R - Multiple variables in for loop?

It seems like you're asking about a nested for loop

for (i in 1:10){
  for(j in 1:10){
    ...
  }
}

But I would recommend a different approach

Vectors <- expand.grid(vector1 = vector1,
                       vector2 = vector2)
Vectors$comparison <- with(Vectors, vector1 == vector2)

You can use outer to perform this and get a 10x10 matrix with the results:

vector <- 1:10
vector2 <- sample(vector)
outer(vector,vector2,"==")
       [,1]  [,2]  [,3]  [,4]  [,5]  [,6]  [,7]  [,8]  [,9] [,10]
 [1,] FALSE FALSE FALSE FALSE FALSE  TRUE FALSE FALSE FALSE FALSE
 [2,] FALSE FALSE  TRUE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
 [3,] FALSE FALSE FALSE FALSE  TRUE FALSE FALSE FALSE FALSE FALSE
 [4,] FALSE FALSE FALSE FALSE FALSE FALSE  TRUE FALSE FALSE FALSE
 [5,]  TRUE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
 [6,] FALSE  TRUE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
 [7,] FALSE FALSE FALSE  TRUE FALSE FALSE FALSE FALSE FALSE FALSE
 [8,] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE  TRUE FALSE
 [9,] FALSE FALSE FALSE FALSE FALSE FALSE FALSE  TRUE FALSE FALSE
[10,] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE  TRUE

You could do this with nested for loops:

for (i in 1:10) {
  for (j in 1:10) {
    # Your logic in here
  }
}

Your code works if you write two nested loops:

for(i in 1:10} {
  for (j in 1:10) {
   if vector[j] == vector2[i]
    print(variable)
  else 
    print(NA)
 }
}

The following example shows a different possibility to obtain this result more quickly:

set.seed(1234)
vector <- sample(20,10)
vector2 <- sample(20,10)
same <- vector[vector %in% vector2]
m <- matrix(NA,ncol=10,nrow=10)
for (i in 1:length(same)) m[vector==same[i], vector2==same[i]] <- same[i]
#> vector
# [1]  3 12 11 18 14 10  1  4  8  6
#> vector2
# [1] 14 11  6 16  5 13 17  4  3 12
#> m
#      [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
# [1,]   NA   NA   NA   NA   NA   NA   NA   NA    3    NA
# [2,]   NA   NA   NA   NA   NA   NA   NA   NA   NA    12
# [3,]   NA   11   NA   NA   NA   NA   NA   NA   NA    NA
# [4,]   NA   NA   NA   NA   NA   NA   NA   NA   NA    NA
# [5,]   14   NA   NA   NA   NA   NA   NA   NA   NA    NA
# [6,]   NA   NA   NA   NA   NA   NA   NA   NA   NA    NA
# [7,]   NA   NA   NA   NA   NA   NA   NA   NA   NA    NA
# [8,]   NA   NA   NA   NA   NA   NA   NA    4   NA    NA
# [9,]   NA   NA   NA   NA   NA   NA   NA   NA   NA    NA
#[10,]   NA   NA    6   NA   NA   NA   NA   NA   NA    NA 
#> same
#[1]  3 12 11 14  4  6

The advantage is here that you have just one small loop that runs only over those entries which are equal in both vectors (six numbers in this case), instead of the nested loops which run over the entire 100 entries of the matrix.

If you want to print the entries of this matrix sequentially, like in your nested loops, you can type:

print(c(m))

As far as the changes connected with your edit are concerned, I assume that this should work:

same1 <- st231_eq1_alg$Output[st231_eq1_alg$Output %in% st231_eq1_alg$Input]
idx2 <- which(duplicated(st231_eq1_alg$Output))
same2 <- st231_eq1_alg$Output[idx2]
m <- matrix(NA, ncol = 10, nrow = 10)
for(i in 1:length(same1)) m[st231_eq1_alg$Output==same1[i], st231_eq1_alg$Input==same1[i]] <- same1[i] 
for(i in 1:length(same2)) m[st231_eq1_alg$Output==same2[i], st231_eq1_alg$Output==same2[i]] <- st231_eq1_alg_inv_f[idx2[i]]
print(c(m))

I realized that the solutions above work when i and j are equal. For the situations where i and j are not equal, I use the code below:

a <- 5 
for(i in 4:6){
  for(j in a:7){
    print(i*j)
    a <- a+1
    break
  }
}

The result is 20,30,42

I found a rather easy solution if you would like to still stick with your For Loop and have multiple variables.

You simply need to use the loop to call the variables based on the index rather than directly calling the variable itself. The vectors will need to be of the same length.

so the loop will look something like this:

for (h in 1:length(Vector1)) {

i <- Vector2[h]

j <- Vector3[h]

k <- Vector4[h] ....etc
}

Hope that helps!