[C] If, For, While, and Switch

If statement is composed of conditional decision statement and execution statement. If the conditional expression in If() is true, the subsequent execution statement is executed, and if it is false, it is not executed. The else used with If is executed when the If() conditional expression is false or the higher conditional expression is not executed. When searching for multiple conditions, you can execute each execution statement corresponding to multiple conditions by adding an else if() statement.

  #include <stdio.h>

    int main(){
        int a=1;
        int b=1;

        if(a==1){
            b=3;            
        }
        printf("%d\n", b);

        if(a==2){
            b=2;
        }
        else{
            b=4;
        }
        printf("%d\n", b);

        if(a==2){
            b=2;
        }
        else if (a==4){
            b=4;
        }
        else{
            b=6;
        }
        printf("%d\n", b);

        if(a==1) printf("%d\n",b);


        return 0;
    }

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The Switch statement is composed of conditional expressions and execution statements in the same way as the If statement. Various conditional expressions can be implemented more simply than If statements, but the types of conditional expressions and execution statements are limited. Variable expressions are possible in conditional expressions, but they must be integer expressions, and case comparison values in execution statements cannot be used as variables and only constant expressions are possible. In other words, it is not available in various ways compared to If statement, but it is highly useful when there are multiple result values for one conditional expression.

    #include <stdio.h>

    int main(){
        int a=1;
        int b=1;
        int c=1;

        switch(c){
            case 1: printf("%d\n", a);
            case 2: printf("%d\n", b); break;
            case 3: printf("%d\n", c);
        }

        return 0;
    }

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The For statement is the most frequently used loop along with the If statement. For statement is composed of loop statement setting and execution statement. The loop setting consists of for(initialization expression; conditional expression; post-processing expression). The conditional expression is judged every time starting from the initial value, and if true, the execution statement is executed. In the case of a post-processing expression, it is executed after the execution statement (cf. for(::) forms an infinite loop).

In the case of break and continue, it can be used in loop statements such as For and While statements. In the case of Break, the loop is ended and exited. In the case of continue, the execution of the rest of the code is stopped, and the process returns to the post-processing formula and continues executing the loop.

  #include <stdio.h>

    int main(){
        int i=0;
        int N=10;

        // N회 반복
        for(i=0; i<N; i++){
        }

        // j==N 까지 실행 
        for(int j=0; j<=N; j++){
        }

        // k=3 부터 N 번 실행
        for(int k=3; k<N+3; k++ ){
        }


        for(int m=0; m<10; m++){
            if(m==5) break;
            if(m % 2==0) printf("%d\n", m); 
        }

        return 0;
    }

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Unlike the For statement that sets the number of loops, the While statement is the most used loop when the number of times cannot be set. While statement consists of a conditional expression and an execution statement, and is repeated while the conditional expression remains true. Conditional expressions are mainly set as upper and lower limits of a value, or created by using matching conditions. In the case of the Do-While statement, the execution statement is executed first and the conditional statement is checked, so the execution statement is executed at least once. Like for loops, break and continue statements can be used (cf. while(1) forms an infinite loop, just like for(::)).

    #include <stdio.h>
   
    int main(){
        int i=0;

        while (i<10){
            i++;
        }
        printf("%d\n", i);

        while(!(i==20)){
            i++;
        }
        printf("%d\n",i);

        do{
            printf("%d\n",i);
            i++;
        } while(i<25);

    return 0;
    }
    

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