​享​到​这​里​了​，​记​得​一​键​三​连​哦​(​˵​¯​͒​〰​¯​͒​˵​)​