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C language learning
2022-07-01 12:53:00 【51CTO】
Branches and loops （1）
Text
One . Branch statements and loop statements
Branch statement
if switch
Loop statement
while for do while go to sentence
What is a sentence ？
c There is a semicolon in language （;） Separated by a statement .
1. Branch statement （ Selection structure ）
1）.if sentence
if The grammatical structure of a sentence ：
if（ expression ） or if（ expression ）
sentence ; person sentence 1;
else
sentence 2;
Multiple branches
if（ expression 1）
sentence 1;
else if（ expression 2）
sentence 2;
else
sentence 3;
If the result of the expression is true , Then the statement executes （0 Said the false , Nonzero means true ）
int
main()
{
int
age
=
111;
if (
age
<
18)
printf(
" A minor \n");
else
if (
age
>=
18
&&
age
<
20)
//&& Representation logic and , If the result of the expression is true , Then the statement executes （0 Said the false , Nonzero means true ）
printf(
" teenagers \n");
else
if (
age
>=
20
&&
age
<
50)
printf(
" Prime of life \n");
else
if (
age
>=
50
&&
age
<
90)
printf(
" aged \n");
else
printf(
" Old man does not die \n");
return
0;
}
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If the condition holds , To execute multiple statements , Code blocks should be used
#include <stdio.h>
int main()
{
if( expression )
{
Statement list 1;
}
else
{
Statement list 2
}
return 0;
}
int
main()
{
int
age
=
111;
if (
age
<
18)
{
printf(
" A minor \n");
// Statement list 1
printf(
" Can't fall in love \n");
// You can print multiple printf
}
else
{
if (
age
>=
18
&&
age
<
20)
printf(
" teenagers \n");
else
if (
age
>=
20
&&
age
<
50)
// Statement list 2
printf(
" Prime of life \n");
else
if (
age
>=
50
&&
age
<
90)
printf(
" aged \n");
else
printf(
" Old man does not die \n");
}
return
0;
}
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there {} It's just a code block
In the air else
// Use properly {} It can make the logic of the code clearer
// Code style is important
practice ： Output 1~100 Between the odd numbers
2）.switch sentence
switch The grammatical structure of a sentence ：
switch（ Integer expression ）
{
Statement item ;
}
What are statement items ？
// It's some case sentence
// as follows ：
case Integral constant expression ：
sentence ;
//switch There can be if sentence
int
main()
{
int
day
=
0;
scanf(
"%d",
&
day);
//& Fetch address , Take out variables day
if (
1
==
day)
// Judgment variable day Is it equal to 1, If yes, output on Monday
printf(
" Monday \n");
else
if(
2
==
day)
printf(
" Tuesday \n");
else
if (
3
==
day)
printf(
" Wednesday \n");
else
if (
4
==
day)
printf(
" Thursday \n");
else
if (
5
==
day)
printf(
" Friday \n");
else
if (
6
==
day)
printf(
" Saturday \n");
else
if (
7
==
day)
printf(
" Sunday \n");
return
0;
}
// use else if...else if More complicated , use switch Multi branch statements are relatively simple
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After the change
int
main()
{
int
day
=
0;
//day For integer variables
scanf(
"%d",
&
day);
switch (
day)
//day Is an integer expression
{
case
1:
//case 1（1 Is a constant variable expression ）:
printf(
" Monday \n");
break;
//break It means to end the cycle
case
2:
printf(
" Tuesday \n");
break;
case
3:
printf(
" Wednesday \n");
break;
case
4:
printf(
" Thursday \n");
break;
case
5:
printf(
" Friday \n");
break;
case
6:
printf(
" Saturday \n");
break;
case
7:
printf(
" Sunday \n");
break;
}
return
0;
}
// summary ： Easy to understand
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stay switch Statement break:
stay switch In the sentence , We can't Branch , With break You can realize the real Branch .
practice ： Input 1~5, Output “weekday”; Output 6~7, Input “weekend”
int
main()
{
int
day
=
0;
scanf(
"%d",
&
day);
switch(
day)
{
case
1:
case
2:
case
3:
case
4:
case
5:
printf(
"weekday\n");
break;
case
6:
case
7:
printf(
"weekend\n");
break;
default:
//default Can be placed in switch Anywhere , When switch The value of the expression is not equal to case The value of the tag , This is the time default The following statement will execute .
printf(
" Input error \n");
break;
}
return
0;
}
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break The actual effect of the statement is to divide the statement list into different parts .
default Clause （default and case There is no order , It can be used anywhere ）
If the expressed value is the same as all case What if the value of the tag is different ？
It's nothing , The result is that all statements are skipped ; The program will not terminate , No errors reported , Because this situation is c It is not suitable to report errors .
What if you don't want to ignore the values of expressions that don't match all tags ？
You can add an item to the statement list default Clause , Put the label below default： Write in any one case Where labels can appear （ When switch The value of the expression does not match all case The value of the tag , This default The statement after the clause will be executed . all , Every switch Only one... Can appear in the statement default Clause , But it can appear anywhere in the statement list , And the statement flow will run through case The label runs through default Clause .）
practice ：
int
main()
{
int
n
=
1;
int
m
=
2;
switch (
n)
{
case
1:
m
++;
case
2:
n
++;
case
3:
switch (
n)
{
//switch Allow nesting
case
1:
n
++;
case
2:
m
++;
n
++;
break;
}
case
4:
m
++;
break;
default:
break;
}
printf(
"m=%d,n=%d\n",
m,
n);
return
0;
}
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1. Loop statement
while for do while
1）.while sentence
while Grammatical structure ：（while sentence , Loop can be realized ）
while（ expression ）
Loop statement ;
practice ： Print... On the screen 1~10
break stop it
continue skip , Make the entry judgment of the next cycle
Conclusion ：continue It means a dead cycle （ The cycle is not over ）
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